Fungal and Protozoan Diseases of the Digestive System

Ergot Poisoning

Ergot poisoning is caused by ingestion of the alkaloids produced by the ergot fungi.

Learning Objectives

List the causes and effects of ergot poisoning

Key Takeaways

Key Points

  • Ergot fungi typically infect rye, wheat and forage plants.
  • Ingestion of the alkaloid toxins produced by the fungi can result in ergot poisoning in humans and foraging animals.
  • Ergot poisoning can have either convulsive central nervous system effects or gangrenous symptoms caused by vasoconstriction effects.

Key Terms

  • alkaloids: A class of organic heterocyclic bases.
  • gangrenous: To be afflicted with gangrene.

Ergot poisoning is a type of illness associated with the ingestion of alkaloids produced by the fungi Claviceps purpurea (C. purpurea). Claviceps purpurea is a fungus classified under the fungi genus Claviceps. This specific type of fungus is found on rye, and also on crops like wheat and barley. In addition, Claviceps purpurea can effect plants and crops that are typically considered forage plants. Thus, this type of fungus can also result in diseases within livestock.

The life cycle of C. purpurea begins when an ergot kernel, called a sclerotium, infects the host. The fungi continues to undergo proliferation and destroys the plant ovary. The first stage of ergot infection is a white soft tissue, called Sphacelia segetum, that drops out of the host. The white soft tissue contains asexual spores which infect additional host plants. This tissue, present within the host, is then converted into a hard Sclerotium clavus within the husk. At this specific stage, the alkaloids and lipids accumulate.

image

Example of Ergot on Wheat: This image shows ergot on a wheat spike.

The alkaloids, responsible for the ergot poisoning, are naturally occurring compounds that are mainly comprised of basic nitrogen atoms. Alkaloids are produced within various organisms as a secondary metabolite. Secondary metabolites are most commonly produced in plants as a defense system. The alkaloids produced by fungi are often toxic. Specifically, the alkaloid produced by Claviceps purpurea is ergoline based.

In cases of ergot poisoning (also known as ergotoxicosis or traditionally, Saint Anthony’s Fire) alkaloids accumulate in the system due to the consumption of contaminated grain products. The symptoms which present in individuals with ergot poisoning can be classified as convulsive symptoms and gangrenous symptoms. The convulsive symptoms include seizures and effects on the central nervous system that range from hallucinations to psychotic episodes. The gangrenous symptoms are a result of vasoconstriction induced by the alkaloids. Peripheral systems, such as fingers and toes, are typically affected. More recently, ergot poisoning has been associated with an increased intake of ergot-based drugs. These drugs include those that promote vasoconstriction for treating migraines and Parkinson’s disease.

Aflatoxin Poisoning

Aflatoxin poisoning is a result of ingestion of aflatoxins produced by Aspergillus that have contaminated a food source.

Learning Objectives

Summarize the causes and effects of alfatoxin poisoning

Key Takeaways

Key Points

  • Aspergillus flavus and Aspergillus parasiticus are two commonly known species of Aspergillus that can aflatoxin poisoning.
  • Aflatoxin poisoning can result in either acute aflatoxicosis, which is a result of moderate to high level ingestion; or chronic aflatoxicosis, which results from ingestion of low to moderate levels of aflatoxins.
  • Aspergillus species which cause aflatoxin poisoning are often found in crops in underdeveloped countries, due to lack of detection techniques and inadequate harvesting and storage.

Key Terms

  • mycotoxins: a substance produced by a mold or fungus.

Aflatoxins are categorized as mycotoxins that are typically produced by species of Aspergillus. Aflatoxins, although only synthesized by a few Aspergillus species, are considered to be one of the most important mycotoxins identified to date. Both Aspergillus flavus and Aspergillus parasiticus are well known for their production of aflatoxins. Aflatoxins are most commonly transmitted to humans through the diet. Aflatoxins grow on whole grains and contaminate food supplies during processing, storage, or transport when there are favorable conditions for mold growth, specifically for the Aspergillus species.

image

Aspergillus flavus: A species of Aspergillus that causes aflatoxin poisoning.

Aflatoxin poisoning, or aflatoxicosis, occurs when there is ingestion of aflatoxin contaminated foods. Upon ingestion or exposure to aflatoxin, it is common to see injury to the liver. Aflatoxicosis is a primarily hepatic disease, as the liver is the target organ for this toxin in mammals. Although the liver demonstrates the ability to metabolize the ingested aflatoxins, the intermediate formed is a reactive epoxide or a less harmful hydroxylated form referred to as M1. There have been various studies stating that metabolic activation of aflatoxins is required for the aflatoxin to exert its carcinogenic effects. These metabolites are harmful to the liver and have been implicated in liver cancer development. The aflatoxins produced by these Aspergillus species have been show to produce adducts (altered forms of DNA). These adducts are now used as a diagnostic factor to test for aflatoxin exposure by testing blood and urine.

Aflatoxin poisoning or aflatoxicosis is rarely diagnosed in developed countries but continues to be an issue in underdeveloped countries. In developed countries, commercial crops are screened for the presence of aflatoxins. However, in underdeveloped or developing countries, screening methods are lacking, or are in the process of being introduced. Interestingly, a rise in homegrown food has been correlated with a slight increase in aflatoxin exposure via diet.

Aflatoxin poisoning can be diagnosed as either acute or chronic. In cases of acute aflatoxicosis, an individual has been exposed to moderate to high levels of aflatoxins. Acute aflatoxicosis is characterized by symptoms such as hemorrhaging; acute liver damage and issues with digestion; and absorption and metabolism of nutrients. In cases of chronic aflatoxicosis, an individual has been exposed to low to moderate levels of aflatoxins. Chronic aflatoxicosis is characterized by symptoms such as dysfunctional food conversion and slow growth rates.

Giardiasis

Giardiasis, sometimes referred to as beaver fever, is caused by the protozoan Giardia lamblia and results in diarrheal illness.

Learning Objectives

Summarize the life cycle and route of transmission for Giardia lamblia

Key Takeaways

Key Points

  • Giardia lamblia is transmitted by exposure or ingestion of fecal contaminated sources such as soil, food, and water.
  • Giardiasis is a common worldwide cause of gastroenteritis.
  • The structure and life cycle of Giardia lamblia allow for survival in harsh environments and resistance against numerous types of disinfectants.

Key Terms

  • giardiasis: an infectious diarrheal disease caused by the Giardia lamblia parasite
  • zoonotic: of or relating to zoonosis, the transmission of an infectious disease between species.
  • hematuria: The presence of blood in the urine.

Giardiasis is a protozoan disease caused by Giardia lamblia. Giardiasis, referred to as beaver fever, is a common cause of gastroenteritis worldwide. The protozoa, Giardia lamblia, also referred to as Giardia intestinalis or Giardia duodenalis, infects humans via the fecal-oral route and is also suspected to be zoonotic. The organism is commonly found in soil, food, or water that has been contaminated with fecal matter from infected humans or animals. Beavers typically spread the parasite in their fecal matter in rivers and streams hence, giardiasis is commonly referred to as beaver fever. Individuals susceptible to infection by Giardia lamblia are those who come in frequent contact with individuals already infected. Travelers that spend time in wilderness area are at an increased risk due to ingestion of contaminated food or water sources and a lack of medical care or supplies.

image

Giardia infecting a small intestine: A scanning electron micrograph of the surface of the small intestine of a gerbil infested with Giardia sp. protozoal, present in the trophozoite stage.

The life cycle, structure, and organization of Giardia lamblia promotes its survival for long periods of time outside the body. The organism itself is protected by an outer shell that provides protection against numerous harsh environments. In addition, the shell provides protection against disinfectants including chlorine. The cysts and trophozoites, found in the fecal matter, are extremely resistant to harsh environments. It is the cysts that are ingested and passed from exposure to contaminated food, water, or by the fecal-oral route. Once in the host, the trophozoites multiply via binary fission. They can either remain free within the lumen or attach to the mucosa by a sucking disk. Once the parasites move towards the colon, the encystation phase occurs and the cysts are infectious when passed in the stool.

Giardiasis is characterized as a disease of the gastrointestinal system. The symptoms include from fever, diarrhea, hematuria, stomach cramping, vomiting, flatulence, and loose stool. The symptoms are typically present one to two weeks post infection and can disappear and reappear cyclically. The pathogenecity of Giardia lamblia is characterized by its ability to coat the inside of the intestinal wall and inhibit nutrient absorption. The ability of the protozoan to block nutrient absorption can result in vitamin B12 deficiency. Additionally, a development of lactose intolerance is often associated with giardiasis infection.

Cryptosporidiosis

Cryptosporidiosis is a type of parasitic disease caused by Cryptosporidium that infects the gastrointestinal system.

Learning Objectives

Outline the life cycle of Cryptosporidium

Key Takeaways

Key Points

  • Cryptosporidium is commonly transmitted through food and water that has been contaminated with the feces of an infected individual.
  • Cryptosporidium is commonly found in immunocompromised individuals that exhibit symptoms associated with this disease such as acute or persistent diarrhea.
  • The life cycle of Cryptosporidium involves both asexual and sexual reproduction.

Key Terms

  • excyst: The break down of a cyst wall.
  • schizogony: A form of asexual reproduction in protozoans characterized by multiple divisions.

Cryptosporidiosis is a type of parasitic disease caused by the parasite Cryptosporidium. Cryptosporidiosis is typically spread through the fecal-oral route and can be spread through contaminated water as well. Cryptosporiodiosis is one of most common waterborne diseases identified worldwide. The transmission of Cryptosporidium is based on successful ingestion of oocysts which are able to implant and infect the epithelial tissue of the intestine, hence, the gastrointestinal symptoms associated with cryptosporidiosis.

Cryptosporidium is classified as a protozoan within the Phylum Apicomplexa. Other pathogens classified in this phylum include the malaria parasite and the parasite that causes toxoplasmosis. The life cycle of Cryptosporidium allows for growth in a single host. The spore phase of the life cycle, also referred to as the oocyst stage, is the stage that allows survival of the pathogen in numerous harsh environments. The oocyst allows for survival against harsh chemicals including harsh disinfectants such as chlorine. The life cycle is characterized by the presence of both an asexual and sexual stage. The oocysts, once ingested, excyst within the small intestine and release sporozoites which attach to the microvilli. The sporozoites then develop into trophozoites and undergo asexual reproduction via schizogony. The trophozoites then develop into Type 1 and Type 2 merozoites which can either cause auto infection (Type 1) or undergo releasal and attach the epithelial cells (Type 2). Once released and attached, they will either develop in macrogamonts or microgamonts which correlate with male and female forms. Sexual reproduction occurs than zyogotes are developed. The zygote further develops into the oocyst which can either reinfect the host by rupturing and releasing sporozoites or be excreted into the environment.

image

Life cycle of Cryptosporidium: An overview of the life cycle of Cryptosporidium.

The major symptom associated with individuals infected with Cryptosporidium is diarrhea. However, cryptosporidiosis is prevalent in immunocompromised individuals, such as those infected with the HIV virus. The symptoms of cryptosporidiosis in these cases are much more severe and can be fatal. The oocysts can initiate infections by attaching to the brush border of the small intestine and attacking the epithelial cells. Additional symptoms associated with cryptosporidiosis include abdominal cramping, malnutrition, weight loss, and nausea.

Cyclospora Diarrheal Infection

Cyclospora diarrheal infection is commonly referred to as traveler’s diarrhea and is caused by the parasite Cyclospora cayetanensis.

Learning Objectives

Outline the life cycle of Cyclospora cayetanensis

Key Takeaways

Key Points

  • The life cycle of Cyclospora begins as the host ingests the oocyst form.
  • Cyclospora is transmitted through fecal matter and is commonly found on contaminated fruits and vegetables in countries or areas with a lack of stringent health regulations.
  • Cyclospora causes gastroenteritis with symptoms ranging from diarrhea to loss of appetite, weight loss, cramping, nausea and fatigue.

Key Terms

  • meronts: After infecting a host cell, a trophozoite increases in size and during this process, the organism is referred to as a meront.
  • sporocysts: a cysts that develops from a sporoblasts and results in the production of sporozoites.

Cyclospora diarrheal infection, commonly referred to as travelers diarrhea or cyclosporiasis, is caused by a specific species of Cyclospora. The protozoan that are categorized as cyclospora are defined by the spherical shape of the sporocysts. Specifically, Cyclospora cayetanensis is the species associated with the disease in both humans and primates. Cyclospora can be transmitted by consuming contaminated food or water. In 2012, cyclospora caused about 500 infections in 2012 in the US from a salad mix imported from Mexico and used in restaurants in Texas and Arkansas.

Life Cycle of Cyclospora

The life cycle of Cyclospora begins when the host is exposed and ingests the pathogen either in its oocyst or spore form. The oocyst is comprised of sporocytes which contain sporozoites. Upon release of the sporozoites, the epithelial cells of the intestine are penetrated. Within the intestinal cells, the sporozoites undergo multiple fission and develop into meronts which contain merozoites. The merozoites then undergo division and produce micro- and macro-gametes representing male and female gametes, respectively.

These gametes then reproduce and result in the formation of oocysts. It is the oocysts which pass through the intestinal tract and are released into the feces. The oocysts demonstrate the ability to undergo sporulation in a crop and water host as well beginning with the oocyst stage. It is during the oocyst stage that cyclospora exhibit a high resistance to disinfectants.

image

Life cycle of cyclospora: Cyclosporiasis (Cyclospora cayetanesis) When freshly passed in stools, the oocyst is not infective (1) (thus, direct fecal-oral transmission cannot occur; this differentiates Cyclospora from another important coccidian parasite, Cryptosporidium). In the environment (2), sporulation occurs after days or weeks at temperatures between 22°C to 32°C, resulting in division of the sporont into two sporocysts, each containing two elongate sporozoites (3). Fresh produce and water can serve as vehicles for transmission (4) and the sporulated oocysts are ingested (in contaminated food or water) (5). The oocysts excyst in the gastrointestinal tract, freeing the sporozoites which invade the epithelial cells of the small intestine (6). Inside the cells they undergo asexual multiplication and sexual development to mature into oocysts, which will be shed in stools (7). The potential mechanisms of contamination of food and water are still under investigation.

Symptoms of Cyclospora

The symptoms associated with this disease are categorized as gastroenteritis based issues. The symptoms range from watery, loose stool, weight loss, cramping, fatigue, vomiting, fever and nausea. The symptoms can be extremely severe if presented in an immunocompromised patient, such as a patient living with AIDS. The transmission of cyclospora to humans most often occurs by ingesting contaminated foods. In regions of the world where there is lack of health regulations, the chances of cyclospora exposure is increased. Often times, the contaminants include fresh fruits and vegetables which have been exposed to contaminated soil. Individuals exposed to these pathogens in these of regions are at high risk for developing cyclosporiasis, hence, the origin of the commonly known name, traveler’s diarrhea.

image

Cyclospora cayetanensis oocysts: A photomicrograph of oocysts from Cyclospora cayetanensis derived from a fresh stool sample.

Amoebic Dysentery (Amoebiasis)

Amoebic dysentery is caused by the parasite Entamoeba histolytica and infected individuals suffer from severe diarrhea, cramps, and fever.

Learning Objectives

Outline the life cycle of Entamoeba histolytica

Key Takeaways

Key Points

  • Amoebiasis is transmitted via food or water contaminated with fecal matter from an infected individual.
  • The pathogen, Entamoeba histolytica, is mainly found in tropical areas and is protected from degradation and destruction by its protective shell, formed during its cyst stage.
  • The cyst stage is typically passed in the feces and then ingested to cause infection.
  • The amoebas are able to burrow into the walls of the intestines, causing damage.

Key Terms

  • trophozoites: A protozoan in the feeding stage of its life cycle.

Amoebic dysentery, also referred to as amoebiasis, is caused by the ameoba Entamoeba histolytica. Dysentery is characterized as an inflammatory disorder of the intestine that results in severe diarrhea containing both mucus and blood in the feces, often accompanied with fever and abdominal pain.

The route of transmission for ameobic dysentery is the fecal-oral route. Transmission and infection occur upon exposure or ingestion of contaminated food and water. The infective cysts are passed via infected stool. The ameoba also demonstrates the ability to spread as free amoebae or trophozoites, meaning the cysts are not absolutely necessary; however, these states do not survive long outside of the host.

Ameobic dysentery is seen in both developing and industrialized countries, although it is most common in tropical areas. Ameobic cysts are often found in areas of the world where the use of human feces for fertilizer is common, often referred to as ‘night soil’. Upon ingestion of contaminated foods or water, the cysts will move into the intestinal area. These cysts are protected from stomach acids and are able to evade destruction. Once in the intestine, the cyst breaks open and releases the amoebas which then burrow into and damage the intestinal walls.

The amoebae or trophozoites are able to divide via binary fission and and produce cysts. If they are passed in the feces as is, instead of developing into cysts, their survival rate decreases as they are unable to survive in harsh environments. Interestingly, individuals can be asymptomatic if infected with trophozoites and can function as carriers by passing cysts in their stool.

image

Life Cycle of Entamoeba histolytica: An overview of the life cycle of E. histolytica, the source of amoebic dysentery.

Symptoms of individuals infected with Entamoeba histolytica include ulcers, abdominal cramps, diarrhea, bloody stools, liquid stools, fever and vomiting.

Legionellosis

Legionellosis is most commonly caused by the Gram-negative bacteria Legionella pneumophila which is an aquatic organism.

Learning Objectives

Discuss the mode of infection for the bacteria Legionella

Key Takeaways

Key Points

  • Legionella pneumophila is an aquatic organism that is transmitted via aerosols.
  • Legionellosis is caused by exposure to contaminated water sources including water towers, spas, fountains, swimming pools and cooling towers.
  • Legionellosis-infected individuals have pneumonia -like symptoms with moderate to high fever ranges, chills and a dry cough.

Key Terms

  • macrophages: A type of white blood cell that targets foreign material, including bacteria and viruses.
  • Legionnaire’s disease: The more severe form of legionellosis which produces high fever and pneumonia.

Legionellosis, commonly referred to as Legionnaire’s disease, is caused by the pathogenic, gram-negative bacteria Legionella. It is characterized by flu- and pneumonia -like symptoms, including fevers and chills. In advanced stages of the disease, there are gastrointestinal and nervous system issues which result in diarrhea and nausea.

Legionella are a type of bacterium that reside within amoebae in the natural environment. The specific species most associated with legionellosis is Legionella pneumophilia, an aquatic organism. Legionella transmission occurs via aerosols and infection occurs when upon inhalation of the bacteria. After being inhaled, the bacteria infect the macrophages of the alveolar and exploit the host machinery to create an environment that promotes bacterial replication. However, the bacteria are not spread from one person to another. In the 1970s, the CDC investigated a large outbreak of legionellosis at Baptist Hospital that was spread through its air conditioner.

image

Legionella pneumophila: A microscopic image of Legionella pneumophila, the cause of Legionellosis.

Individuals infected with legionellosis have similar symptoms as those diagnosed with pneumonia. Symptoms include high fevers, chills, cough, muscle aches and headaches. For a diagnosis of legionellosis, x-rays and diagnostic tests are used to identify the bacteria. Individuals particularly at risk are older individuals, those immunocompromised or with chronic lung disease.

Legionellosis can take on two distinct forms commonly referred to as legion fever or pontiac fever. Legion fever resembles acute influenza and is the more severe form of the disease, characterized by high fever and pneumonia. Pontiac fever is a milder version and results in mild respiratory illness without the development of pneumonia.

Common sources of Legionella include swimming pools, cooling towers, hot-water systems such as spas, fountains, freshwater ponds and creeks. As seen, the major source for Legionella bacteria is infected water. The bacteria can become suspended in water droplets which are then inhaled into the lungs.