21.4 Mycoses of the Skin

Fungal Skin Infections

Fungal skin infections (cutaneous mycoses) are among the most common infections worldwide, caused by organisms that have evolved to feed on keratin, the structural protein in your skin, hair, and nails. Understanding the pathogens, how they spread, and how they're diagnosed is essential for clinical management.

Three main groups of fungi cause these infections: dermatophytes, Candida, and Malassezia. Each has a different ecological niche and causes distinct clinical presentations.

Common Fungal Skin Pathogens

Dermatophytes are filamentous fungi that specifically infect keratinized tissues (skin, hair, nails). They're the most clinically significant group for cutaneous mycoses.

• Trichophyton spp. are the most common dermatophytes overall
  • T. rubrum frequently causes athlete's foot (tinea pedis), jock itch (tinea cruris), and nail infections (onychomycosis)
  • T. mentagrophytes often causes ringworm (tinea corporis) and athlete's foot
• Microsporum spp. primarily affect the skin and scalp
  • M. canis is a zoophilic dermatophyte, meaning it's transmitted from animals (especially cats and dogs) to humans
• Epidermophyton spp. mainly infect the skin and nails
  • E. floccosum causes athlete's foot and jock itch but does not infect hair

Candida spp. are yeasts (not filamentous fungi) that cause superficial infections, particularly in warm, moist skin folds (axillae, groin, under breasts).

• C. albicans is the most common species causing cutaneous candidiasis
• C. parapsilosis often infects the skin and nails

Malassezia spp. are lipophilic yeasts (they require lipids for growth) that live as normal flora on most people's skin but can become pathogenic under certain conditions.

• M. globosa and M. restricta are the main species associated with dandruff, seborrheic dermatitis, and tinea versicolor (pityriasis versicolor), which causes patchy discoloration of the skin

Fungal Structures and Growth

Dermatophytes grow as hyphae, which are thread-like filaments that branch and interweave to form a network called a mycelium. This is what you'd see if you cultured a dermatophyte on an agar plate.

Fungi reproduce and spread through spores, which are highly resistant structures that can survive on surfaces and in the environment for extended periods. This durability is a big reason why fungal infections spread so easily in shared spaces like locker rooms.

To obtain nutrients, dermatophytes secrete keratinases (keratinolytic enzymes) that break down keratin. This enzymatic degradation of host tissue is what produces the characteristic scaling, crumbling, and tissue damage you see in these infections.

Symptoms and Transmission of Tinea

"Tinea" is the clinical term for dermatophyte infections, followed by a Latin word indicating the body site. The same organism can cause different tinea types depending on where it infects.

• Tinea pedis (athlete's foot) affects the feet, especially the interdigital spaces (between the toes)
  • Symptoms: itching, burning, cracking, and scaling of the skin
  • Transmission: direct contact with infected skin or contaminated surfaces like locker room floors and shared showers
• Tinea cruris (jock itch) affects the groin, inner thighs, and buttocks
  • Presents as a red, itchy, scaly rash with a characteristic raised border
  • Transmission: warm, moist environments; close skin-to-skin contact; shared towels or clothing
• Tinea corporis (ringworm) can occur on any part of the body
  • Appears as circular, red, scaly patches with a clear center and raised edges (the "ring" shape that gives it its common name; no actual worm is involved)
  • Transmission: contact with infected people, animals (cats, dogs), or contaminated fomites (combs, brushes)
• Tinea capitis (scalp ringworm) affects the scalp and hair follicles
  • Causes scaly patches, hair loss (alopecia), and itching on the scalp; most common in children
  • Transmission: sharing combs, brushes, or hats with infected individuals
• Tinea unguium (onychomycosis) is a fungal infection of the nails
  • Leads to thickening, yellowing/discoloration, and separation of the nail from the nail bed (onycholysis)
  • Transmission: warm, moist environments; contact with infected nail debris. This form is notoriously difficult to treat because the nail plate shields the fungus from topical medications

Diagnosis and Treatment of Mycoses

Diagnosis relies on a combination of clinical examination and lab confirmation:

1. Physical examination of the affected area to identify characteristic signs (ring-shaped lesions, scaling patterns, nail changes)
2. KOH preparation: skin scrapings or nail clippings are treated with potassium hydroxide (KOH), which dissolves keratin and other cellular material, leaving fungal hyphae and spores visible under the microscope
3. Fungal culture on selective media (such as Sabouraud dextrose agar) to identify the specific pathogen. Cultures can take 1-4 weeks because fungi grow slowly

Treatment depends on the severity, location, and extent of infection:

• Topical antifungals for localized, mild-to-moderate infections:
  • Azoles (clotrimazole, miconazole) inhibit ergosterol synthesis by blocking the enzyme lanosterol 14-alpha-demethylase. Ergosterol is a critical component of the fungal cell membrane (analogous to cholesterol in human cell membranes), so blocking it destabilizes the membrane.
  • Allylamines (terbinafine) also inhibit ergosterol synthesis, but at an earlier step (squalene epoxidase). They do not disrupt cell wall synthesis.
  • Ciclopirox has broad-spectrum activity against dermatophytes, yeasts, and molds through a different mechanism involving chelation of metal ions needed for fungal enzymes
• Oral antifungals for severe, widespread, or resistant infections (and almost always required for tinea capitis and onychomycosis):
  • Terbinafine is highly effective against dermatophytes and is often the first-line oral agent
  • Itraconazole and fluconazole are azoles with broader activity that includes yeasts, making them useful when Candida is involved
• Adjunctive measures to prevent recurrence:
  • Keep affected areas clean and dry to reduce the moist conditions fungi need
  • Avoid tight-fitting clothing or occlusive shoes to minimize friction and improve ventilation
  • Use antifungal powders or sprays in shoes to prevent reinfection with tinea pedis
  • Avoid sharing personal items (towels, combs, nail clippers)

Antifungal resistance can develop, particularly in chronic or recurrent infections. This is an increasing clinical concern and may require susceptibility testing and alternative treatment strategies.