{"id":729,"date":"2016-11-04T03:35:01","date_gmt":"2016-11-04T03:35:01","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/microbiology\/?post_type=chapter&#038;p=729"},"modified":"2017-03-24T15:58:21","modified_gmt":"2017-03-24T15:58:21","slug":"modes-of-disease-transmission","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/chapter\/modes-of-disease-transmission\/","title":{"raw":"Modes of Disease Transmission","rendered":"Modes of Disease Transmission"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<ul>\r\n \t<li>Describe the different types of disease reservoirs<\/li>\r\n \t<li>Compare contact, vector, and vehicle modes of transmission<\/li>\r\n \t<li>Identify important disease vectors<\/li>\r\n \t<li>Explain the prevalence of nosocomial infections<\/li>\r\n<\/ul>\r\n<\/div>\r\nUnderstanding how infectious pathogens spread is critical to preventing infectious disease. Many pathogens require a living host to survive, while others may be able to persist in a dormant state outside of a living host. But having infected one host, all pathogens must also have a mechanism of transfer from one host to another or they will die when their host dies. Pathogens often have elaborate adaptations to exploit host biology, behavior, and ecology to live in and move between hosts. Hosts have evolved defenses against pathogens, but because their rates of evolution are typically slower than their pathogens (because their generation times are longer), hosts are usually at an evolutionary disadvantage. This section will explore where pathogens survive\u2014both inside and outside hosts\u2014and some of the many ways they move from one host to another.\r\n<h2>Reservoirs and Carriers<\/h2>\r\nFor pathogens to persist over long periods of time they require <strong>reservoir<\/strong><strong>s<\/strong> where they normally reside. Reservoirs can be living organisms or nonliving sites. Nonliving reservoirs can include soil and water in the environment. These may naturally harbor the organism because it may grow in that environment. These environments may also become contaminated with pathogens in human feces, pathogens shed by intermediate hosts, or pathogens contained in the remains of intermediate hosts.\r\n\r\nPathogens may have mechanisms of dormancy or resilience that allow them to survive (but typically not to reproduce) for varying periods of time in nonliving environments. For example, <strong><em>Clostridium tetani<\/em><\/strong> survives in the soil and in the presence of oxygen as a resistant endospore. Although many viruses are soon destroyed once in contact with air, water, or other non-physiological conditions, certain types are capable of persisting outside of a living cell for varying amounts of time. For example, a study that looked at the ability of <strong>influenza viruses<\/strong> to infect a cell culture after varying amounts of time on a banknote showed survival times from 48 hours to 17 days, depending on how they were deposited on the banknote.[footnote]Yves Thomas, Guido Vogel, Werner Wunderli, Patricia Suter, Mark Witschi, Daniel Koch, Caroline Tapparel, and Laurent Kaiser. \"Survival of Influenza Virus on Banknotes.\" <em>Applied and Environmental Microbiology<\/em> 74, no. 10 (2008): 3002\u20133007.[\/footnote] On the other hand, cold-causing <strong>rhinoviruses<\/strong> are somewhat fragile, typically surviving less than a day outside of physiological fluids.\r\n\r\nA human acting as a reservoir of a pathogen may or may not be capable of transmitting the pathogen, depending on the stage of infection and the pathogen. To help prevent the spread of disease among school children, the CDC has developed guidelines based on the risk of transmission during the course of the disease. For example, children with chickenpox are considered contagious for five days from the start of the rash, whereas children with most gastrointestinal illnesses should be kept home for 24 hours after the symptoms disappear.\r\n\r\nAn individual capable of transmitting a pathogen without displaying symptoms is referred to as a carrier. A <strong>passive carrier<\/strong> is contaminated with the pathogen and can mechanically transmit it to another host; however, a passive carrier is not infected. For example, a health-care professional who fails to wash his hands after seeing a patient harboring an infectious agent could become a passive carrier, transmitting the pathogen to another patient who becomes infected.\r\n\r\nBy contrast, an <strong>active carrier<\/strong> is an infected individual who can transmit the disease to others. An active carrier may or may not exhibit signs or symptoms of infection. For example, active carriers may transmit the disease during the <strong>incubation period<\/strong> (before they show signs and symptoms) or the <strong>period of convalescence<\/strong> (after symptoms have subsided). Active carriers who do not present signs or symptoms of disease despite infection are called <strong>asymptomatic carrier<\/strong><strong>s<\/strong>. Pathogens such as <strong>hepatitis B virus<\/strong>, <strong>herpes simplex virus<\/strong>, and <strong>HIV<\/strong> are frequently transmitted by asymptomatic carriers. <strong>Mary Mallon<\/strong>, better known as <strong>Typhoid Mary<\/strong>, is a famous historical example of an asymptomatic carrier. An Irish immigrant, Mallon worked as a cook for households in and around New York City between 1900 and 1915. In each household, the residents developed <strong>typhoid fever<\/strong> (caused by <strong><em>Salmonella typhi<\/em><\/strong>) a few weeks after Mallon started working. Later investigations determined that Mallon was responsible for at least 122 cases of typhoid fever, five of which were fatal.[footnote]Filio Marineli, Gregory Tsoucalas, Marianna Karamanou, and George Androutsos. \"Mary Mallon (1869\u20131938) and the History of Typhoid Fever.\" <em>Annals of Gastroenterology<\/em> 26 (2013): 132\u2013134. http:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC3959940\/pdf\/AnnGastroenterol-26-132.pdf.[\/footnote] See <a href=\".\/chapter\/bacterial-infections-of-the-gastrointestinal-tract\/\" target=\"_blank\">\"Typhoid Mary\" in Bacterial Infections of the Gastrointestinal Tract<\/a> for more about the Mallon case.\r\n\r\nA pathogen may have more than one living reservoir. In zoonotic diseases, animals act as reservoirs of human disease and transmit the infectious agent to humans through direct or indirect contact. In some cases, the disease also affects the animal, but in other cases the animal is asymptomatic.\r\n\r\nIn parasitic infections, the parasite\u2019s preferred host is called the <strong>definitive host<\/strong>. In parasites with complex life cycles, the definitive host is the host in which the parasite reaches sexual maturity. Some parasites may also infect one or more <strong>intermediate host<\/strong><strong>s<\/strong> in which the parasite goes through several immature life cycle stages or reproduces asexually.\r\n<div class=\"textbox\"><a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC1911442\/\" target=\"_blank\">George Soper, the sanitary engineer who traced the typhoid outbreak to Mary Mallon, gives an account of his investigation<\/a>, an example of descriptive epidemiology, in \"The Curious Career of Typhoid Mary.\"<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>List some nonliving reservoirs for pathogens.<\/li>\r\n \t<li>Explain the difference between a passive carrier and an active carrier.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>Transmission<\/h2>\r\nRegardless of the reservoir, <strong>transmission<\/strong> must occur for an infection to spread. First, transmission from the reservoir to the individual must occur. Then, the individual must transmit the infectious agent to other susceptible individuals, either directly or indirectly. Pathogenic microorganisms employ diverse transmission mechanisms.\r\n<h3>Contact Transmission<\/h3>\r\n<strong>Contact transmission<\/strong> includes direct contact or indirect contact. <strong>Person-to-person transmission<\/strong> is a form of <strong>direct contact transmission<\/strong>. Here the agent is transmitted by physical contact between two individuals (Figure\u00a01) through actions such as touching, kissing, sexual intercourse, or <strong>droplet sprays<\/strong>. Direct contact can be categorized as vertical, horizontal, or droplet transmission. <strong>Vertical direct contact transmission<\/strong> occurs when pathogens are transmitted from mother to child during pregnancy, birth, or breastfeeding. Other kinds of direct contact transmission are called <strong>horizontal direct contact transmission<\/strong>. Often, contact between <strong>mucous membranes<\/strong> is required for entry of the pathogen into the new host, although skin-to-skin contact can lead to mucous membrane contact if the new host subsequently touches a mucous membrane. Contact transmission may also be site-specific; for example, some diseases can be transmitted by sexual contact but not by other forms of contact.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"900\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165455\/OSC_Microbio_16_03_Contact.jpg\" alt=\"Photo of person kissing a child and one of person playing arm wrestling with a child.\" width=\"900\" height=\"350\" data-media-type=\"image\/jpeg\" \/> Figure\u00a01. Direct contact transmission of pathogens can occur through physical contact. Many pathogens require contact with a mucous membrane to enter the body, but the host may transfer the pathogen from another point of contact (e.g., hand) to a mucous membrane (e.g., mouth or eye). (credit left: modification of work by Lisa Doehnert)[\/caption]\r\n\r\nWhen an individual coughs or sneezes, small droplets of mucus that may contain pathogens are ejected. This leads to direct <strong>droplet transmission<\/strong>, which refers to droplet transmission of a pathogen to a new host over distances of one meter or less. A wide variety of diseases are transmitted by droplets, including <strong>influenza<\/strong> and many forms of <strong>pneumonia<\/strong>. Transmission over distances greater than one meter is called <strong>airborne transmission<\/strong>.\r\n\r\n<strong>Indirect contact transmission<\/strong> involves inanimate objects called <strong>fomites<\/strong> that become contaminated by pathogens from an infected individual or reservoir (Figure\u00a02). For example, an individual with the common cold may sneeze, causing droplets to land on a fomite such as a tablecloth or carpet, or the individual may wipe her nose and then transfer mucus to a fomite such as a doorknob or towel. Transmission occurs indirectly when a new susceptible host later touches the fomite and transfers the contaminated material to a susceptible portal of entry. Fomites can also include objects used in clinical settings that are not properly sterilized, such as syringes, needles, catheters, and surgical equipment. Pathogens transmitted indirectly via such fomites are a major cause of healthcare-associated infections (see <a href=\".\/chapter\/controlling-microbial-growth\/\" target=\"_blank\">Controlling Microbial Growth<\/a>).\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1100\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165458\/OSC_Microbio_16_03_Indirect.jpg\" alt=\"Photos of person touching doorknob, towel on a hook, and end of a syringe.\" width=\"1100\" height=\"237\" data-media-type=\"image\/jpeg\" \/> Figure\u00a02. Fomites are nonliving objects that facilitate the indirect transmission of pathogens. Contaminated doorknobs, towels, and syringes are all common examples of fomites. (credit left: modification of work by Kate Ter Haar; credit middle: modification of work by Vernon Swanepoel; credit right: modification of work by \"Zaldylmg\"\/Flickr)[\/caption]\r\n<h3>Vehicle Transmission<\/h3>\r\nThe term <strong>vehicle transmission<\/strong> refers to the transmission of pathogens through vehicles such as water, food, and air. Water contamination through poor sanitation methods leads to <strong>waterborne transmission<\/strong> of disease. Waterborne disease remains a serious problem in many regions throughout the world. The World Health Organization (WHO) estimates that contaminated drinking water is responsible for more than 500,000 deaths each year.[footnote]World Health Organization.\u00a0Fact sheet No. 391<em>\u2014Drinking Water.<\/em> June 2005. http:\/\/www.who.int\/mediacentre\/factsheets\/fs391\/en.[\/footnote] Similarly, food contaminated through poor handling or storage can lead to <strong>foodborne transmission<\/strong> of disease (Figure\u00a03).\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"400\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165501\/OSC_Microbio_16_03_Food.jpg\" alt=\"Photo of food at a cafeteria with glass shielding over the food.\" width=\"400\" height=\"288\" data-media-type=\"image\/jpeg\" \/> Figure\u00a03. Food is an important vehicle of transmission for pathogens, especially of the gastrointestinal and upper respiratory systems. Notice the glass shield above the food trays, designed to prevent pathogens ejected in coughs and sneezes from entering the food. (credit: Fort George G. Meade Public Affairs Office)[\/caption]\r\n\r\nDust and fine particles known as <strong>aerosols<\/strong>, which can float in the air, can carry pathogens and facilitate the <strong>airborne transmission<\/strong> of disease. For example, dust particles are the dominant mode of transmission of <strong>hantavirus<\/strong> to humans. Hantavirus is found in mouse feces, urine, and saliva, but when these substances dry, they can disintegrate into fine particles that can become airborne when disturbed; inhalation of these particles can lead to a serious and sometimes fatal respiratory infection.\r\n\r\nAlthough droplet transmission over short distances is considered contact transmission as discussed above, longer distance transmission of droplets through the air is considered vehicle transmission. Unlike larger particles that drop quickly out of the air column, fine mucus droplets produced by coughs or sneezes can remain suspended for long periods of time, traveling considerable distances. In certain conditions, droplets desiccate quickly to produce a <strong>droplet nucleus<\/strong> that is capable of transmitting pathogens; air temperature and humidity can have an impact on effectiveness of airborne transmission.\r\n\r\nTuberculosis is often transmitted via airborne transmission when the causative agent, <em>Mycobacterium tuberculosis<\/em>, is released in small particles with coughs. Because <strong>tuberculosis<\/strong> requires as few as 10 microbes to initiate a new infection, patients with tuberculosis must be treated in rooms equipped with special ventilation, and anyone entering the room should wear a mask.\r\n<div class=\"textbox examples\">\r\n<h3>Clinical Focus: Florida, Resolution<\/h3>\r\nThis example continues the\u00a0story that started on <a href=\"https:\/\/courses.lumenlearning.com\/microbiology\/chapter\/tracking-infectious-diseases\/chapter\/the-language-of-epidemiologists\/\" target=\"_blank\">The Language of\u00a0Epidemiologists<\/a>\u00a0and <a href=\".\/chapter\/tracking-infectious-diseases\/\" target=\"_blank\">Tracking Infectious Diseases<\/a>.\r\n\r\nAfter identifying the source of the contaminated turduckens, the Florida public health office notified the CDC, which requested an expedited inspection of the facility by state inspectors. Inspectors found that a machine used to process the chicken was contaminated with <em>Salmonella<\/em> as a result of substandard cleaning protocols. Inspectors also found that the process of stuffing and packaging the turduckens prior to refrigeration allowed the meat to remain at temperatures conducive to bacterial growth for too long. The contamination and the delayed refrigeration led to vehicle (food) transmission of the bacteria in turduckens.\r\n\r\nBased on these findings, the plant was shut down for a full and thorough decontamination. All turduckens produced in the plant were recalled and pulled from store shelves ahead of the December holiday season, preventing further outbreaks.\r\n\r\n<\/div>\r\n<h3>Vector Transmission<\/h3>\r\nDiseases can also be transmitted by a mechanical or biological <strong>vector<\/strong>, an animal (typically an <strong>arthropod<\/strong>) that carries the disease from one host to another. <strong>Mechanical transmission<\/strong> is facilitated by a <strong>mechanical vector<\/strong>, an animal that carries a pathogen from one host to another without being infected itself. For example, a fly may land on fecal matter and later transmit bacteria from the feces to food that it lands on; a human eating the food may then become infected by the bacteria, resulting in a case of diarrhea or dysentery (Figure\u00a04).\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1300\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165504\/OSC_Microbio_16_03_Vector.jpg\" alt=\"a) Step 1: fly picks up pathogen from fecal matter and carries it on its body. 2: Fly transfers pathogen to food. 3: Person eats contaminated food and gets sick. B) Step 1: Infected mosquito bites uninfected person. 2: Infections spreads through body and into red blood cells. 3: Second mosquito bites infected person. Mosquito may now transmit infection to another person.\" width=\"1300\" height=\"718\" data-media-type=\"image\/jpeg\" \/> Figure\u00a04. (a) A mechanical vector carries a pathogen on its body from one host to another, not as an infection. (b) A biological vector carries a pathogen from one host to another after becoming infected itself.[\/caption]\r\n\r\n<strong>Biological transmission<\/strong> occurs when the pathogen reproduces within a <strong>biological vector<\/strong> that transmits the pathogen from one host to another (Figure\u00a04). Arthropods are the main vectors responsible for biological transmission (Table 1). Most arthropod vectors transmit the pathogen by biting the host, creating a wound that serves as a portal of entry. The pathogen may go through part of its reproductive cycle in the gut or salivary glands of the arthropod to facilitate its transmission through the bite. For example, hemipterans (called \"kissing bugs\" or \"assassin bugs\") transmit <strong>Chagas disease<\/strong> to humans by defecating when they bite, after which the human scratches or rubs the infected feces into a mucous membrane or break in the skin.\r\n\r\nBiological insect vectors include <strong>mosquitoes<\/strong>, which transmit <strong>malaria<\/strong> and other diseases, and <strong>lice<\/strong>, which transmit <strong>typhus<\/strong>. Other arthropod vectors can include arachnids, primarily <strong>ticks<\/strong>, which transmit <strong>Lyme disease<\/strong> and other diseases, and <strong>mites<\/strong>, which transmit <strong>scrub typhus<\/strong> and <strong>rickettsial pox<\/strong>. Biological transmission, because it involves survival and reproduction within a parasitized vector, complicates the biology of the pathogen and its transmission. There are also important non-arthropod vectors of disease, including mammals and birds. Various species of mammals can transmit <strong>rabies<\/strong> to humans, usually by means of a bite that transmits the rabies virus. Chickens and other domestic poultry can transmit <strong>avian influenza<\/strong> to humans through direct or indirect contact with <strong>avian influenza virus A<\/strong> shed in the birds\u2019 saliva, mucous, and feces.\r\n<table>\r\n<thead>\r\n<tr>\r\n<th colspan=\"4\">Table 1.\u00a0Common Arthropod Vectors and Selected Pathogens[footnote]Image credits: \"Black fly\", \"Tick\", \"Tsetse fly\": modification of work by USDA; \"Flea\": modification of work by Centers for Disease Control and Prevention; \"Louse\", \"Mosquito\", \"Sand fly\": modification of work by James Gathany, Centers for Disease Control and Prevention; \"Kissing bug\": modification of work by Glenn Seplak; \"Mite\": modification of work by Michael Wunderli[\/footnote]<\/th>\r\n<\/tr>\r\n<tr>\r\n<th>Vector<\/th>\r\n<th>Species<\/th>\r\n<th>Pathogen<\/th>\r\n<th>Disease<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td>\n\n\r\n\r\n[caption id=\"attachment_1982\" align=\"alignnone\" width=\"251\"]<img class=\" wp-image-1982\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24154842\/01_Blackfly.png\" alt=\"A black fly on a human hand\" width=\"251\" height=\"171\" \/> Black fly[\/caption]<\/td>\r\n<td><em>Simulium<\/em> spp.<\/td>\r\n<td><em>Onchocerca volvulus<\/em><\/td>\r\n<td>Onchocerciasis (river blindness)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td rowspan=\"2\">\n\n\r\n\r\n[caption id=\"attachment_1983\" align=\"alignnone\" width=\"250\"]<img class=\" wp-image-1983\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155104\/02_Flea.png\" alt=\"Flea\" width=\"250\" height=\"198\" \/> Flea[\/caption]<\/td>\r\n<td rowspan=\"2\"><em>Xenopsylla cheopis<\/em><\/td>\r\n<td><em>Rickettsia typhi<\/em><\/td>\r\n<td>Murine typhus<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Yersinia pestis<\/em><\/td>\r\n<td>Plague<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>\n\n\r\n\r\n[caption id=\"attachment_1984\" align=\"alignnone\" width=\"250\"]<img class=\" wp-image-1984\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155144\/03_Kissingbug.png\" alt=\"a Kissing bug on a human hand\" width=\"250\" height=\"187\" \/> Kissing bug[\/caption]<\/td>\r\n<td><em>Triatoma<\/em> spp.<\/td>\r\n<td><em>Trypanosoma cruzi<\/em><\/td>\r\n<td>Chagas disease<\/td>\r\n<\/tr>\r\n<tr>\r\n<td rowspan=\"3\">\n\n\r\n\r\n[caption id=\"attachment_1985\" align=\"alignnone\" width=\"250\"]<img class=\" wp-image-1985\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155217\/04_Louse.png\" alt=\"A louse on a human hand\" width=\"250\" height=\"160\" \/> Louse[\/caption]<\/td>\r\n<td rowspan=\"3\"><em>Pediculus humanus humanus<\/em><\/td>\r\n<td><em>Bartonella quintana<\/em><\/td>\r\n<td>Trench fever<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Borrelia recurrentis<\/em><\/td>\r\n<td>Relapsing fever<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Rickettsia prowazekii<\/em><\/td>\r\n<td>Typhus<\/td>\r\n<\/tr>\r\n<tr>\r\n<td rowspan=\"2\">\n\n\r\n\r\n[caption id=\"attachment_1986\" align=\"alignnone\" width=\"250\"]<img class=\" wp-image-1986\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155252\/05_Mite.png\" alt=\"A micrograph of a mite\" width=\"250\" height=\"253\" \/> Mite (chigger)[\/caption]<\/td>\r\n<td><em>Leptotrombidium<\/em> spp.<\/td>\r\n<td><em>Orientia tsutsugamushi<\/em><\/td>\r\n<td>Scrub typhus<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Liponyssoides sanguineus<\/em><\/td>\r\n<td><em>Rickettsia akari<\/em><\/td>\r\n<td>Rickettsialpox<\/td>\r\n<\/tr>\r\n<tr>\r\n<td rowspan=\"3\">\n\n\r\n\r\n[caption id=\"attachment_1987\" align=\"alignnone\" width=\"250\"]<img class=\" wp-image-1987\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155347\/06_Mosquito.png\" alt=\"A mosquito drinking blood from a human\" width=\"250\" height=\"180\" \/> Mosquito[\/caption]<\/td>\r\n<td><em>Aedes<\/em> spp., <em>Haemogogus<\/em> spp.<\/td>\r\n<td>Yellow fever virus<\/td>\r\n<td>Yellow fever<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Anopheles<\/em> spp.<\/td>\r\n<td>Plasmodium falciparum<\/td>\r\n<td>Malaria<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Cutex pipiens<\/em><\/td>\r\n<td>West nile virus<\/td>\r\n<td>West nile disease<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>\n\n\r\n\r\n[caption id=\"attachment_1988\" align=\"alignnone\" width=\"250\"]<img class=\"wp-image-1988\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155447\/07_Sandfly.png\" alt=\"A sand fly drinking blood from a human\" width=\"250\" height=\"192\" \/> Sand fly[\/caption]<\/td>\r\n<td><em>Phlebotomus<\/em> spp.<\/td>\r\n<td><em>Leishmania<\/em> spp.<\/td>\r\n<td>Leishmaniasis<\/td>\r\n<\/tr>\r\n<tr>\r\n<td rowspan=\"2\">\n\n\r\n\r\n[caption id=\"attachment_1989\" align=\"alignnone\" width=\"249\"]<img class=\" wp-image-1989\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155530\/08_Tick.png\" alt=\"A tick sitting on a leaf\" width=\"249\" height=\"191\" \/> Tick[\/caption]<\/td>\r\n<td><em>Ixodes<\/em> spp.<\/td>\r\n<td><em>Borrelia<\/em> spp.<\/td>\r\n<td>Lyme disease<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Dermacentor<\/em> spp. and others<\/td>\r\n<td><em>Rickettsi rickettsia<\/em><\/td>\r\n<td>Rocky Mountain spotted fever<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>\n\n\r\n\r\n[caption id=\"attachment_1990\" align=\"alignnone\" width=\"250\"]<img class=\" wp-image-1990\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155620\/09_Tsetsefly.png\" alt=\"A tsetse fly\" width=\"250\" height=\"166\" \/> Tsetse fly[\/caption]<\/td>\r\n<td><em> Glossina<\/em> spp.<\/td>\r\n<td><em>Trypanosoma brucei\u00a0<\/em><\/td>\r\n<td>African trypanosomiasis (sleeping sickness)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>Describe how diseases can be transmitted through the air.<\/li>\r\n \t<li>Explain the difference between a mechanical vector and a biological vector.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"textbox shaded\">\r\n<h3>Using GMOs to Stop the Spread of Zika<\/h3>\r\nIn 2016, an epidemic of the <strong>Zika virus<\/strong> was linked to a high incidence of birth defects in South America and Central America. As winter turned to spring in the northern hemisphere, health officials correctly predicted the virus would spread to North America, coinciding with the breeding season of its major vector, the <strong><em>Aedes aegypti<\/em><\/strong> mosquito.\r\n\r\nThe range of the <em>A. aegypti<\/em> mosquito extends well into the southern United States (Figure\u00a05). Because these same mosquitoes serve as vectors for other problematic diseases (<strong>dengue fever<\/strong>, <strong>yellow fever<\/strong>, and others), various methods of mosquito control have been proposed as solutions. Chemical pesticides have been used effectively in the past, and are likely to be used again; but because chemical pesticides can have negative impacts on the environment, some scientists have proposed an alternative that involves genetically engineering <em>A. aegypti<\/em> so that it cannot reproduce. This method, however, has been the subject of some controversy.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1200\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165512\/OSC_Microbio_16_03_Mosquito.jpg\" alt=\"Micrograph of brown dots of about 50 nm inside cells; dots re labeled Zika virus. Photo of mosquito labeled Aedes aegypti. Map of where mosquitoes are found in the US. Aedes aegypti and Aedes albopictus are both found in the lower half of the US, reaching up to Connecticut, Missouri, and California. Aedes albopictus reaches further north in the eastern part o the country; through Minnosota. Aedes aegypti reaches a bit further into Utah and is in Puerto Rico.\" width=\"1200\" height=\"608\" data-media-type=\"image\/jpeg\" \/> Figure\u00a05. The Zika virus is an enveloped virus transmitted by mosquitoes, especially <em>Aedes aegypti<\/em>. The range of this mosquito includes much of the United States, from the Southwest and Southeast to as far north as the Mid-Atlantic. The range of <em>A. albopictus<\/em>, another vector, extends even farther north to New England and parts of the Midwest. (credit micrograph: modification of work by Cynthia Goldsmith, Centers for Disease Control and Prevention; credit photo: modification of work by James Gathany, Centers for Disease Control and Prevention; credit map: modification of work by Centers for Disease Control and Prevention)[\/caption]\r\n\r\nOne method that has worked in the past to control pests, with little apparent downside, has been sterile male introductions. This method controlled the screw-worm fly pest in the southwest United States and fruit fly pests of fruit crops. In this method, males of the target species are reared in the lab, sterilized with radiation, and released into the environment where they mate with wild females, who subsequently bear no live offspring. Repeated releases shrink the pest population.\r\n\r\nA similar method, taking advantage of recombinant DNA technology,[footnote]Blandine Massonnet-Bruneel, Nicole Corre-Catelin, Renaud Lacroix, Rosemary S. Lees, Kim Phuc Hoang, Derric Nimmo, Luke Alphey, and Paul Reiter. \"Fitness of Transgenic Mosquito <em>Aedes aegypti<\/em> Males Carrying a Dominant Lethal Genetic System.\" <em>PLOS ONE<\/em> 8, no. 5 (2013): e62711.[\/footnote] introduces a dominant lethal allele into male mosquitoes that is suppressed in the presence of tetracycline (an antibiotic) during laboratory rearing. The males are released into the environment and mate with female mosquitoes. Unlike the sterile male method, these matings produce offspring, but they die as larvae from the lethal gene in the absence of tetracycline in the environment. As of 2016, this method has yet to be implemented in the United States, but a UK company tested the method in Piracicaba, Brazil, and found an 82% reduction in wild <em>A. aegypti<\/em> larvae and a 91% reduction in dengue cases in the treated area.[footnote]Richard Levine. \"Cases of Dengue Drop 91 Percent Due to Genetically Modified\u00a0Mosquitoes.\" <em>Entomology Today.<\/em> https:\/\/entomologytoday.org\/2016\/07\/14\/cases-of-dengue-drop-91-due-to-genetically-modified-mosquitoes.[\/footnote] In August 2016, amid news of Zika infections in several Florida communities, the FDA gave the UK company permission to test this same mosquito control method in Key West, Florida, pending compliance with local and state regulations and a referendum in the affected communities.\r\n\r\nThe use of <strong>genetically modified organisms (GMOs)<\/strong> to control a disease vector has its advocates as well as its opponents. In theory, the system could be used to drive the <em>A. aegypti<\/em> mosquito extinct\u2014a noble goal according to some, given the damage they do to human populations.[footnote]Olivia Judson. \"A Bug\u2019s Death.\" <em>The New York Times<\/em>, September 25, 2003. http:\/\/www.nytimes.com\/2003\/09\/25\/opinion\/a-bug-s-death.html.[\/footnote] But opponents of the idea are concerned that the gene could escape the species boundary of <em>A. aegypti<\/em> and cause problems in other species, leading to unforeseen ecological consequences. Opponents are also wary of the program because it is being administered by a for-profit corporation, creating the potential for conflicts of interest that would have to be tightly regulated; and it is not clear how any unintended consequences of the program could be reversed.\r\n\r\nThere are other epidemiological considerations as well. <em>Aedes aegypti<\/em> is apparently not the only vector for the Zika virus. <strong><em>Aedes albopictus<\/em><\/strong>, the Asian tiger mosquito, is also a vector for the Zika virus.[footnote]Gilda Grard, M\u00e9lanie Caron, Illich Manfred Mombo, Dieudonn\u00e9 Nkoghe, Statiana Mboui Ondo, Davy Jiolle, Didier Fontenille, Christophe Paupy, and Eric Maurice Leroy. \"Zika Virus in Gabon (Central Africa)\u20132007: A New Threat from <em>Aedes albopictus<\/em>?\" <em>PLOS Neglected Tropical Diseases<\/em> 8, no. 2 (2014): e2681.[\/footnote] <em>A. albopictus<\/em> is now widespread around the planet including much of the United States (Figure\u00a05). Many other mosquitoes have been found to harbor Zika virus, though their capacity to act as vectors is unknown.[footnote]Const\u00e2ncia F.J. Ayres. \"Identification of Zika Virus Vectors and Implications for Control.\" <em>The Lancet Infectious Diseases<\/em> 16, no. 3 (2016): 278\u2013279.[\/footnote] Genetically modified strains of <em>A. aegypti<\/em> will not control the other species of vectors. Finally, the Zika virus can apparently be transmitted sexually between human hosts, from mother to child, and possibly through blood transfusion. All of these factors must be considered in any approach to controlling the spread of the virus.\r\n\r\nClearly there are risks and unknowns involved in conducting an open-environment experiment of an as-yet poorly understood technology. But allowing the Zika virus to spread unchecked is also risky. Does the threat of a Zika epidemic justify the ecological risk of genetically engineering mosquitos? Are current methods of mosquito control sufficiently ineffective or harmful that we need to try untested alternatives? These are the questions being put to public health officials now.\r\n\r\n<\/div>\r\n<h2>Quarantining<\/h2>\r\nIndividuals suspected or known to have been exposed to certain contagious pathogens may be <strong>quarantined<\/strong>, or isolated to prevent transmission of the disease to others. Hospitals and other health-care facilities generally set up special wards to isolate patients with particularly hazardous diseases such as tuberculosis or Ebola (Figure\u00a06). Depending on the setting, these wards may be equipped with special air-handling methods, and personnel may implement special protocols to limit the risk of transmission, such as personal protective equipment or the use of chemical disinfectant sprays upon entry and exit of medical personnel.\r\n\r\nThe duration of the quarantine depends on factors such as the <strong>incubation period<\/strong> of the disease and the evidence suggestive of an infection. The patient may be released if signs and symptoms fail to materialize when expected or if preventive treatment can be administered in order to limit the risk of transmission. If the infection is confirmed, the patient may be compelled to remain in isolation until the disease is no longer considered contagious.\r\n\r\nIn the United States, public health authorities may only quarantine patients for certain diseases, such as <strong>cholera<\/strong>, <strong>diphtheria<\/strong>, infectious <strong>tuberculosis<\/strong>, and strains of <strong>influenza<\/strong> capable of causing a <strong>pandemic<\/strong>. Individuals entering the United States or moving between states may be quarantined by the CDC if they are suspected of having been exposed to one of these diseases. Although the CDC routinely monitors entry points to the United States for crew or passengers displaying illness, quarantine is rarely implemented.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1100\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165515\/OSC_Microbio_16_03_Isolate.jpg\" alt=\"a) Photo of a plastic tent next to an airplane b) Photo of beds in a room.\" width=\"1100\" height=\"460\" data-media-type=\"image\/jpeg\" \/> Figure\u00a06. (a) The Aeromedical Biological Containment System (ABCS) is a module designed by the CDC and Department of Defense specifically for transporting highly contagious patients by air. (b) An isolation ward for <strong>Ebola<\/strong> patients in Lagos, Nigeria. (credit a: modification of work by Centers for Disease Control and Prevention; credit b: modification of work by CDC Global)[\/caption]\r\n<h2>Healthcare-Associated (Nosocomial) Infections<\/h2>\r\nHospitals, retirement homes, and prisons attract the attention of epidemiologists because these settings are associated with increased incidence of certain diseases. Higher rates of transmission may be caused by characteristics of the environment itself, characteristics of the population, or both. Consequently, special efforts must be taken to limit the risks of infection in these settings.\r\n\r\nInfections acquired in health-care facilities, including hospitals, are called <strong>nosocomial infections<\/strong> or <strong>healthcare-associated infections (HAI)<\/strong>. HAIs are often connected with surgery or other invasive procedures that provide the pathogen with access to the portal of infection. For an infection to be classified as an HAI, the patient must have been admitted to the health-care facility for a reason other than the infection. In these settings, patients suffering from primary disease are often afflicted with compromised immunity and are more susceptible to secondary infection and opportunistic pathogens.\r\n\r\nIn 2011, more than 720,000 HAIs occurred in hospitals in the United States, according to the CDC. About 22% of these HAIs occurred at a surgical site, and cases of <strong>pneumonia<\/strong> accounted for another 22%; <strong>urinary tract infections<\/strong> accounted for an additional 13%, and primary <strong>bloodstream infections<\/strong> 10%.[footnote]Centers for Disease Control and Prevention. \"HAI Data and Statistics.\" 2016. http:\/\/www.cdc.gov\/hai\/surveillance. Accessed Jan 2, 2016.[\/footnote] Such HAIs often occur when pathogens are introduced to patients\u2019 bodies through contaminated surgical or medical equipment, such as <strong>catheters<\/strong> and respiratory ventilators. Health-care facilities seek to limit nosocomial infections through training and hygiene protocols such as those described in <a href=\".\/chapter\/introduction-12\/\" target=\"_blank\">Control of Microbial Growth<\/a>.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>Give some reasons why HAIs occur.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Key Concepts and Summary<\/h3>\r\n<ul>\r\n \t<li><strong>Reservoirs<\/strong> of human disease can include the human and animal populations, soil, water, and inanimate objects or materials.<\/li>\r\n \t<li><strong>Contact transmission<\/strong> can be <strong>direct<\/strong> or <strong>indirect<\/strong> through physical contact with either an infected host (direct) or contact with a fomite that an infected host has made contact with previously (indirect).<\/li>\r\n \t<li>Vector transmission occurs when a living organism carries an infectious agent on its body (<strong>mechanical<\/strong>) or as an infection host itself (<strong>biological<\/strong>), to a new host.<\/li>\r\n \t<li><strong>Vehicle transmission<\/strong> occurs when a substance, such as soil, water, or air, carries an infectious agent to a new host.<\/li>\r\n \t<li><strong>Healthcare-associated infections (HAI)<\/strong>, or <strong>nosocomial infections<\/strong>, are acquired in a clinical setting. Transmission is facilitated by medical interventions and the high concentration of susceptible, immunocompromised individuals in clinical settings.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Multiple Choice<\/h3>\r\nWhich is the most common type of biological vector of human disease?\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>viruses<\/li>\r\n \t<li>bacteria<\/li>\r\n \t<li>mammals<\/li>\r\n \t<li>arthropods<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"727911\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"727911\"]Answer d. Arthropods are\u00a0the most common type of biological vector of human disease.[\/hidden-answer]\r\n\r\nA mosquito bites a person who subsequently develops a fever and abdominal rash. What type of transmission would this be?\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>mechanical vector transmission<\/li>\r\n \t<li>biological vector transmission<\/li>\r\n \t<li>direct contact transmission<\/li>\r\n \t<li>vehicle transmission<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"600518\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"600518\"]Answer b. This would be\u00a0biological vector transmission.[\/hidden-answer]\r\n\r\nCattle are allowed to pasture in a field that contains the farmhouse well, and the farmer\u2019s family becomes ill with a gastrointestinal pathogen after drinking the water. What type of transmission of infectious agents would this be?\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>biological vector transmission<\/li>\r\n \t<li>direct contact transmission<\/li>\r\n \t<li>indirect contact transmission<\/li>\r\n \t<li>vehicle transmission<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"858091\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"858091\"]Answer d. This would be vehicle transmission.[\/hidden-answer]\r\n\r\nA blanket from a child with chickenpox is likely to be contaminated with the virus that causes chickenpox (Varicella-zoster virus). What is the blanket called?\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>fomite<\/li>\r\n \t<li>host<\/li>\r\n \t<li>pathogen<\/li>\r\n \t<li>vector<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"686339\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"686339\"]Answer a.\u00a0The blanket is a\u00a0fomite.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Fill in the Blank<\/h3>\r\nA patient in the hospital with a urinary catheter develops a bladder infection. This is an example of a(n) ________ infection.\r\n[reveal-answer q=\"978282\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"978282\"]A patient in the hospital with a urinary catheter develops a bladder infection. This is an example of a\u00a0<strong>nosocomial or healthcare-associated<\/strong> infection.[\/hidden-answer]\r\n\r\nA ________ is an animal that can transfer infectious pathogens from one host to another.\r\n[reveal-answer q=\"227054\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"227054\"]A <strong>vector<\/strong> is an animal that can transfer infectious pathogens from one host to another.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\nDifferentiate between droplet vehicle transmission and airborne transmission.\r\n\r\nMany people find that they become ill with a cold after traveling by airplane. The air circulation systems of commercial aircraft use HEPA filters that should remove any infectious agents that pass through them. What are the possible reasons for increased incidence of colds after flights?\r\n\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<ul>\n<li>Describe the different types of disease reservoirs<\/li>\n<li>Compare contact, vector, and vehicle modes of transmission<\/li>\n<li>Identify important disease vectors<\/li>\n<li>Explain the prevalence of nosocomial infections<\/li>\n<\/ul>\n<\/div>\n<p>Understanding how infectious pathogens spread is critical to preventing infectious disease. Many pathogens require a living host to survive, while others may be able to persist in a dormant state outside of a living host. But having infected one host, all pathogens must also have a mechanism of transfer from one host to another or they will die when their host dies. Pathogens often have elaborate adaptations to exploit host biology, behavior, and ecology to live in and move between hosts. Hosts have evolved defenses against pathogens, but because their rates of evolution are typically slower than their pathogens (because their generation times are longer), hosts are usually at an evolutionary disadvantage. This section will explore where pathogens survive\u2014both inside and outside hosts\u2014and some of the many ways they move from one host to another.<\/p>\n<h2>Reservoirs and Carriers<\/h2>\n<p>For pathogens to persist over long periods of time they require <strong>reservoir<\/strong><strong>s<\/strong> where they normally reside. Reservoirs can be living organisms or nonliving sites. Nonliving reservoirs can include soil and water in the environment. These may naturally harbor the organism because it may grow in that environment. These environments may also become contaminated with pathogens in human feces, pathogens shed by intermediate hosts, or pathogens contained in the remains of intermediate hosts.<\/p>\n<p>Pathogens may have mechanisms of dormancy or resilience that allow them to survive (but typically not to reproduce) for varying periods of time in nonliving environments. For example, <strong><em>Clostridium tetani<\/em><\/strong> survives in the soil and in the presence of oxygen as a resistant endospore. Although many viruses are soon destroyed once in contact with air, water, or other non-physiological conditions, certain types are capable of persisting outside of a living cell for varying amounts of time. For example, a study that looked at the ability of <strong>influenza viruses<\/strong> to infect a cell culture after varying amounts of time on a banknote showed survival times from 48 hours to 17 days, depending on how they were deposited on the banknote.<a class=\"footnote\" title=\"Yves Thomas, Guido Vogel, Werner Wunderli, Patricia Suter, Mark Witschi, Daniel Koch, Caroline Tapparel, and Laurent Kaiser. &quot;Survival of Influenza Virus on Banknotes.&quot; Applied and Environmental Microbiology 74, no. 10 (2008): 3002\u20133007.\" id=\"return-footnote-729-1\" href=\"#footnote-729-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a> On the other hand, cold-causing <strong>rhinoviruses<\/strong> are somewhat fragile, typically surviving less than a day outside of physiological fluids.<\/p>\n<p>A human acting as a reservoir of a pathogen may or may not be capable of transmitting the pathogen, depending on the stage of infection and the pathogen. To help prevent the spread of disease among school children, the CDC has developed guidelines based on the risk of transmission during the course of the disease. For example, children with chickenpox are considered contagious for five days from the start of the rash, whereas children with most gastrointestinal illnesses should be kept home for 24 hours after the symptoms disappear.<\/p>\n<p>An individual capable of transmitting a pathogen without displaying symptoms is referred to as a carrier. A <strong>passive carrier<\/strong> is contaminated with the pathogen and can mechanically transmit it to another host; however, a passive carrier is not infected. For example, a health-care professional who fails to wash his hands after seeing a patient harboring an infectious agent could become a passive carrier, transmitting the pathogen to another patient who becomes infected.<\/p>\n<p>By contrast, an <strong>active carrier<\/strong> is an infected individual who can transmit the disease to others. An active carrier may or may not exhibit signs or symptoms of infection. For example, active carriers may transmit the disease during the <strong>incubation period<\/strong> (before they show signs and symptoms) or the <strong>period of convalescence<\/strong> (after symptoms have subsided). Active carriers who do not present signs or symptoms of disease despite infection are called <strong>asymptomatic carrier<\/strong><strong>s<\/strong>. Pathogens such as <strong>hepatitis B virus<\/strong>, <strong>herpes simplex virus<\/strong>, and <strong>HIV<\/strong> are frequently transmitted by asymptomatic carriers. <strong>Mary Mallon<\/strong>, better known as <strong>Typhoid Mary<\/strong>, is a famous historical example of an asymptomatic carrier. An Irish immigrant, Mallon worked as a cook for households in and around New York City between 1900 and 1915. In each household, the residents developed <strong>typhoid fever<\/strong> (caused by <strong><em>Salmonella typhi<\/em><\/strong>) a few weeks after Mallon started working. Later investigations determined that Mallon was responsible for at least 122 cases of typhoid fever, five of which were fatal.<a class=\"footnote\" title=\"Filio Marineli, Gregory Tsoucalas, Marianna Karamanou, and George Androutsos. &quot;Mary Mallon (1869\u20131938) and the History of Typhoid Fever.&quot; Annals of Gastroenterology 26 (2013): 132\u2013134. http:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC3959940\/pdf\/AnnGastroenterol-26-132.pdf.\" id=\"return-footnote-729-2\" href=\"#footnote-729-2\" aria-label=\"Footnote 2\"><sup class=\"footnote\">[2]<\/sup><\/a> See <a href=\".\/chapter\/bacterial-infections-of-the-gastrointestinal-tract\/\" target=\"_blank\">&#8220;Typhoid Mary&#8221; in Bacterial Infections of the Gastrointestinal Tract<\/a> for more about the Mallon case.<\/p>\n<p>A pathogen may have more than one living reservoir. In zoonotic diseases, animals act as reservoirs of human disease and transmit the infectious agent to humans through direct or indirect contact. In some cases, the disease also affects the animal, but in other cases the animal is asymptomatic.<\/p>\n<p>In parasitic infections, the parasite\u2019s preferred host is called the <strong>definitive host<\/strong>. In parasites with complex life cycles, the definitive host is the host in which the parasite reaches sexual maturity. Some parasites may also infect one or more <strong>intermediate host<\/strong><strong>s<\/strong> in which the parasite goes through several immature life cycle stages or reproduces asexually.<\/p>\n<div class=\"textbox\"><a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC1911442\/\" target=\"_blank\">George Soper, the sanitary engineer who traced the typhoid outbreak to Mary Mallon, gives an account of his investigation<\/a>, an example of descriptive epidemiology, in &#8220;The Curious Career of Typhoid Mary.&#8221;<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>List some nonliving reservoirs for pathogens.<\/li>\n<li>Explain the difference between a passive carrier and an active carrier.<\/li>\n<\/ul>\n<\/div>\n<h2>Transmission<\/h2>\n<p>Regardless of the reservoir, <strong>transmission<\/strong> must occur for an infection to spread. First, transmission from the reservoir to the individual must occur. Then, the individual must transmit the infectious agent to other susceptible individuals, either directly or indirectly. Pathogenic microorganisms employ diverse transmission mechanisms.<\/p>\n<h3>Contact Transmission<\/h3>\n<p><strong>Contact transmission<\/strong> includes direct contact or indirect contact. <strong>Person-to-person transmission<\/strong> is a form of <strong>direct contact transmission<\/strong>. Here the agent is transmitted by physical contact between two individuals (Figure\u00a01) through actions such as touching, kissing, sexual intercourse, or <strong>droplet sprays<\/strong>. Direct contact can be categorized as vertical, horizontal, or droplet transmission. <strong>Vertical direct contact transmission<\/strong> occurs when pathogens are transmitted from mother to child during pregnancy, birth, or breastfeeding. Other kinds of direct contact transmission are called <strong>horizontal direct contact transmission<\/strong>. Often, contact between <strong>mucous membranes<\/strong> is required for entry of the pathogen into the new host, although skin-to-skin contact can lead to mucous membrane contact if the new host subsequently touches a mucous membrane. Contact transmission may also be site-specific; for example, some diseases can be transmitted by sexual contact but not by other forms of contact.<\/p>\n<div style=\"width: 910px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165455\/OSC_Microbio_16_03_Contact.jpg\" alt=\"Photo of person kissing a child and one of person playing arm wrestling with a child.\" width=\"900\" height=\"350\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a01. Direct contact transmission of pathogens can occur through physical contact. Many pathogens require contact with a mucous membrane to enter the body, but the host may transfer the pathogen from another point of contact (e.g., hand) to a mucous membrane (e.g., mouth or eye). (credit left: modification of work by Lisa Doehnert)<\/p>\n<\/div>\n<p>When an individual coughs or sneezes, small droplets of mucus that may contain pathogens are ejected. This leads to direct <strong>droplet transmission<\/strong>, which refers to droplet transmission of a pathogen to a new host over distances of one meter or less. A wide variety of diseases are transmitted by droplets, including <strong>influenza<\/strong> and many forms of <strong>pneumonia<\/strong>. Transmission over distances greater than one meter is called <strong>airborne transmission<\/strong>.<\/p>\n<p><strong>Indirect contact transmission<\/strong> involves inanimate objects called <strong>fomites<\/strong> that become contaminated by pathogens from an infected individual or reservoir (Figure\u00a02). For example, an individual with the common cold may sneeze, causing droplets to land on a fomite such as a tablecloth or carpet, or the individual may wipe her nose and then transfer mucus to a fomite such as a doorknob or towel. Transmission occurs indirectly when a new susceptible host later touches the fomite and transfers the contaminated material to a susceptible portal of entry. Fomites can also include objects used in clinical settings that are not properly sterilized, such as syringes, needles, catheters, and surgical equipment. Pathogens transmitted indirectly via such fomites are a major cause of healthcare-associated infections (see <a href=\".\/chapter\/controlling-microbial-growth\/\" target=\"_blank\">Controlling Microbial Growth<\/a>).<\/p>\n<div style=\"width: 1110px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165458\/OSC_Microbio_16_03_Indirect.jpg\" alt=\"Photos of person touching doorknob, towel on a hook, and end of a syringe.\" width=\"1100\" height=\"237\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a02. Fomites are nonliving objects that facilitate the indirect transmission of pathogens. Contaminated doorknobs, towels, and syringes are all common examples of fomites. (credit left: modification of work by Kate Ter Haar; credit middle: modification of work by Vernon Swanepoel; credit right: modification of work by &#8220;Zaldylmg&#8221;\/Flickr)<\/p>\n<\/div>\n<h3>Vehicle Transmission<\/h3>\n<p>The term <strong>vehicle transmission<\/strong> refers to the transmission of pathogens through vehicles such as water, food, and air. Water contamination through poor sanitation methods leads to <strong>waterborne transmission<\/strong> of disease. Waterborne disease remains a serious problem in many regions throughout the world. The World Health Organization (WHO) estimates that contaminated drinking water is responsible for more than 500,000 deaths each year.<a class=\"footnote\" title=\"World Health Organization.\u00a0Fact sheet No. 391\u2014Drinking Water. June 2005. http:\/\/www.who.int\/mediacentre\/factsheets\/fs391\/en.\" id=\"return-footnote-729-3\" href=\"#footnote-729-3\" aria-label=\"Footnote 3\"><sup class=\"footnote\">[3]<\/sup><\/a> Similarly, food contaminated through poor handling or storage can lead to <strong>foodborne transmission<\/strong> of disease (Figure\u00a03).<\/p>\n<div style=\"width: 410px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165501\/OSC_Microbio_16_03_Food.jpg\" alt=\"Photo of food at a cafeteria with glass shielding over the food.\" width=\"400\" height=\"288\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a03. Food is an important vehicle of transmission for pathogens, especially of the gastrointestinal and upper respiratory systems. Notice the glass shield above the food trays, designed to prevent pathogens ejected in coughs and sneezes from entering the food. (credit: Fort George G. Meade Public Affairs Office)<\/p>\n<\/div>\n<p>Dust and fine particles known as <strong>aerosols<\/strong>, which can float in the air, can carry pathogens and facilitate the <strong>airborne transmission<\/strong> of disease. For example, dust particles are the dominant mode of transmission of <strong>hantavirus<\/strong> to humans. Hantavirus is found in mouse feces, urine, and saliva, but when these substances dry, they can disintegrate into fine particles that can become airborne when disturbed; inhalation of these particles can lead to a serious and sometimes fatal respiratory infection.<\/p>\n<p>Although droplet transmission over short distances is considered contact transmission as discussed above, longer distance transmission of droplets through the air is considered vehicle transmission. Unlike larger particles that drop quickly out of the air column, fine mucus droplets produced by coughs or sneezes can remain suspended for long periods of time, traveling considerable distances. In certain conditions, droplets desiccate quickly to produce a <strong>droplet nucleus<\/strong> that is capable of transmitting pathogens; air temperature and humidity can have an impact on effectiveness of airborne transmission.<\/p>\n<p>Tuberculosis is often transmitted via airborne transmission when the causative agent, <em>Mycobacterium tuberculosis<\/em>, is released in small particles with coughs. Because <strong>tuberculosis<\/strong> requires as few as 10 microbes to initiate a new infection, patients with tuberculosis must be treated in rooms equipped with special ventilation, and anyone entering the room should wear a mask.<\/p>\n<div class=\"textbox examples\">\n<h3>Clinical Focus: Florida, Resolution<\/h3>\n<p>This example continues the\u00a0story that started on <a href=\"https:\/\/courses.lumenlearning.com\/microbiology\/chapter\/tracking-infectious-diseases\/chapter\/the-language-of-epidemiologists\/\" target=\"_blank\">The Language of\u00a0Epidemiologists<\/a>\u00a0and <a href=\".\/chapter\/tracking-infectious-diseases\/\" target=\"_blank\">Tracking Infectious Diseases<\/a>.<\/p>\n<p>After identifying the source of the contaminated turduckens, the Florida public health office notified the CDC, which requested an expedited inspection of the facility by state inspectors. Inspectors found that a machine used to process the chicken was contaminated with <em>Salmonella<\/em> as a result of substandard cleaning protocols. Inspectors also found that the process of stuffing and packaging the turduckens prior to refrigeration allowed the meat to remain at temperatures conducive to bacterial growth for too long. The contamination and the delayed refrigeration led to vehicle (food) transmission of the bacteria in turduckens.<\/p>\n<p>Based on these findings, the plant was shut down for a full and thorough decontamination. All turduckens produced in the plant were recalled and pulled from store shelves ahead of the December holiday season, preventing further outbreaks.<\/p>\n<\/div>\n<h3>Vector Transmission<\/h3>\n<p>Diseases can also be transmitted by a mechanical or biological <strong>vector<\/strong>, an animal (typically an <strong>arthropod<\/strong>) that carries the disease from one host to another. <strong>Mechanical transmission<\/strong> is facilitated by a <strong>mechanical vector<\/strong>, an animal that carries a pathogen from one host to another without being infected itself. For example, a fly may land on fecal matter and later transmit bacteria from the feces to food that it lands on; a human eating the food may then become infected by the bacteria, resulting in a case of diarrhea or dysentery (Figure\u00a04).<\/p>\n<div style=\"width: 1310px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165504\/OSC_Microbio_16_03_Vector.jpg\" alt=\"a) Step 1: fly picks up pathogen from fecal matter and carries it on its body. 2: Fly transfers pathogen to food. 3: Person eats contaminated food and gets sick. B) Step 1: Infected mosquito bites uninfected person. 2: Infections spreads through body and into red blood cells. 3: Second mosquito bites infected person. Mosquito may now transmit infection to another person.\" width=\"1300\" height=\"718\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a04. (a) A mechanical vector carries a pathogen on its body from one host to another, not as an infection. (b) A biological vector carries a pathogen from one host to another after becoming infected itself.<\/p>\n<\/div>\n<p><strong>Biological transmission<\/strong> occurs when the pathogen reproduces within a <strong>biological vector<\/strong> that transmits the pathogen from one host to another (Figure\u00a04). Arthropods are the main vectors responsible for biological transmission (Table 1). Most arthropod vectors transmit the pathogen by biting the host, creating a wound that serves as a portal of entry. The pathogen may go through part of its reproductive cycle in the gut or salivary glands of the arthropod to facilitate its transmission through the bite. For example, hemipterans (called &#8220;kissing bugs&#8221; or &#8220;assassin bugs&#8221;) transmit <strong>Chagas disease<\/strong> to humans by defecating when they bite, after which the human scratches or rubs the infected feces into a mucous membrane or break in the skin.<\/p>\n<p>Biological insect vectors include <strong>mosquitoes<\/strong>, which transmit <strong>malaria<\/strong> and other diseases, and <strong>lice<\/strong>, which transmit <strong>typhus<\/strong>. Other arthropod vectors can include arachnids, primarily <strong>ticks<\/strong>, which transmit <strong>Lyme disease<\/strong> and other diseases, and <strong>mites<\/strong>, which transmit <strong>scrub typhus<\/strong> and <strong>rickettsial pox<\/strong>. Biological transmission, because it involves survival and reproduction within a parasitized vector, complicates the biology of the pathogen and its transmission. There are also important non-arthropod vectors of disease, including mammals and birds. Various species of mammals can transmit <strong>rabies<\/strong> to humans, usually by means of a bite that transmits the rabies virus. Chickens and other domestic poultry can transmit <strong>avian influenza<\/strong> to humans through direct or indirect contact with <strong>avian influenza virus A<\/strong> shed in the birds\u2019 saliva, mucous, and feces.<\/p>\n<table>\n<thead>\n<tr>\n<th colspan=\"4\">Table 1.\u00a0Common Arthropod Vectors and Selected Pathogens<a class=\"footnote\" title=\"Image credits: &quot;Black fly&quot;, &quot;Tick&quot;, &quot;Tsetse fly&quot;: modification of work by USDA; &quot;Flea&quot;: modification of work by Centers for Disease Control and Prevention; &quot;Louse&quot;, &quot;Mosquito&quot;, &quot;Sand fly&quot;: modification of work by James Gathany, Centers for Disease Control and Prevention; &quot;Kissing bug&quot;: modification of work by Glenn Seplak; &quot;Mite&quot;: modification of work by Michael Wunderli\" id=\"return-footnote-729-4\" href=\"#footnote-729-4\" aria-label=\"Footnote 4\"><sup class=\"footnote\">[4]<\/sup><\/a><\/th>\n<\/tr>\n<tr>\n<th>Vector<\/th>\n<th>Species<\/th>\n<th>Pathogen<\/th>\n<th>Disease<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>\n<div id=\"attachment_1982\" style=\"width: 261px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1982\" class=\"wp-image-1982\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24154842\/01_Blackfly.png\" alt=\"A black fly on a human hand\" width=\"251\" height=\"171\" \/><\/p>\n<p id=\"caption-attachment-1982\" class=\"wp-caption-text\">Black fly<\/p>\n<\/div>\n<\/td>\n<td><em>Simulium<\/em> spp.<\/td>\n<td><em>Onchocerca volvulus<\/em><\/td>\n<td>Onchocerciasis (river blindness)<\/td>\n<\/tr>\n<tr>\n<td rowspan=\"2\">\n<div id=\"attachment_1983\" style=\"width: 260px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1983\" class=\"wp-image-1983\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155104\/02_Flea.png\" alt=\"Flea\" width=\"250\" height=\"198\" \/><\/p>\n<p id=\"caption-attachment-1983\" class=\"wp-caption-text\">Flea<\/p>\n<\/div>\n<\/td>\n<td rowspan=\"2\"><em>Xenopsylla cheopis<\/em><\/td>\n<td><em>Rickettsia typhi<\/em><\/td>\n<td>Murine typhus<\/td>\n<\/tr>\n<tr>\n<td><em>Yersinia pestis<\/em><\/td>\n<td>Plague<\/td>\n<\/tr>\n<tr>\n<td>\n<div id=\"attachment_1984\" style=\"width: 260px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1984\" class=\"wp-image-1984\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155144\/03_Kissingbug.png\" alt=\"a Kissing bug on a human hand\" width=\"250\" height=\"187\" \/><\/p>\n<p id=\"caption-attachment-1984\" class=\"wp-caption-text\">Kissing bug<\/p>\n<\/div>\n<\/td>\n<td><em>Triatoma<\/em> spp.<\/td>\n<td><em>Trypanosoma cruzi<\/em><\/td>\n<td>Chagas disease<\/td>\n<\/tr>\n<tr>\n<td rowspan=\"3\">\n<div id=\"attachment_1985\" style=\"width: 260px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1985\" class=\"wp-image-1985\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155217\/04_Louse.png\" alt=\"A louse on a human hand\" width=\"250\" height=\"160\" \/><\/p>\n<p id=\"caption-attachment-1985\" class=\"wp-caption-text\">Louse<\/p>\n<\/div>\n<\/td>\n<td rowspan=\"3\"><em>Pediculus humanus humanus<\/em><\/td>\n<td><em>Bartonella quintana<\/em><\/td>\n<td>Trench fever<\/td>\n<\/tr>\n<tr>\n<td><em>Borrelia recurrentis<\/em><\/td>\n<td>Relapsing fever<\/td>\n<\/tr>\n<tr>\n<td><em>Rickettsia prowazekii<\/em><\/td>\n<td>Typhus<\/td>\n<\/tr>\n<tr>\n<td rowspan=\"2\">\n<div id=\"attachment_1986\" style=\"width: 260px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1986\" class=\"wp-image-1986\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155252\/05_Mite.png\" alt=\"A micrograph of a mite\" width=\"250\" height=\"253\" \/><\/p>\n<p id=\"caption-attachment-1986\" class=\"wp-caption-text\">Mite (chigger)<\/p>\n<\/div>\n<\/td>\n<td><em>Leptotrombidium<\/em> spp.<\/td>\n<td><em>Orientia tsutsugamushi<\/em><\/td>\n<td>Scrub typhus<\/td>\n<\/tr>\n<tr>\n<td><em>Liponyssoides sanguineus<\/em><\/td>\n<td><em>Rickettsia akari<\/em><\/td>\n<td>Rickettsialpox<\/td>\n<\/tr>\n<tr>\n<td rowspan=\"3\">\n<div id=\"attachment_1987\" style=\"width: 260px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1987\" class=\"wp-image-1987\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155347\/06_Mosquito.png\" alt=\"A mosquito drinking blood from a human\" width=\"250\" height=\"180\" \/><\/p>\n<p id=\"caption-attachment-1987\" class=\"wp-caption-text\">Mosquito<\/p>\n<\/div>\n<\/td>\n<td><em>Aedes<\/em> spp., <em>Haemogogus<\/em> spp.<\/td>\n<td>Yellow fever virus<\/td>\n<td>Yellow fever<\/td>\n<\/tr>\n<tr>\n<td><em>Anopheles<\/em> spp.<\/td>\n<td>Plasmodium falciparum<\/td>\n<td>Malaria<\/td>\n<\/tr>\n<tr>\n<td><em>Cutex pipiens<\/em><\/td>\n<td>West nile virus<\/td>\n<td>West nile disease<\/td>\n<\/tr>\n<tr>\n<td>\n<div id=\"attachment_1988\" style=\"width: 260px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1988\" class=\"wp-image-1988\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155447\/07_Sandfly.png\" alt=\"A sand fly drinking blood from a human\" width=\"250\" height=\"192\" \/><\/p>\n<p id=\"caption-attachment-1988\" class=\"wp-caption-text\">Sand fly<\/p>\n<\/div>\n<\/td>\n<td><em>Phlebotomus<\/em> spp.<\/td>\n<td><em>Leishmania<\/em> spp.<\/td>\n<td>Leishmaniasis<\/td>\n<\/tr>\n<tr>\n<td rowspan=\"2\">\n<div id=\"attachment_1989\" style=\"width: 259px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1989\" class=\"wp-image-1989\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155530\/08_Tick.png\" alt=\"A tick sitting on a leaf\" width=\"249\" height=\"191\" \/><\/p>\n<p id=\"caption-attachment-1989\" class=\"wp-caption-text\">Tick<\/p>\n<\/div>\n<\/td>\n<td><em>Ixodes<\/em> spp.<\/td>\n<td><em>Borrelia<\/em> spp.<\/td>\n<td>Lyme disease<\/td>\n<\/tr>\n<tr>\n<td><em>Dermacentor<\/em> spp. and others<\/td>\n<td><em>Rickettsi rickettsia<\/em><\/td>\n<td>Rocky Mountain spotted fever<\/td>\n<\/tr>\n<tr>\n<td>\n<div id=\"attachment_1990\" style=\"width: 260px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1990\" class=\"wp-image-1990\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2017\/03\/24155620\/09_Tsetsefly.png\" alt=\"A tsetse fly\" width=\"250\" height=\"166\" \/><\/p>\n<p id=\"caption-attachment-1990\" class=\"wp-caption-text\">Tsetse fly<\/p>\n<\/div>\n<\/td>\n<td><em> Glossina<\/em> spp.<\/td>\n<td><em>Trypanosoma brucei\u00a0<\/em><\/td>\n<td>African trypanosomiasis (sleeping sickness)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>Describe how diseases can be transmitted through the air.<\/li>\n<li>Explain the difference between a mechanical vector and a biological vector.<\/li>\n<\/ul>\n<\/div>\n<div class=\"textbox shaded\">\n<h3>Using GMOs to Stop the Spread of Zika<\/h3>\n<p>In 2016, an epidemic of the <strong>Zika virus<\/strong> was linked to a high incidence of birth defects in South America and Central America. As winter turned to spring in the northern hemisphere, health officials correctly predicted the virus would spread to North America, coinciding with the breeding season of its major vector, the <strong><em>Aedes aegypti<\/em><\/strong> mosquito.<\/p>\n<p>The range of the <em>A. aegypti<\/em> mosquito extends well into the southern United States (Figure\u00a05). Because these same mosquitoes serve as vectors for other problematic diseases (<strong>dengue fever<\/strong>, <strong>yellow fever<\/strong>, and others), various methods of mosquito control have been proposed as solutions. Chemical pesticides have been used effectively in the past, and are likely to be used again; but because chemical pesticides can have negative impacts on the environment, some scientists have proposed an alternative that involves genetically engineering <em>A. aegypti<\/em> so that it cannot reproduce. This method, however, has been the subject of some controversy.<\/p>\n<div style=\"width: 1210px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165512\/OSC_Microbio_16_03_Mosquito.jpg\" alt=\"Micrograph of brown dots of about 50 nm inside cells; dots re labeled Zika virus. Photo of mosquito labeled Aedes aegypti. Map of where mosquitoes are found in the US. Aedes aegypti and Aedes albopictus are both found in the lower half of the US, reaching up to Connecticut, Missouri, and California. Aedes albopictus reaches further north in the eastern part o the country; through Minnosota. Aedes aegypti reaches a bit further into Utah and is in Puerto Rico.\" width=\"1200\" height=\"608\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a05. The Zika virus is an enveloped virus transmitted by mosquitoes, especially <em>Aedes aegypti<\/em>. The range of this mosquito includes much of the United States, from the Southwest and Southeast to as far north as the Mid-Atlantic. The range of <em>A. albopictus<\/em>, another vector, extends even farther north to New England and parts of the Midwest. (credit micrograph: modification of work by Cynthia Goldsmith, Centers for Disease Control and Prevention; credit photo: modification of work by James Gathany, Centers for Disease Control and Prevention; credit map: modification of work by Centers for Disease Control and Prevention)<\/p>\n<\/div>\n<p>One method that has worked in the past to control pests, with little apparent downside, has been sterile male introductions. This method controlled the screw-worm fly pest in the southwest United States and fruit fly pests of fruit crops. In this method, males of the target species are reared in the lab, sterilized with radiation, and released into the environment where they mate with wild females, who subsequently bear no live offspring. Repeated releases shrink the pest population.<\/p>\n<p>A similar method, taking advantage of recombinant DNA technology,<a class=\"footnote\" title=\"Blandine Massonnet-Bruneel, Nicole Corre-Catelin, Renaud Lacroix, Rosemary S. Lees, Kim Phuc Hoang, Derric Nimmo, Luke Alphey, and Paul Reiter. &quot;Fitness of Transgenic Mosquito Aedes aegypti Males Carrying a Dominant Lethal Genetic System.&quot; PLOS ONE 8, no. 5 (2013): e62711.\" id=\"return-footnote-729-5\" href=\"#footnote-729-5\" aria-label=\"Footnote 5\"><sup class=\"footnote\">[5]<\/sup><\/a> introduces a dominant lethal allele into male mosquitoes that is suppressed in the presence of tetracycline (an antibiotic) during laboratory rearing. The males are released into the environment and mate with female mosquitoes. Unlike the sterile male method, these matings produce offspring, but they die as larvae from the lethal gene in the absence of tetracycline in the environment. As of 2016, this method has yet to be implemented in the United States, but a UK company tested the method in Piracicaba, Brazil, and found an 82% reduction in wild <em>A. aegypti<\/em> larvae and a 91% reduction in dengue cases in the treated area.<a class=\"footnote\" title=\"Richard Levine. &quot;Cases of Dengue Drop 91 Percent Due to Genetically Modified\u00a0Mosquitoes.&quot; Entomology Today. https:\/\/entomologytoday.org\/2016\/07\/14\/cases-of-dengue-drop-91-due-to-genetically-modified-mosquitoes.\" id=\"return-footnote-729-6\" href=\"#footnote-729-6\" aria-label=\"Footnote 6\"><sup class=\"footnote\">[6]<\/sup><\/a> In August 2016, amid news of Zika infections in several Florida communities, the FDA gave the UK company permission to test this same mosquito control method in Key West, Florida, pending compliance with local and state regulations and a referendum in the affected communities.<\/p>\n<p>The use of <strong>genetically modified organisms (GMOs)<\/strong> to control a disease vector has its advocates as well as its opponents. In theory, the system could be used to drive the <em>A. aegypti<\/em> mosquito extinct\u2014a noble goal according to some, given the damage they do to human populations.<a class=\"footnote\" title=\"Olivia Judson. &quot;A Bug\u2019s Death.&quot; The New York Times, September 25, 2003. http:\/\/www.nytimes.com\/2003\/09\/25\/opinion\/a-bug-s-death.html.\" id=\"return-footnote-729-7\" href=\"#footnote-729-7\" aria-label=\"Footnote 7\"><sup class=\"footnote\">[7]<\/sup><\/a> But opponents of the idea are concerned that the gene could escape the species boundary of <em>A. aegypti<\/em> and cause problems in other species, leading to unforeseen ecological consequences. Opponents are also wary of the program because it is being administered by a for-profit corporation, creating the potential for conflicts of interest that would have to be tightly regulated; and it is not clear how any unintended consequences of the program could be reversed.<\/p>\n<p>There are other epidemiological considerations as well. <em>Aedes aegypti<\/em> is apparently not the only vector for the Zika virus. <strong><em>Aedes albopictus<\/em><\/strong>, the Asian tiger mosquito, is also a vector for the Zika virus.<a class=\"footnote\" title=\"Gilda Grard, M\u00e9lanie Caron, Illich Manfred Mombo, Dieudonn\u00e9 Nkoghe, Statiana Mboui Ondo, Davy Jiolle, Didier Fontenille, Christophe Paupy, and Eric Maurice Leroy. &quot;Zika Virus in Gabon (Central Africa)\u20132007: A New Threat from Aedes albopictus?&quot; PLOS Neglected Tropical Diseases 8, no. 2 (2014): e2681.\" id=\"return-footnote-729-8\" href=\"#footnote-729-8\" aria-label=\"Footnote 8\"><sup class=\"footnote\">[8]<\/sup><\/a> <em>A. albopictus<\/em> is now widespread around the planet including much of the United States (Figure\u00a05). Many other mosquitoes have been found to harbor Zika virus, though their capacity to act as vectors is unknown.<a class=\"footnote\" title=\"Const\u00e2ncia F.J. Ayres. &quot;Identification of Zika Virus Vectors and Implications for Control.&quot; The Lancet Infectious Diseases 16, no. 3 (2016): 278\u2013279.\" id=\"return-footnote-729-9\" href=\"#footnote-729-9\" aria-label=\"Footnote 9\"><sup class=\"footnote\">[9]<\/sup><\/a> Genetically modified strains of <em>A. aegypti<\/em> will not control the other species of vectors. Finally, the Zika virus can apparently be transmitted sexually between human hosts, from mother to child, and possibly through blood transfusion. All of these factors must be considered in any approach to controlling the spread of the virus.<\/p>\n<p>Clearly there are risks and unknowns involved in conducting an open-environment experiment of an as-yet poorly understood technology. But allowing the Zika virus to spread unchecked is also risky. Does the threat of a Zika epidemic justify the ecological risk of genetically engineering mosquitos? Are current methods of mosquito control sufficiently ineffective or harmful that we need to try untested alternatives? These are the questions being put to public health officials now.<\/p>\n<\/div>\n<h2>Quarantining<\/h2>\n<p>Individuals suspected or known to have been exposed to certain contagious pathogens may be <strong>quarantined<\/strong>, or isolated to prevent transmission of the disease to others. Hospitals and other health-care facilities generally set up special wards to isolate patients with particularly hazardous diseases such as tuberculosis or Ebola (Figure\u00a06). Depending on the setting, these wards may be equipped with special air-handling methods, and personnel may implement special protocols to limit the risk of transmission, such as personal protective equipment or the use of chemical disinfectant sprays upon entry and exit of medical personnel.<\/p>\n<p>The duration of the quarantine depends on factors such as the <strong>incubation period<\/strong> of the disease and the evidence suggestive of an infection. The patient may be released if signs and symptoms fail to materialize when expected or if preventive treatment can be administered in order to limit the risk of transmission. If the infection is confirmed, the patient may be compelled to remain in isolation until the disease is no longer considered contagious.<\/p>\n<p>In the United States, public health authorities may only quarantine patients for certain diseases, such as <strong>cholera<\/strong>, <strong>diphtheria<\/strong>, infectious <strong>tuberculosis<\/strong>, and strains of <strong>influenza<\/strong> capable of causing a <strong>pandemic<\/strong>. Individuals entering the United States or moving between states may be quarantined by the CDC if they are suspected of having been exposed to one of these diseases. Although the CDC routinely monitors entry points to the United States for crew or passengers displaying illness, quarantine is rarely implemented.<\/p>\n<div style=\"width: 1110px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165515\/OSC_Microbio_16_03_Isolate.jpg\" alt=\"a) Photo of a plastic tent next to an airplane b) Photo of beds in a room.\" width=\"1100\" height=\"460\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a06. (a) The Aeromedical Biological Containment System (ABCS) is a module designed by the CDC and Department of Defense specifically for transporting highly contagious patients by air. (b) An isolation ward for <strong>Ebola<\/strong> patients in Lagos, Nigeria. (credit a: modification of work by Centers for Disease Control and Prevention; credit b: modification of work by CDC Global)<\/p>\n<\/div>\n<h2>Healthcare-Associated (Nosocomial) Infections<\/h2>\n<p>Hospitals, retirement homes, and prisons attract the attention of epidemiologists because these settings are associated with increased incidence of certain diseases. Higher rates of transmission may be caused by characteristics of the environment itself, characteristics of the population, or both. Consequently, special efforts must be taken to limit the risks of infection in these settings.<\/p>\n<p>Infections acquired in health-care facilities, including hospitals, are called <strong>nosocomial infections<\/strong> or <strong>healthcare-associated infections (HAI)<\/strong>. HAIs are often connected with surgery or other invasive procedures that provide the pathogen with access to the portal of infection. For an infection to be classified as an HAI, the patient must have been admitted to the health-care facility for a reason other than the infection. In these settings, patients suffering from primary disease are often afflicted with compromised immunity and are more susceptible to secondary infection and opportunistic pathogens.<\/p>\n<p>In 2011, more than 720,000 HAIs occurred in hospitals in the United States, according to the CDC. About 22% of these HAIs occurred at a surgical site, and cases of <strong>pneumonia<\/strong> accounted for another 22%; <strong>urinary tract infections<\/strong> accounted for an additional 13%, and primary <strong>bloodstream infections<\/strong> 10%.<a class=\"footnote\" title=\"Centers for Disease Control and Prevention. &quot;HAI Data and Statistics.&quot; 2016. http:\/\/www.cdc.gov\/hai\/surveillance. Accessed Jan 2, 2016.\" id=\"return-footnote-729-10\" href=\"#footnote-729-10\" aria-label=\"Footnote 10\"><sup class=\"footnote\">[10]<\/sup><\/a> Such HAIs often occur when pathogens are introduced to patients\u2019 bodies through contaminated surgical or medical equipment, such as <strong>catheters<\/strong> and respiratory ventilators. Health-care facilities seek to limit nosocomial infections through training and hygiene protocols such as those described in <a href=\".\/chapter\/introduction-12\/\" target=\"_blank\">Control of Microbial Growth<\/a>.<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>Give some reasons why HAIs occur.<\/li>\n<\/ul>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Key Concepts and Summary<\/h3>\n<ul>\n<li><strong>Reservoirs<\/strong> of human disease can include the human and animal populations, soil, water, and inanimate objects or materials.<\/li>\n<li><strong>Contact transmission<\/strong> can be <strong>direct<\/strong> or <strong>indirect<\/strong> through physical contact with either an infected host (direct) or contact with a fomite that an infected host has made contact with previously (indirect).<\/li>\n<li>Vector transmission occurs when a living organism carries an infectious agent on its body (<strong>mechanical<\/strong>) or as an infection host itself (<strong>biological<\/strong>), to a new host.<\/li>\n<li><strong>Vehicle transmission<\/strong> occurs when a substance, such as soil, water, or air, carries an infectious agent to a new host.<\/li>\n<li><strong>Healthcare-associated infections (HAI)<\/strong>, or <strong>nosocomial infections<\/strong>, are acquired in a clinical setting. Transmission is facilitated by medical interventions and the high concentration of susceptible, immunocompromised individuals in clinical settings.<\/li>\n<\/ul>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Multiple Choice<\/h3>\n<p>Which is the most common type of biological vector of human disease?<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>viruses<\/li>\n<li>bacteria<\/li>\n<li>mammals<\/li>\n<li>arthropods<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q727911\">Show Answer<\/span><\/p>\n<div id=\"q727911\" class=\"hidden-answer\" style=\"display: none\">Answer d. Arthropods are\u00a0the most common type of biological vector of human disease.<\/div>\n<\/div>\n<p>A mosquito bites a person who subsequently develops a fever and abdominal rash. What type of transmission would this be?<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>mechanical vector transmission<\/li>\n<li>biological vector transmission<\/li>\n<li>direct contact transmission<\/li>\n<li>vehicle transmission<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q600518\">Show Answer<\/span><\/p>\n<div id=\"q600518\" class=\"hidden-answer\" style=\"display: none\">Answer b. This would be\u00a0biological vector transmission.<\/div>\n<\/div>\n<p>Cattle are allowed to pasture in a field that contains the farmhouse well, and the farmer\u2019s family becomes ill with a gastrointestinal pathogen after drinking the water. What type of transmission of infectious agents would this be?<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>biological vector transmission<\/li>\n<li>direct contact transmission<\/li>\n<li>indirect contact transmission<\/li>\n<li>vehicle transmission<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q858091\">Show Answer<\/span><\/p>\n<div id=\"q858091\" class=\"hidden-answer\" style=\"display: none\">Answer d. This would be vehicle transmission.<\/div>\n<\/div>\n<p>A blanket from a child with chickenpox is likely to be contaminated with the virus that causes chickenpox (Varicella-zoster virus). What is the blanket called?<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>fomite<\/li>\n<li>host<\/li>\n<li>pathogen<\/li>\n<li>vector<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q686339\">Show Answer<\/span><\/p>\n<div id=\"q686339\" class=\"hidden-answer\" style=\"display: none\">Answer a.\u00a0The blanket is a\u00a0fomite.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Fill in the Blank<\/h3>\n<p>A patient in the hospital with a urinary catheter develops a bladder infection. This is an example of a(n) ________ infection.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q978282\">Show Answer<\/span><\/p>\n<div id=\"q978282\" class=\"hidden-answer\" style=\"display: none\">A patient in the hospital with a urinary catheter develops a bladder infection. This is an example of a\u00a0<strong>nosocomial or healthcare-associated<\/strong> infection.<\/div>\n<\/div>\n<p>A ________ is an animal that can transfer infectious pathogens from one host to another.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q227054\">Show Answer<\/span><\/p>\n<div id=\"q227054\" class=\"hidden-answer\" style=\"display: none\">A <strong>vector<\/strong> is an animal that can transfer infectious pathogens from one host to another.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<p>Differentiate between droplet vehicle transmission and airborne transmission.<\/p>\n<p>Many people find that they become ill with a cold after traveling by airplane. The air circulation systems of commercial aircraft use HEPA filters that should remove any infectious agents that pass through them. What are the possible reasons for increased incidence of colds after flights?<\/p>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-729\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>OpenStax Microbiology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\">http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-729-1\">Yves Thomas, Guido Vogel, Werner Wunderli, Patricia Suter, Mark Witschi, Daniel Koch, Caroline Tapparel, and Laurent Kaiser. \"Survival of Influenza Virus on Banknotes.\" <em>Applied and Environmental Microbiology<\/em> 74, no. 10 (2008): 3002\u20133007. <a href=\"#return-footnote-729-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><li id=\"footnote-729-2\">Filio Marineli, Gregory Tsoucalas, Marianna Karamanou, and George Androutsos. \"Mary Mallon (1869\u20131938) and the History of Typhoid Fever.\" <em>Annals of Gastroenterology<\/em> 26 (2013): 132\u2013134. http:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC3959940\/pdf\/AnnGastroenterol-26-132.pdf. <a href=\"#return-footnote-729-2\" class=\"return-footnote\" aria-label=\"Return to footnote 2\">&crarr;<\/a><\/li><li id=\"footnote-729-3\">World Health Organization.\u00a0Fact sheet No. 391<em>\u2014Drinking Water.<\/em> June 2005. http:\/\/www.who.int\/mediacentre\/factsheets\/fs391\/en. <a href=\"#return-footnote-729-3\" class=\"return-footnote\" aria-label=\"Return to footnote 3\">&crarr;<\/a><\/li><li id=\"footnote-729-4\">Image credits: \"Black fly\", \"Tick\", \"Tsetse fly\": modification of work by USDA; \"Flea\": modification of work by Centers for Disease Control and Prevention; \"Louse\", \"Mosquito\", \"Sand fly\": modification of work by James Gathany, Centers for Disease Control and Prevention; \"Kissing bug\": modification of work by Glenn Seplak; \"Mite\": modification of work by Michael Wunderli <a href=\"#return-footnote-729-4\" class=\"return-footnote\" aria-label=\"Return to footnote 4\">&crarr;<\/a><\/li><li id=\"footnote-729-5\">Blandine Massonnet-Bruneel, Nicole Corre-Catelin, Renaud Lacroix, Rosemary S. Lees, Kim Phuc Hoang, Derric Nimmo, Luke Alphey, and Paul Reiter. \"Fitness of Transgenic Mosquito <em>Aedes aegypti<\/em> Males Carrying a Dominant Lethal Genetic System.\" <em>PLOS ONE<\/em> 8, no. 5 (2013): e62711. <a href=\"#return-footnote-729-5\" class=\"return-footnote\" aria-label=\"Return to footnote 5\">&crarr;<\/a><\/li><li id=\"footnote-729-6\">Richard Levine. \"Cases of Dengue Drop 91 Percent Due to Genetically Modified\u00a0Mosquitoes.\" <em>Entomology Today.<\/em> https:\/\/entomologytoday.org\/2016\/07\/14\/cases-of-dengue-drop-91-due-to-genetically-modified-mosquitoes. <a href=\"#return-footnote-729-6\" class=\"return-footnote\" aria-label=\"Return to footnote 6\">&crarr;<\/a><\/li><li id=\"footnote-729-7\">Olivia Judson. \"A Bug\u2019s Death.\" <em>The New York Times<\/em>, September 25, 2003. http:\/\/www.nytimes.com\/2003\/09\/25\/opinion\/a-bug-s-death.html. <a href=\"#return-footnote-729-7\" class=\"return-footnote\" aria-label=\"Return to footnote 7\">&crarr;<\/a><\/li><li id=\"footnote-729-8\">Gilda Grard, M\u00e9lanie Caron, Illich Manfred Mombo, Dieudonn\u00e9 Nkoghe, Statiana Mboui Ondo, Davy Jiolle, Didier Fontenille, Christophe Paupy, and Eric Maurice Leroy. \"Zika Virus in Gabon (Central Africa)\u20132007: A New Threat from <em>Aedes albopictus<\/em>?\" <em>PLOS Neglected Tropical Diseases<\/em> 8, no. 2 (2014): e2681. <a href=\"#return-footnote-729-8\" class=\"return-footnote\" aria-label=\"Return to footnote 8\">&crarr;<\/a><\/li><li id=\"footnote-729-9\">Const\u00e2ncia F.J. Ayres. \"Identification of Zika Virus Vectors and Implications for Control.\" <em>The Lancet Infectious Diseases<\/em> 16, no. 3 (2016): 278\u2013279. <a href=\"#return-footnote-729-9\" class=\"return-footnote\" aria-label=\"Return to footnote 9\">&crarr;<\/a><\/li><li id=\"footnote-729-10\">Centers for Disease Control and Prevention. \"HAI Data and Statistics.\" 2016. http:\/\/www.cdc.gov\/hai\/surveillance. Accessed Jan 2, 2016. <a href=\"#return-footnote-729-10\" class=\"return-footnote\" aria-label=\"Return to footnote 10\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":17,"menu_order":4,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"OpenStax Microbiology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-729","chapter","type-chapter","status-publish","hentry"],"part":707,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/pressbooks\/v2\/chapters\/729","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":5,"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/pressbooks\/v2\/chapters\/729\/revisions"}],"predecessor-version":[{"id":1991,"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/pressbooks\/v2\/chapters\/729\/revisions\/1991"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/pressbooks\/v2\/parts\/707"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/pressbooks\/v2\/chapters\/729\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/wp\/v2\/media?parent=729"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/pressbooks\/v2\/chapter-type?post=729"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/wp\/v2\/contributor?post=729"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-microbiology\/wp-json\/wp\/v2\/license?post=729"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}