15.1E: Fungal Skin and Nail Diseases - Biology

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 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.

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

Bristol Royal Infirmary, Bristol Dermatology Centre, Bristol, UK

Brugmann – St Pierre and Children's University Hospitals, Université Libre de Bruxelles, Brussels, Belgium

University of Franche-Comté, Nail Disease Centre, Cannes, France

Bristol Royal Infirmary, Bristol Dermatology Centre, Bristol, UK

Brugmann – St Pierre and Children's University Hospitals, Université Libre de Bruxelles, Brussels, Belgium

University of Franche-Comté, Nail Disease Centre, Cannes, France


This chapter deals with all aspects of the nails in health and disease apart from hereditary nail disorders and fungal infections of the nails which are addressed in Chapters 69 and 32, respectively. Nail biology and growth establishes the basis for an understanding of how the nail may be affected by disease processes and their treatment. The significance of nail signs, which may be an important indicator of systemic diseases, is fully discussed before individual disorders of the nail and perionychium are described. The section on dermatoses affecting the nails includes the main categories of inflammatory and neoplastic diseases seen at other sites in the skin, but as expressed in the nail unit. The diagnostic imaging of the nail and nail surgery sections highlight special considerations required in managing disorders of the nail. Finally, the reader is introduced to cosmetic procedures used on nails and problems which may arise from them.

Direct microscopy of skin scrapings and nail clippings

The material is examined by microscopy by one or more of these methods:

  • Potassium hydroxide (KOH) preparation, stained with blue or black ink
  • Fluorescent staining
  • An unstained wet-mount
  • A stained dried smear
  • Histopathology of biopsy with special stains, such as periodic acid-Schiff (PAS).

Microscopy can identify a dermatophyte by the presence of:

  • Fungal hyphae (branched filaments) making up a mycelium
  • Arthrospores (broken-off spores )
  • Arthroconidia (specialised external spores)
  • Spores inside a hair (endothrix) or outside a hair (ectothrix).

Fungal elements are sometimes difficult to find, especially if the tissue is very inflamed, so a negative result does not rule out fungal infection.

A yeast infection can be identified by the presence of:

  • Yeast cells, which may be dividing by budding
  • Pseudohyphae (branched filaments similar to those of a dermatophyte) forming a pseudomycelium.

PAS stain of aspergillus seen in a skin biopsy


Dermatophytes are a group of pathogenic fungi that cause mostly superficial diseases on humans and other mammals (Kwon-Chung and Bennett 1992 White et al. 2008 Achterman and White 2012a,b). The diseases that result from a dermatophyte infection are known as tineas. The location of the disease on the body further defines the disease, so that tinea pedis are dermatophyte infections of the feet, tinea cruris of the genitals, tinea corporis of the torso, and tinea capitis of the head. There are at least 40 species of dermatophytes that infect humans, and many of these fungi can cause disease in more than one body location. Still the most prevalent cause of tinea pedis is T. rubrum, and the most prevalent causes of tinea capitis are Trichophyton tonsurans and Microsporum canis.

Dermatophytes are ascomycetes with septate hyphae, most closely related to Coccidioides immitis within the Onygenales (Kwon-Chung and Bennett 1992 Graser et al. 2008). There are three genera of dermatophytes, Trichophyton, Microsporum, and Epidermophyton. Although the species were historically divided into these genera by morphology and physical attributes, recent analysis by rRNA sequencing indicates that the dermatophytes as a whole are a cohesive group, with no clear distinction between the three genera. The closest relatives to any one Microsporum species might be two Trichophyton species (Graser et al. 2008). Thus, no comparisons should be made between genera without consulting the rRNA-based phylogenetic tree.


Jane E. Sykes , Catherine A. Outerbridge , in Canine and Feline Infectious Diseases , 2014

Etiology and Epidemiology

Dermatophytosis (ringworm or tinea) is a superficial cutaneous infection with one or more of the keratinophilic fungi that belong to the genera Microsporum, Trichophyton, or Epidermophyton. Transmission of dermatophytes occurs by close contact with other infected animals or through contact with contaminated fomites (which includes the haircoats of animals and arthropods such as fleas or houseflies). Dermatophyte spores survive more than a year in the environment under optimal conditions of temperature and humidity, and they resist most routinely used hospital disinfectants, which facilitates transmission. Dermatophytes are somewhat host species specific and are classified as geophilic, zoophilic, or anthropophilic ( Table 58-1 ). 3-8 Geophilic dermatophytes are soil saprophytes. The most common geophilic dermatophyte that infects dogs or cats is Microsporum gypseum, which is most prevalent in warm, humid tropical and subtropical environments. Zoophilic dermatophytes are adapted to animal hosts and are rarely found in soil. The most common zoophilic dermatophyte that infects dogs and especially cats is Microsporum canis this organism accounts for more than 90% of dermatophyte isolates from cats worldwide and more than 60% of isolates from dogs. 3,9,10 Sylvatic dermatophytes are zoophilic dermatophytes that are adapted to rodents or hedgehogs. The most common sylvatic dermatophyte that infects dogs and cats is Trichophyton mentagrophytes. Anthropophilic dermatophytes are adapted to human hosts and do not survive in the soil they include Microsporum audouinii, Trichophyton tonsurans, Trichophyton rubrum, and Epidermophyton floccosum. Rarely, these species infect or contaminate dogs or cats that have a history of close contact with infected humans. 4,11-16 Mixed infections with multiple dermatophyte species occur rarely in dogs. 10

Risk factors for dermatophytosis in dogs and cats include young age and concurrent immunosuppressive disorders, especially endogenous or iatrogenic hyperadrenocorticism. Dermatophytosis is more common in cats than in dogs, but the prevalence varies with geographic location. Positive dermatophyte cultures occurred in 6% of submissions from dogs or cats in the southern United States (3.8% for dogs and 14.9% for cats), 17 and in the United Kingdom, 16% were positive (10% for dogs and 16% for cats). 9 Higher prevalences of detection (19% to 43%) have been reported in some studies of cats and dogs, with and without skin lesions, from parts of continental Europe. 3,5,10 Dermatophytosis (especially geophilic dermatophytosis) is more prevalent in regions with high warmth and humidity. Shelter cats, the vast majority of which lack skin lesions, are more likely to carry dermatophytes on their haircoats than healthy pet cats. Dermatophytes were cultured from 5.5% and 19% of all shelter cats depending on geographical region, 18,19 whereas the prevalence of M. canis isolation from healthy pet cats is generally less than 2.5%. 18 Animals admitted to breeding or boarding facilities that have a history of dermatophytosis are also at risk of infection, whereas dermatophytes are generally not detectable on the haircoats of animals in facilities without a history of infection. 20 Retrovirus-infected cats are no more likely to carry dermatophytes than retrovirus-negative cats, 21 but retrovirus-infected cats carry a greater range of species and may be more likely to develop skin lesions (dermatophytosis). Genetic factors may also be important. Persian and Himalayan cats and Yorkshire and Jack Russell terriers appear to be predisposed to dermatophytosis. 3,9,17 In particular, Persian cats are thought to be at greater risk for development of dermatophytic mycetomas (see Clinical Features). Dogs that hunt and burrow in soil may be at increased risk for infection by geophilic or sylvatic dermatophytes, which may explain the common distribution of lesions on the face and distal thoracic limbs for these dermatophyte species ( Figure 58-1 ).

Global and Multi-National Prevalence of Fungal Diseases-Estimate Precision

Fungal diseases kill more than 1.5 million and affect over a billion people. However, they are still a neglected topic by public health authorities even though most deaths from fungal diseases are avoidable. Serious fungal infections occur as a consequence of other health problems including asthma, AIDS, cancer, organ transplantation and corticosteroid therapies. Early accurate diagnosis allows prompt antifungal therapy however this is often delayed or unavailable leading to death, serious chronic illness or blindness. Recent global estimates have found 3,000,000 cases of chronic pulmonary aspergillosis,

223,100 cases of cryptococcal meningitis complicating HIV/AIDS,

700,000 cases of invasive candidiasis,

500,000 cases of Pneumocystis jirovecii pneumonia,

250,000 cases of invasive aspergillosis,

100,000 cases of disseminated histoplasmosis, over 10,000,000 cases of fungal asthma and

1,000,000 cases of fungal keratitis occur annually. Since 2013, the Leading International Fungal Education (LIFE) portal has facilitated the estimation of the burden of serious fungal infections country by country for over 5.7 billion people (>80% of the world's population). These studies have shown differences in the global burden between countries, within regions of the same country and between at risk populations. Here we interrogate the accuracy of these fungal infection burden estimates in the 43 published papers within the LIFE initiative.

Keywords: estimate precision fungal diseases global prevalence.

Conflict of interest statement

Denning and family hold Founder shares in F2G Ltd., a University of Manchester spin-out antifungal discovery company. He acts or has recently acted as a consultant to Astellas, Sigma Tau, Basilea, Scynexis, Cidara, Biosergen, Quintiles, Pulmatrix, Pulmocide and Zambon. In the last 3 years, he has been paid for talks on behalf of Astellas, Dynamiker, Gilead, Merck and Pfizer. He is a longstanding member of the Infectious Disease Society of America Aspergillosis Guidelines group, the European Society for Clinical Microbiology and Infectious Diseases Aspergillosis Guidelines group and the British Society for Medical Mycology Standards of Care committee. Felix Bongomin, Sara Gago and Rita O. Oladele declare no conflicts of interest.

Start Here

  • Fungal Culture Test(National Library of Medicine) Also in Spanish
  • Rash Evaluation(National Library of Medicine) Also in Spanish
  • Skin Rashes and Other Problems (American Academy of Family Physicians) Also in Spanish
  • Sputum Culture(National Library of Medicine) Also in Spanish

Candida Species

The genus Candida are yeasts and the most commonly known species is Candida albicans. Other lesser known species includes C. tropicalis, C. parapsilosis, C. glabrata and C. guilliermondii. It is naturally found on humans, primarily in the digestive tract, but does not usually cause an infection unless a person is immunocompromised. Sometimes an overgrowth can occur when the natural environment where it exists is altered in a way that allows it to thrive unchecked.

Candidiasis of the skin is not common when compared to fungal skin infections caused by dermatophytes (dermatophytosis). When candidal skin infections do occur, it is mainly at sites where the deeper living tissue is exposed. This may be seen at the body folds where the skin is inflamed and even eroded in severe cases. Apart from a weakened immune system as in AIDS, these candidal skin infections also commonly arises in diabetics and people who are obese. It may also be responsible for some cases of diaper rash.

CARD9 Deficiency and Other Syndromes of Susceptibility to Candidiasis

CARD9 deficiency is a genetic immune disorder characterized by susceptibility to fungal infections like candidiasis, which is caused by the yeast fungus Candida. Typically, Candida does not cause severe problems in healthy people, but it can take advantage of those with a weakened immune system.

NIAID researchers are exploring how mutations that cause Candida susceptibility impact the function of immune cells by studying people with genetic disorders such as CARD9 deficiency SCID and deficiencies in STAT1, STAT3 and DOCK8. NIAID researchers also are studying chronic mucocutaneous candidiasis (CMC) and systemic candidiasis—two types of Candida infection—in mouse models of these genetic disorders. By identifying the genetic defects responsible for a person’s immunodeficiency, researchers may be able to develop targeted therapies to prevent these opportunistic infections.

CARD9 deficiency, an autosomal recessive disorder, differs from many other genetic immune disorders that cause fungal infections. Most genetic immune disorders cause either CMC or systemic candidiasis, but people with CARD9 deficiency experience both CMC and systemic candidiasis. CMC is a localized Candida infection that causes lesions and scaling on the skin and nails. The infection also occurs in areas where the skin transitions to other tissues, such as the genital area, eyelids and mouth, and may progress along the throat. Normally, cells of the adaptive immune system, like T cells, stop this infection from taking hold. Systemic candidiasis is an invasive infection that involves the bloodstream and deep-seated organs such as the kidneys, brain, liver and/or spleen, and requires innate immune cells to prevent. CARD9 is required for anti-fungal responses in both innate and adaptive immune cells, explaining why people with this deficiency experience both conditions.

Systemic candidiasis may lead to life-threatening sepsis or meningitis, an infection of the brain and its linings. Many people with CARD9 deficiency develop systemic candidiasis that targets the central nervous system, or CNS. NIAID researchers have described the crucial role of CARD9 in recruiting infection-fighting neutrophils to the CNS during fungal infection, helping explain why people with CARD9 deficiency are highly susceptible to fungal CNS infections.

Fungal infections are diagnosed by physical examination of the infected sites and by using laboratory tests to confirm the presence of Candida or other fungi. If warranted, genetic testing can confirm the presence of mutations associated with these infections, like CARD9 deficiency.

Patients with fungal infections may be prescribed antifungal medications.

To learn more about CARD9 deficiency and other syndromes of susceptibility to candidiasis, visit the National Library of Medicine, Genetics Home Reference familial candidiasis site.

4 Easy Questions on Fungal Diseases in Humans

The main human diseases caused by fungi in immunocompetent patients are coccidioidomycosis, histoplasmosis, blastomycosis, paracoccidioidomycosis, or South American blastomycosis, sporotrichosis and onychomycosis (nail mycosis).

In immuno-deficient patients, in addition to the diseases mentioned above, other fungal diseases such as systemic candidiasis, aspergillosis, cryptococcosis and other opportunistic diseases may occur.


More Bite-Sized Q&As Below

2. Moniliasis is one of the most common opportunistic diseases in AIDS patients. What is the etiological agent of moniliasis and what is the other name for the disease? Why is monilia also common in healthy newborns?

The etiological agent of moniliasis is Candida albicans, a fungus. Moniliasis is also known as mucocutaneous candidiasis. In AIDS, moniliasis can complicate and turn into systemic candidiasis, affecting many organs.

The immune system of newborns does not yet work with complete efficiency and, as a result, they are more susceptible to candidiasis, which generally appears in the mouth and in the genital mucosae, and disappears naturally.

Fungal Zoonoses

3. What are some fungal diseases transmitted by animal feces?

Bat and pigeon feces can carry Histoplasma capsulatum, the fungal agent of histoplasmosis. The infection is transmitted through the inhalation of contaminated dust in places visited by these animals (caves, tunnels, squares, roofs, etc.). Cryptococcosis is another fungal disease transmitted by pigeon feces.

Select any question to share it on FB or Twitter

Just select (or double-click) a question to share. Challenge your Facebook and Twitter friends.


4. What are some antibiotics used against fungi?

Topical or systemic azoles (such as itraconazole, fluconazole and others), amphotericin B, echinocandins (caspofungin, micafungin), terbinafine and griseofulvin are examples of antifungal drugs.

Now that you have finished studying Fungal Diseases, these are your options: