Mycoses of the Skin

Many fungal infections of the skin involve fungi that are found in the normal skin microbiota. Some of these fungi can cause infection when they gain entry through a wound; others mainly cause opportunistic infections in immunocompromised patients. Other fungal pathogens primarily cause infection in unusually moist environments that promote fungal growth; for example, sweaty shoes, communal showers, and locker rooms provide excellent breeding grounds that promote the growth and transmission of fungal pathogens.

Fungal infections, also called mycoses, can be divided into classes based on their invasiveness. Mycoses that cause superficial infections of the epidermis, hair, and nails, are called cutaneous mycoses. Mycoses that penetrate the epidermis and the dermis to infect deeper tissues are called subcutaneous mycoses. Mycoses that spread throughout the body are called systemic mycoses.

Tineas

A group of cutaneous mycoses called tineas are caused by dermatophytes, fungal molds that require keratin, a protein found in skin, hair, and nails, for growth. There are three genera of dermatophytes, all of which can cause cutaneous mycoses: Trichophyton, Epidermophyton, and Microsporum. Tineas on most areas of the body are generally called ringworm, but tineas in specific locations may have distinctive names and symptoms (see the table and figure below). Keep in mind that these names—even though they are Latinized—refer to locations on the body, not causative organisms. Tineas can be caused by different dermatophytes in most areas of the body.

Some Common Tineas and Location on the Body

Dermatophytes are commonly found in the environment and in soils and are frequently transferred to the skin via contact with other humans and animals. Fungal spores can also spread on hair. Many dermatophytes grow well in moist, dark environments. For example, tinea pedis (athlete’s foot) commonly spreads in public showers, and the causative fungi grow well in the dark, moist confines of sweaty shoes and socks. Likewise, tinea cruris (jock itch) often spreads in communal living environments and thrives in warm, moist undergarments.

Tineas on the body (tinea corporis) often produce lesions that grow radially and heal towards the center. This causes the formation of a red ring, leading to the misleading name of ringworm (recall the Clinical Focus case in The Eukaryotes of Microbiology).

Several approaches may be used to diagnose tineas. A Wood’s lamp (also called a black lamp) with a wavelength of 365 nm is often used. When directed on a tinea, the ultraviolet light emitted from the Wood’s lamp causes the fungal elements (spores and hyphae) to fluoresce. Direct microscopic evaluation of specimens from skin scrapings, hair, or nails can also be used to detect fungi. Generally, these specimens are prepared in a wet mount using a potassium hydroxide solution (10%–20% aqueous KOH), which dissolves the keratin in hair, nails, and skin cells to allow for visualization of the hyphae and fungal spores. The specimens may be grown on Sabouraud dextrose CC (chloramphenicol/cyclohexamide), a selective agar that supports dermatophyte growth while inhibiting the growth of bacteria and saprophytic fungi (see the figure below). Macroscopic colony morphology is often used to initially identify the genus of the dermatophyte; identification can be further confirmed by visualizing the microscopic morphology using either a slide culture or a sticky tape prep stained with lactophenol cotton blue.

Various antifungal treatments can be effective against tineas. Allylamine ointments that include terbinafine are commonly used; miconazole and clotrimazole are also available for topical treatment, and griseofulvin is used orally.

Check Your Understanding

Cutaneous Aspergillosis

Another cause of cutaneous mycoses is Aspergillus, a genus consisting of molds of many different species, some of which cause a condition called aspergillosis. Primary cutaneous aspergillosis, in which the infection begins in the skin, is rare but does occur. More common is secondary cutaneous aspergillosis, in which the infection begins in the respiratory system and disseminates systemically. Both primary and secondary cutaneous aspergillosis result in distinctive eschars that form at the site or sites of infection (see the figure below). Pulmonary aspergillosis will be discussed more thoroughly in Respiratory Mycoses.

Primary cutaneous aspergillosis usually occurs at the site of an injury and is most often caused by Aspergillus fumigatus or Aspergillus flavus. It is usually reported in patients who have had an injury while working in an agricultural or outdoor environment. However, opportunistic infections can also occur in health-care settings, often at the site of intravenous catheters, venipuncture wounds, or in association with burns, surgical wounds, or occlusive dressing. After candidiasis, aspergillosis is the second most common hospital-acquired fungal infection and often occurs in immunocompromised patients, who are more vulnerable to opportunistic infections.

Cutaneous aspergillosis is diagnosed using patient history, culturing, histopathology using a skin biopsy. Treatment involves the use of antifungal medications such as voriconazole (preferred for invasive aspergillosis), itraconazole, and amphotericin B if itraconazole is not effective. For immunosuppressed individuals or burn patients, medication may be used and surgical or immunotherapy treatments may be needed.

Check Your Understanding

Candidiasis of the Skin and Nails

Candida albicans and other yeasts in the genus Candida can cause skin infections referred to as cutaneous candidiasis. Candida spp. are sometimes responsible for intertrigo, a general term for a rash that occurs in a skin fold, or other localized rashes on the skin. Candida can also infect the nails, causing them to become yellow and harden (see the figure below).

Candidiasis of the skin and nails is diagnosed through clinical observation and through culture, Gram stain, and KOH wet mounts. Susceptibility testing for anti-fungal agents can also be done. Cutaneous candidiasis can be treated with topical or systemic azole antifungal medications. Because candidiasis can become invasive, patients suffering from HIV/AIDS, cancer, or other conditions that compromise the immune system may benefit from preventive treatment. Azoles, such as clotrimazole, econazole, fluconazole, ketoconazole, and miconazole; nystatin; terbinafine; and naftifine may be used for treatment. Long-term treatment with medications such as itraconazole or ketoconazole may be used for chronic infections. Repeat infections often occur, but this risk can be reduced by carefully following treatment recommendations, avoiding excessive moisture, maintaining good health, practicing good hygiene, and having appropriate clothing (including footwear).

Candida also causes infections in other parts of the body besides the skin. These include vaginal yeast infections (see Fungal Infections of the Reproductive System) and oral thrush (see Microbial Diseases of the Mouth and Oral Cavity).

Check Your Understanding

Sporotrichosis

Whereas cutaneous mycoses are superficial, subcutaneous mycoses can spread from the skin to deeper tissues. In temperate regions, the most common subcutaneous mycosis is a condition called sporotrichosis, caused by the fungus Sporothrix schenkii and commonly known as rose gardener’s disease or rose thorn disease (recall Case in Point: Every Rose Has Its Thorn). Sporotrichosis is often contracted after working with soil, plants, or timber, as the fungus can gain entry through a small wound such as a thorn-prick or splinter. Sporotrichosis can generally be avoided by wearing gloves and protective clothing while gardening and promptly cleaning and disinfecting any wounds sustained during outdoor activities.

Sporothrix infections initially present as small ulcers in the skin, but the fungus can spread to the lymphatic system and sometimes beyond. When the infection spreads, nodules appear, become necrotic, and may ulcerate. As more lymph nodes become affected, abscesses and ulceration may develop over a larger area (often on one arm or hand). In severe cases, the infection may spread more widely throughout the body, although this is relatively uncommon.

Sporothrix infection can be diagnosed based upon histologic examination of the affected tissue. Its macroscopic morphology can be observed by culturing the mold on potato dextrose agar, and its microscopic morphology can be observed by staining a slide culture with lactophenol cotton blue. Treatment with itraconazole is generally recommended.

Check Your Understanding

Summary

• Mycoses can be cutaneous, subcutaneous, or systemic.
• Common cutaneous mycoses include tineas caused by dermatophytes of the genera Trichophyton, Epidermophyton, and Microsporum. Tinea corporis is called ringworm. Tineas on other parts of the body have names associated with the affected body part.
• Aspergillosis is a fungal disease caused by molds of the genus Aspergillus. Primary cutaneous aspergillosis enters through a break in the skin, such as the site of an injury or a surgical wound; it is a common hospital-acquired infection. In secondary cutaneous aspergillosis, the fungus enters via the respiratory system and disseminates systemically, manifesting in lesions on the skin.
• The most common subcutaneous mycosis is sporotrichosis (rose gardener’s disease), caused by Sporothrix schenkii.
• Yeasts of the genus Candida can cause opportunistic infections of the skin called candidiasis, producing intertrigo, localized rashes, or yellowing of the nails.

Key terms

• cutaneous mycoses — mycoses that cause superficial infections of the epidermis, hair, and nails.
• subcutaneous mycoses — mycoses that penetrate the epidermis and the dermis to infect deeper tissues.
• systemic mycoses — mycoses that spread throughout the body.
• tineas — any cutaneous fungal infection caused by dermatophytes, such as tinea corporis, tinea capitis, tinea cruris, and tinea pedis.
• dermatophytes — any fungus of the genera Microsporum, Epidermophyton, or Trichophyton, which feed on keratin (a protein found in skin, hair, and nails) and can cause cutaneous infections.
• ringworm — a tinea (cutaneous mycosis of the skin), typically characterized by a round, red, slightly raised lesion that heals outward from the center, giving it the appearance of a round worm.
• tinea pedis — cutaneous mycosis of the feet; also known as athlete’s foot.
• tinea cruris — cutaneous mycosis of the groin region; also known as jock itch.
• tinea corporis — cutaneous mycosis of the body; also known as ringworm of the body.
• intertrigo — a rash that occurs in a skin fold.
• sporotrichosis — subcutaneous infection caused by the fungus Sporothrix schenkii, which causes skin lesions and can potentially spread to the lymphatic system; also known as rose gardener’s disease or rose thorn disease.

Practice

Identify the most common fungal pathogens associated with cutaneous and subcutaneous mycoses

Compare the major characteristics of specific fungal diseases affecting the skin

This section is adapted from Microbiology, Section 21.4: Mycoses of the Skin by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: all four of the module’s non-Disease-Profile figures are re-encoded as WebP and rendered as mediafigures after image and PDF inspection, all kind="photo" (every panel in all four is a photograph or photomicrograph, none a drawn diagram), with the tineas figure carrying eager="true" as the page’s first figure; three source-alt corrections are made with no inline note — the dermatophyte-colony alt’s “blackn” to “black,” the aspergillosis alt’s “think strands” to “thin strands,” and the candidiasis-plate alt’s “fuzzy” reworded to “smooth, lobed” after comparing the image, whose colonies are glossy and lobed rather than cottony — and each figure’s alt lettering is normalized to a lowercase “(a)/(b)/(c)” style matching its caption; two punctuation corrections restore the source’s own convention rather than the raw CNXML’s printed form: the ringworm sentence’s missing parentheses around “recall the Clinical Focus case in [The Eukaryotes of Microbiology]” are added, matching this page’s own later “(recall [Case in Point: Every Rose Has Its Thorn])” construction and the parallel form used elsewhere in the book, in place of the raw CNXML’s unpunctuated “ringworm recall the Clinical Focus case in”; and the stray, unmatched closing parenthesis the source prints after the Pulmonary-aspergillosis cross-reference to “Respiratory Mycoses)” is dropped; the module’s fifth figure, the Disease Profile’s OSC_Microbio_21_04_DMycoses table image, is not vendored and is instead transcribed as a Markdown table inside the Disease Profile callout, from the image, checked cell by cell against the PDF page — the Candidiasis row’s Transmission cell is transcribed as “Overgrowth of normal skin microbiota, especially in moist, dark areas,” which the image and the PDF both print, correcting the source alt attribute’s own mistaken text for that cell (“Opportunistic infections in immunocompromised patients,” copied from the Signs-and-Symptoms phrasing used elsewhere in the same alt); the module’s one table with a summary (the six-row tineas/location table) is transcribed as Markdown and, per this run’s decision for a one-item-per-bin table, is not a sortbins — instead three of its rows (feet, groin, body) become per-row multiplechoice items in Practice offering all six tinea names as options in the table’s own order, the correct option’s position varying row to row on its own, in the shape of a converted Matching set; the cross-reference to Pulmonary aspergillosis is linked to Respiratory Mycoses, a section of this same two-chapter authoring run; the cross-reference to the ringworm Clinical Focus case is linked to this book’s authored Unicellular Eukaryotic Parasites; the cross-reference to the sporotrichosis Case in Point is linked to this book’s authored Physical Defenses, which contains that box; the cross-references to vaginal candidiasis and oral thrush stay plain text naming their (not yet authored) sections, Fungal Infections of the Reproductive System and Microbial Diseases of the Mouth and Oral Cavity; of the module’s four body Check Your Understanding bullets, one — “Identify the sources of infection for primary and secondary cutaneous aspergillosis” — is graded as a two-bin sortbins built from the module’s own primary/secondary compare-and-contrast paragraph, and the remaining three (the ringworm-naming question, the candidiasis signs-and-symptoms question, and the Sporothrix progression question) stay body self-checks, since each needs more than one module sentence to answer honestly; the module’s unkeyed Short Answer question (“What yeasts commonly cause opportunistic infections?”) is graded as a textin keyed Candida albicans (accept="C. albicans|Candida") from the candidiasis figure’s own caption, which names the yeast; the module’s unkeyed Critical Thinking question (steps to reduce toenail fungal infection risk) stays a selfcheck, whose model answer discloses that the section’s only relevant prevention sentence in fact describes repeat cutaneous candidiasis infections rather than toenail infections specifically; the module’s four Multiple Choice and one Fill in the Blank items are rendered exactly as keyed, in source order and source option order (the Fill in the Blank as a textin); key terms are compiled from the module’s eleven <term> elements, giving eleven distinct bullets; three of these — cutaneous, subcutaneous, and systemic mycoses — have no entry in the book’s Glossary appendix and are defined from the module’s own defining sentence rather than a glossary quotation; no source exercise, table, or Check Your Understanding bullet is omitted.