Microbial Diseases of the Skin

Structure of the Skin: Epidermis

The epidermis includes five main layers: the stratum corneum, stratum lucidium, stratum granulosum, stratum spinosum, and stratum germinativum.

Learning Objectives

Describe the layers of the epidermis

Key Takeaways

Key Points

  • The epidermis provides a protective waterproof barrier that also keeps pathogens at bay and regulates body temperature.
  • The main layers of the epidermis are: stratum corneum, stratum lucidium, stratum granulosm, stratum spinosum, stratum germinativum (also called stratum basale).
  • Keratinocytes in the stratum basale proliferate during mitosis and the daughter cells move up the strata, changing shape and composition as they undergo multiple stages of cell differentiation.

Key Terms

  • keratinocyte: the predominant cell type in the epidermis, the outermost layer of the skin, constituting 95% of the cells found there. Those keratinocytes found in the basal layer (stratum germinativum) of the skin are sometimes referred to as basal cells or basal keratinocytes.
  • stratum germinativum: the basal layer—sometimes referred to as stratum basale—is the deepest of the five layers of the epidermis.
  • stratum corneum: the most superficial layer of the epidermis from which dead skin sheds.
  • epidermis: the outermost layer of skin.
  • stratum lucidum: a layer of our skin that is found on the palms of our hands and the soles of our feet.

Layers of the Epidermis

The epidermis is the outermost layer of our skin. It is the layer we see with our eyes. It contains no blood supply of its own—which is why you can shave your skin and not cause any bleeding despite losing many cells in the process. Assuming, that is, you don’t nick your skin to deep, where the blood supply is actually found. The epidermis is itself divided into at least four separate parts. A fifth part is present in some areas of our body. In order from the deepest layer of the epidermis to the most superficial, these layers (strata) are the:

  • Stratum basale
  • Stratum spinosum
  • Stratum granulosum
  • Stratum lucidum
  • Stratum corneum

Stratum Basale

The stratum basale, also called the stratum germinativum, is the basal (base) layer of the epidermis. It is the layer that’s closest to the blood supply lying underneath the epidermis.

This layer is one of the most important layers of our skin. This is because it contains the only cells of the epidermis that can divide via the process of mitosis, which means that skin cells germinate here, hence the word germinativum.

In this layer, the most numerous cells of the epidermis, called keratinocytes, arise thanks to mitosis. Keratinocytes produce the most important protein of the epidermis.

This protein is appropriately called keratin. Keratin makes our skin tough and provides us with much-needed protection from microorganisms, physical harm, and chemical irritation.

Millions of these new cells arise in the stratum basale on a daily basis. The newly produced cells push older cells into the upper layers of the epidermis with time. As these older cells move up toward the surface, they change their shape, nuclear, and chemical composition. These changes are, in part, what give the strata their unique characteristics.

Stratum Spinosum and Granulosum

From the stratum basale, the keratinocytes move into the stratum spinosum, a layer so called because its cells are spiny-shaped cells. The stratum spinosum is partly responsible for the skin’s strength and flexibility. From there the keratinocytes move into the next layer, called the stratum granulosum. This layer gets its name from the fact that the cells located here contain many granules. The keratinocytes produce a lot of keratin in this layer—they become filled with keratin. This process is known as keratinization. The keratinocytes become flatter, more brittle, and lose their nuclei in the stratum granulosum as well.

Stratum Lucidum

Once the keratinocytes leave the stratum granulosum, they die and help form the stratum lucidum. This death occurs largely as a result of the distance the keratinocytes find themselves from the rich blood supply the cells of the stratum basale lie on top off. Devoid of nutrients and oxygen, the keratinocytes die as they are pushed towards the surface of our skin. The stratum lucidum is a layer that derives its name from the lucid (clear/transparent) appearance it gives off under a microscope. This layer is only easily found in certain hairless parts of our body, namely the palms of our hands and the soles of our feet. Meaning, the places where our skin is usually the thickest.

Stratum Corneum

From the stratum lucidum, the keratinocytes enter the next layer, called the stratum corneum (the horny layer filled with cornified cells). This the only layer of skin we see with our eyes. The keratinocytes in this layer are called corneocytes. They are devoid of almost all of their water and they are completely devoid of a nucleus at this point. They are dead skin cells filled with the tough protein keratin. In essence, they are a protein mass more so than they are a cell. The corneocytes serve as a hard protective layer against environmental trauma, such as abrasions, light, heat, chemicals, and microorganism. The cells of the stratum corneum are also surrounded by lipids (fats) that help repel water as well. These corneocytes are eventually shed into the environment and become part of the dandruff in our hair or the dust around us, which dust mites readily munch on. This entire cycle, from new keratinocyte in the straum basale to a dead cell flaked off into the air, takes between 25–45 days.

 This is an image of the layers beneath thin (hairy) and thick (hairy) skin.

Skin overview: skin layers, of both hairy and hairless skin.

This image details the parts of the integumentary system. Starting with a hair shaft and sweat pore on top, this picture identifies the contents of the epidermis, which include the arrector pili musicle, sebaceious gland, hair follicle, papilla of hair, nerve fiber, blood and lymph vessels, sweat glan, pacinian corpuscle, vein and artery.

Human skin: This image details the parts of the integumentary system.

This image details the layers of the epidermis. Starting at the deepest level and rising to the top, it depicts the: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.

Layers of the epidermis: The epidermis is made up of 95% keratinocytes but also contains melanocytes, Langerhans cells, Merkel cells, and inflammatory cells. The stratum basale is primarily made up of basal keratinocyte cells, which can be considered the stem cells of the epidermis. They divide to form the keratinocytes of the stratum spinosum, which migrate superficially.

Microbiota of the Skin

The skin flora, more properly referred to as the skin microbiome or skin microbiota, are the microorganisms that reside on the skin.

Learning Objectives

Describe the types of skin flora and how they can be beneficial for the organism

Key Takeaways

Key Points

  • Most bacteria on the skin are found in the superficial layers of the epidermis and the upper parts of hair follicles.
  • Skin flora are usually non-pathogenic, and either commensals (are not harmful to their host) or mutualistic (offer a benefit).
  • The benefits bacteria can offer include preventing transient pathogenic organisms from colonizing the skin surface, either by competing for nutrients, secreting chemicals against them, or stimulating the skin’s immune system.
  • Resident microbes can cause skin diseases and enter the blood system creating life-threatening diseases particularly in immunosuppressed people.

Key Terms

  • skin flora: the skin flora, more properly referred to as the skin microbiome or skin microbiota, are the microorganisms which reside on the skin.
  • commensal: a term for a form of symbiosis in which one organism derives a benefit while the other is unaffected
  • mutualistic: mutually beneficial.

The skin flora, more properly referred to as the skin microbiome or skin microbiota, are the microorganisms that reside on the skin. Most bacteria on the skin are found in the superficial layers of the epidermis and the upper parts of hair follicles. Skin flora are usually non-pathogenic, and either commensals (are not harmful to their host) or mutualistic (offer a benefit). The benefits bacteria can offer include preventing transient pathogenic organisms from colonizing the skin surface, either by competing for nutrients, secreting chemicals against them, or stimulating the skin’s immune system. However, resident microbes can cause skin diseases and enter the blood system creating life-threatening diseases particularly in immunosuppressed people. Hygiene to control such flora is important in preventing the transmission of antibiotic resistant hospital-acquired infections.

A major nonhuman skin flora is Batrachochytrium dendrobatidis, a chytrid and non-hyphal zoosporic fungus that causes chytridiomycosis, an infectious disease thought to be responsible for the decline in amphibian populations. The estimate of the number of species present on skin bacteria has been radically changed by the use of 16S ribosomal RNA to identify bacterial species present on skin samples direct from their genetic material. Previously such identification had depended upon microbiological culture upon which many varieties of bacteria did not grow and so were hidden to science. Staphylococcus epidermidis and Staphylococcus aureus were thought from cultural based research to be dominant. However, 16S ribosomal RNA research found that while common these species make up only 5% of skin bacteria. However, skin variety provides a rich and diverse habitat for bacteria. Most come from four phyla: Actinobacteria (51.8%), Firmicutes (24.4%), Proteobacteria (16.5%), and Bacteroidetes (6.3%).

There are three main ecological areas for skin flora: sebaceous, moist, and dry. Propionibacteria and Staphylococci species are the main species in sebaceous areas. In moist places on the body Corynebacteria together with Staphylococci dominate. In dry areas, there is a mixture of species, but b-Proteobacteria and Flavobacteriales are dominant. Ecologically, sebaceous areas have greater species richness than moist and dry ones. The areas with least similarity between people in species are the spaces between fingers, the spaces between toes, axillae, and umbilical cord stump. Most similar are beside the nostril, nares (inside the nostril), and on the back.

Skin microflora can be commensals, mutualistic, or pathogens. Often they can be all three depending upon the strength of the person’s immune system. Research on the immune system in the gut and lungs has shown that microflora aids immunity development. However, such research has only started upon whether this is the case with the skin. Pseudomonas aeruginosa is an example of a mutualistic bacterium that can turn into a pathogen and cause disease. If it gains entry into the blood system it can result in infections in bone, joint, gastrointestinal, and respiratory systems and it can also cause dermatitis. However,

Pseudomonas aeruginosa produces antimicrobial substances such as pseudomonic acid (that are exploited commercially such as Mupirocin). This works against staphylococcal and streptococcal infections. Pseudomonas aeruginosa also produces substances that inhibit the growth of fungus species such as Candida krusei, Candida albicans, Torulopsis glabrata, Saccharomyces cerevisiae, and Aspergillus fumigatus. It can also inhibit the growth of Helicobacter pylori. Fatty acids (caproic acid) on the skin inhibit bacteria, especially after puberty, when undecylic acid becomes the primary fatty acid on the skin. Undecylic acid provides resistance to ringworm fungus and other skin infections.

Another aspect of bacteria is the generation of body odor. Sweat is odorless. However, several bacteria may consume it and create byproducts which may be considered putrid by man (as in contrast to flies, for example, that may find them attractive/appealing). For example, Propionibacteria in adolescent and adult produce propionic acid in sebaceous glands.

Staphylococcus epidermidis: Scanning electron microscope image of Staphylococcus epidermidis one of roughly 1,000 bacteria species present on human skin. Though usually not pathogenic, it can cause skin infections and even life threatening illnesses in those that are immunocompromised.

Bacterial Skin Diseases

Bacterial skin infections include impetigo, erysipelas, and cellulitis.

Learning Objectives

Describe how impetigo, erysipelas and cellulitis are acquired and the treatment options available

Key Takeaways

Key Points

  • Impetigo is a highly contagious bacterial skin infection most common among pre-school children primarily caused by Staphylococcus aureus and sometimes by Streptococcus pyogenes.
  • Erysipelas is an acute streptococcus bacterial infection of the upper dermis and superficial lymphatics.
  • Cellulitis is a diffuse inflammation of connective tissue with severe inflammation of dermal and subcutaneous layers of the skin.
  • Antimicrobial therapy is available for impetigo, erysipelas, and cellulitis.

Key Terms

  • erysipelas: a severe skin disease caused by streptococcus infection in surface and surrounding tissue, marked by continued spreading inflammation
  • impetigo: a contagious bacterial skin disease forming pustules and yellow crusty sores, chiefly on the face and hands. It is common in children. Infection is often through cuts or insect bites.
  • cellulitis: an inflammation of subcutaneous or connective tissue caused by a bacterial infection

Facial impetigo.

Common Bacterial Skin Infections

Bacterial skin infections include impetigo, erysipelas, and cellulitis.

IMPETIGO

Impetigo is a highly contagious bacterial skin infection most common among pre-school children. It is primarily caused by Staphylococcus aureus and sometimes by Streptococcus pyogenes. The infection is spread by direct contact with lesions or with nasal carriers. The incubation period is 1–3 days. Dried streptococci in the air are not infectious to intact skin. Scratching may spread the lesions. Impetigo generally appears as honey-colored scabs formed from dried serum and is often found on the arms, legs, or face . For generations, the disease was treated with an application of the antiseptic gentian violet. Today, topical or oral antibiotics are usually prescribed.

Facial erysipelas.  Erysipelas of the face due to invasive Streptococcus.

ERYSIPELAS

Erysipelas is an acute streptococcus bacterial infection of the upper dermis and superficial lymphatics. This disease is most common among the elderly, infants, and children. People with immune deficiency, diabetes, alcoholism, skin ulceration, fungal infections, and impaired lymphatic drainage (e.g., after mastectomy, pelvic surgery, bypass grafting) are also at increased risk. Patients typically develop symptoms including high fevers, shaking, chills, fatigue, headaches, vomiting, and general illness within 48 hours of the initial infection. The erythematous skin lesion enlarges rapidly and has a sharply demarcated raised edge. It appears as a red, swollen, warm, hardened and painful rash, similar in consistency to an orange peel. More severe infections can result in vesicles, bullae, and petechiae, with possible skin necrosis. Lymph nodes may be swollen and lymphedema may occur. Occasionally, a red streak extending to the lymph node can be seen. Most cases of erysipelas are due to Streptococcus pyogenes (also known as beta-hemolytic group A streptococci), although non-group A streptococci can also be the causative agent. Beta-hemolytic, non-group A streptococci include Streptococcus agalactiae, also known as group B strep or GBS. Depending on the severity, treatment involves either oral or intravenous antibiotics, using penicillins, clindamycin, or erythromycin. While illness symptoms resolve in a day or two, the skin may take weeks to return to normal.

Cellulitis.  Left shin infected with cellulitis.

CELLULITIS

Cellulitis is a diffuse inflammation of connective tissue with severe inflammation of dermal and subcutaneous layers of the skin. Cellulitis can be caused by normal skin flora or by exogenous bacteria, and often occurs where the skin has previously been broken. Common points of infection include cracks in the skin, cuts, blisters, burns, insect bites, surgical wounds, intravenous drug injection, or sites of intravenous catheter insertion. Group A Streptococcus and Staphylococcus are the most common of these bacteria, which are part of the normal flora of the skin, but normally cause no actual infection while on the skin’s outer surface. Skin on the face or lower legs is most commonly affected by this infection, though cellulitis can occur on any part of the body. The mainstay of therapy remains treatment with appropriate antibiotics Recovery periods last from 48 hours to six months. The typical signature symptom of cellulitis is an area which is red, hot, and tender . Cellulitis is most often a clinical diagnosis, and local cultures do not always identify the causative organism. Blood cultures usually are positive only if the patient develops generalized sepsis.Treatment consists of resting the affected area, cutting away dead tissue, and administration of antibiotics (either oral or intravenous).

Viral Skin Diseases

Virus-related cutaneous conditions include cold sores, shingles, and warts.

Learning Objectives

Describe what causes cold sores, shingles and warts and the treatment options available

Key Takeaways

Key Points

  • Oral herpes, the visible symptoms of which are colloquially called cold sores or fever blisters, is an infection of the face or mouth and is the most common form of infection by herpes simplex.
  • Herpes zoster (or simply zoster), commonly known as shingles, is a viral disease caused by reactivation of latent varizella zoster virus and characterized by a painful skin rash with blisters in a limited area on one side of the body, often in a stripe.
  • A wart is generally a small, rough growth, typically on a human’s hands or feet that can resemble a cauliflower or a solid blister and it is caused by infection by the human papilloma virus.

Key Terms

  • shingles: a viral disease characterized by a painful skin rash with blisters in a limited area on one side of the body, often in a stripe
  • wart: a type of deformed growth occurring on the skin caused by the human papillomavirus (HPV).
  • cold sore: a small bump on the lips resulting from infection by the herpes virus.
  • zoster: the disease called herpes zoster (from the typically beltlike pattern of its rash); shingles.

Virus-related cutaneous conditions are caused by two main groups of viruses–DNA and RNA types–both of which are obligatory intracellular parasites.

A cutaneous condition is any medical condition that affects the integumentary system — the organ system that comprises the entire surface of the body and includes skin, hair, nails, and related muscle and glands. Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses. While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described. Three common skin conditions that result from viral infections are cold sores, shingles, and warts.

Herpes

Herpes simplex is a viral disease from the herpesviridae family caused by both Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2). Infection with the herpes virus is categorized into one of several distinct disorders based on the site of infection. Oral herpes , the visible symptoms of which are colloquially called cold sores or fever blisters, is an infection of the face or mouth and is the most common form of infection. Genital herpes, known simply as herpes, is the second most common form of herpes. Herpes simplex is most easily transmitted by direct contact with a lesion or the body fluid of an infected individual. Transmission may also occur through skin-to-skin contact during periods of asymptomatic shedding. Barrier protection methods are the most reliable method of preventing transmission of herpes, but they merely reduce rather than eliminate risk. Oral herpes is easily diagnosed if the patient presents with visible sores or ulcers. Once infected, the virus remains in the body for life. Recurrent infections (outbreaks) may occur from time to time, especially in times of immune impairment such as HIV and cancer-related immune suppression. However, after several years, outbreaks become less severe and more sporadic, and some people will become perpetually asymptomatic and will no longer experience outbreaks, though they may still be contagious to others. Treatments with antivirals can reduce viral shedding and alleviate the severity of symptomatic episodes.

Herpes simplex. Herpes labialis or cold sore of the lower lip. Note the blisters in a group marked by an arrow.

Herpes Zoster

Herpes zoster (or simply zoster), commonly known as shingles, is a viral disease characterized by a painful skin rash with blisters in a limited area on one side of the body, often in a stripe. The initial infection with varicella zoster virus (VZV) causes the acute (short-lived) illness chickenpox which generally occurs in children and young people. Once an episode of chickenpox has resolved, the virus is not eliminated from the body but remains latent and can go on to cause shingles—an illness with very different symptoms—often many years after the initial infection. Although the zoster rash usually heals within two to four weeks, some sufferers experience residual nerve pain for months or years, a condition called postherpetic neuralgia. Exactly how the virus remains latent in the body, and subsequently re-activates is not understood. The earliest symptoms of herpes zoster, which include headache, fever, and malaise, are nonspecific, and may result in an incorrect diagnosis. In most cases after 1–2 days, but sometimes as long as three weeks, the initial phase is followed by the appearance of the characteristic skin rash. The pain and rash most commonly occurs on the torso, but can appear on the face, eyes, or other parts of the body.

At first the rash appears similar to the first appearance of hives. However, unlike hives, herpes zoster causes skin changes limited to a dermatome, normally resulting in a stripe or belt-like pattern that is limited to one side of the body and does not cross the midline. The goals of treatment are to limit the severity and duration of pain, shorten the duration of a shingles episode, and reduce complications. Symptomatic treatment is often needed for the complication of postherpetic neuralgia. Topical lotions containing calamine can be used on the rash or blisters and may be soothing. Antiviral drugs inhibit VZV replication and reduce the severity and duration of herpes zoster with minimal side effects, but do not reliably prevent postherpetic neuralgia. Of these drugs, acyclovir has been the standard treatment.

Herpes Zoster. Neck Herpes zoster blisters on the neck and shoulder.

A wart is generally a small, rough growth, typically on a human’s hands or feet , but often other locations, that can resemble a cauliflower or a solid blister. They are caused by a viral infection, specifically by one of the many types of human papillomavirus (HPV). It is possible to get warts from others. They are contagious and usually enter the body in an area of broken skin. They typically disappear after a few months but can last for years and can recur. Gardasil is an HPV vaccine aimed at preventing cervical cancers and genital warts. There are many treatments and procedures associated with wart removal.

Warts. Warts on the big toe.

Fungal Skin and Nail Diseases

Common fungal skin diseases include athlete’s foot, jock itch, and ringworm.

Learning Objectives

Describe how fungal skin and nail diseases arise, their characteristic symptoms and the treatment options available

Key Takeaways

Key Points

  • Athlete’s foot (also known as ringworm of the foot and tinea pedis) is an infection of the skin that causes scaling, flaking, and itching of affected areas and is caused by a fungi in the genus Trichophyton.
  • Tinea cruris, also known as jock itch, is a dermatophyte fungal infection of the groin region in any sex, though more often seen in males.
  • Dermatophytosis or ringworm is a clinical condition caused by fungal infection of the skin in humans, pets such as cats, and domesticated animals such as sheep and cattle.

Key Terms

  • jock itch: a fungal infection, tinea cruris, of the groin region, due to the fungus Trichophyton rubrum and others.
  • ringworm: a contagious fungal affliction of the skin, characterized by ring-shaped discoloured patches, covered by vesicles or scales.
  • athlete’s foot: a fungal infection of the skin of the foot, usually between the toes, caused by the pathogen fungi. Scientific name: tinea pedis.

A cutaneous condition is any medical condition that affects the integumentary system — the organ system that comprises the entire surface of the body and includes skin, hair, nails, and related muscle and glands. Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). Common fungal skin and nail diseases include athlete’s foot, jock itch, and ringworm.

Athlete’s foot (also known as ringworm of the foot and tinea pedis; ) is an infection of the skin that is caused by a fungi in the genus Trichophyton. While it is typically transmitted in moist communal areas where people walk barefoot, the disease requires a warm moist environment, such as the inside of a shoe, in order to incubate. Athlete’s foot causes scaling, flaking, and itching of the affected skin. Blisters and cracked skin may also occur, leading to exposed raw tissue, pain, swelling, and inflammation. Secondary bacterial infection can accompany the fungal infection, sometimes requiring a course of oral antibiotics. Athlete’s foot can usually be diagnosed by visual inspection of the skin, but where the diagnosis is in doubt direct microscopy of a potassium hydroxide preparation (known as a KOH test) may help rule out other possible causes, such as eczema or psoriasis. Without medication athlete’s foot resolves in 30–40% of cases and topical antifungal medication consistently produce much higher percentages of a cure. Conventional treatment typically involves daily or twice daily application of a topical medication in conjunction with hygiene measures outlined in the above section on prevention. Keeping feet dry and practicing good hygiene is crucial to preventing reinfection. Severe or prolonged fungal skin infections may require treatment with oral antifungal medication.

Athlete's Foot

Athlete’s Foot. A severe case of athlete’s foot.

Tinea cruris, also known as crotch itch, crotch rot, Dhobie itch, eczema marginatum, gym itch, jock itch, jock rot, and ringworm of the groin is a dermatophyte fungal infection of the groin region in any sex, though more often seen in males. As the common name for this condition implies, it causes itching or a burning sensation in the groin area, thigh skin folds, or anus. It may involve the inner thighs and genital areas, as well as extending back to the perineum and perianal areas. Affected areas may appear red, tan, or brown, with flaking, rippling, peeling, or cracking skin. Opportunistic infections (infections that are caused by a diminished immune system) are frequent. Fungus from other parts of the body (commonly tinea pedis or ‘athlete’s foot’) can contribute to this itch. A warm, damp environment allowing the fungus to cultivate greatly contributes; especially with tight, sweaty, or rubbing clothing such as a jockstrap. Medical professionals suggest keeping the groin area clean and dry by drying off thoroughly after bathing and putting on dry clothing right away after swimming or perspiring. Other recommendations to prevent this infection are: not sharing clothing or towels with others, showering immediately after athletic activities, wearing loose cotton underwear, avoiding tight-fitting clothes, and using antifungal powders. Tinea cruris is best treated with topical antifungal medications of the allylamine or azole type.

Dermatophytosis or ringworm is a clinical condition caused by fungal infection of the skin in humans, pets such as cats, and domesticated animals such as sheep and cattle. The term “ringworm” is a misnomer, since the condition is caused by fungi of several different species and not by parasitic worms. The fungi that cause parasitic infection (dermatophytes) feed on keratin, the material found in the outer layer of skin, hair, and nails. These fungi thrive on skin that is warm and moist, but may also survive directly on the outsides of hair shafts or in their interiors. In pets, the fungus responsible for the disease survives in skin and on the outer surface of hairs. Advice often given to prevent this infection includes: avoiding sharing clothing, sports equipment, towels, or sheets and washing clothes in hot water with fungicidal soap after suspected exposure to ringworm. After being exposed to places where the potential of being infected is high, one should wash with an antibacterial and anti-fungal soap or one that contains tea tree oil, which contains terpinen-4-ol. Antifungal treatments include topical agents such as miconazole, terbinafine, clotrimazole, ketoconazole, or tolnaftate applied twice daily until symptoms resolve — usually within one or two weeks

Parasitic Skin Diseases

Parasites can cause skin infections and common examples include creeping eruption, lice, and scabies.

Learning Objectives

Describe how the parasitic skin infections creeping eruption, lice and scabies arise and the treatment options available

Key Takeaways

Key Points

  • Cutaneous larva migrans is a skin disease in humans caused by the larvae of various nematode parasites of the hookworm family (Ancylostomatidae) and characterized by a red, intense itching eruption.
  • Humans host three different kinds of lice (head lice, body lice, and pubic lice). Lice infestations can be controlled with lice combs and medicated shampoos or washes.
  • Scabies, known colloquially as the seven-year itch, is a contagious skin infection that occurs among humans and other animals.

Key Terms

  • scabies: an infestation of parasitic mites, Sarcoptes scabiei, causing intense itching caused by the mites burrowing into the skin of humans and other animals. It is easily transmissible from human to human; secondary skin infection may occur.
  • cutaneous larva migrans: a skin disease in humans, caused by the larvae of various nematode parasites of the hookworm family (Ancylostomatidae).
  • louse: a small parasitic wingless insect of the order Phthiraptera.

A cutaneous condition is any medical condition that affects the integumentary system — the organ system that comprises the entire surface of the body and includes skin, hair, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment. Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). Common parasitic skin diseases include creeping eruption, lice, and scabies.

Cutaneous larva migrans (abbreviated CLM) is a skin disease in humans caused by the larvae of various nematode parasites of the hookworm family (Ancylostomatidae). The most common species that cayse this disease in the Americas is Ancylostoma braziliense. Colloquially called creeping eruption due to the way it looks, the disease is also somewhat ambiguously known as “ground itch” or (in some parts of the southern U.S.) “sandworms,” as the larvae like to live in sandy soil. Another vernacular name is plumber’s itch. The medical term CLM literally means “wandering larvae in the skin. ” These parasites are found in dog and cat feces and although they are able to infect the deeper tissues of these animals (through to the lungs and then the intestinal tract), in humans they are only able to penetrate the outer layers of the skin and thus create the typical wormlike burrows visible underneath the skin . The parasites apparently lack the collagenase enzymes required to penetrate through the basement membrane deeper into the skin. The infection causes a red, intense itching eruption. The itching can become very painful and if scratched may allow a secondary bacterial infection to develop but it will stop after the parasites are dead. Systemic (oral) agents to treat this infection include albendazole (trade name Albenza) and ivermectin (trade name Stromectol).

Larva Migrans Cutanea

Larva Migrans Cutanea. Typical “creeping eruption” associated with cutaneous larva migrans.

Louse (plural: lice) is the common name for members of over 3,000 species of wingless insects of the order Phthiraptera, three of which are classified as human disease agents. They are obligate ectoparasites of every avian and mammalian order except for monotremes (the platypus and echidnas), bats, whales, dolphins, porpoises, and pangolins. Most lice are scavengers, feeding on skin and other debris found on the host’s body, but some species feed on sebaceous secretions and blood. Most are found only on specific types of animals, and, in some cases, only to a particular part of the body. Some animals are known to host up to 15 different species, although one to three is typical for mammals, and two to six for birds. For example, in humans, different species of louse inhabit the scalp and pubic hair. Lice generally cannot survive for long if removed from their host. Humans host three different kinds of lice: head lice, body lice, and pubic lice. Lice infestations can be controlled with lice combs and medicated shampoos or washes.

Scabies (from Latin: scabere, “to scratch”), known colloquially as the seven-year itch, is a contagious skin infection that occurs among humans and other animals. The disease may be transmitted from objects, but is most often transmitted by direct skin-to-skin contact, with a higher risk with prolonged contact. Initial infections require four to six weeks to become symptomatic. Reinfection, however, may manifest symptoms within as little as 24 hours. Because the symptoms are allergic, their delay in onset is often mirrored by a significant delay in relief after the parasites have been eradicated. The characteristic symptoms of a scabies infection include intense itching and superficial burrows . The burrow tracks are often linear, to the point that a neat “line” of four or more closely placed and equally developed mosquito-like “bites” is almost diagnostic of the disease. Scabies may be diagnosed clinically in geographical areas where it is common when diffuse itching presents along with either lesions in two typical spots or there is itchiness of another household member. The classical sign of scabies is the burrows made by the mites within the skin. To detect the burrow, the suspected area is rubbed with ink from a fountain pen or a topical tetracycline solution, which glows under a special light. A number of medications are effective in treating scabies with permethrin being the most effective treatment. However, treatment must often involve the entire household or community to prevent re-infection. Options to improve itchiness include antihistamines.

Acarodermatitis

Acarodermatitis.  Scabies of the foot.