Cutaneous & Subcutaneous Mycoses
From keratin to lymphatics: name the fungus by site and scraping
Dermatophytes: biology and diagnosis
Keratin specialists. Trichophyton, Microsporum, and Epidermophyton are molds that digest keratin, so they live in the stratum corneum, hair, and nails without invading living tissue in healthy hosts. Trichophyton infects all three sites, Microsporum skin and hair, and Epidermophyton skin and nails. They are caught from other people, from animals (Microsporum canis from cats and dogs), or from soil, and each infection is named "tinea" plus the site.
Diagnosis. Scrape the active, scaly border onto a slide with KOH, which dissolves keratin and exposes branching septate hyphae. Some Microsporum species fluoresce blue-green under a Wood lamp; Trichophyton tonsurans, the main cause of US tinea capitis, does not. Culture takes weeks.
Topical steroids blunt the scale and redness ("tinea incognito"); scrape and KOH any scaly ring before treating it.
The tineas by site
| Tinea | Clinical clue | Treatment |
|---|---|---|
| Capitis | Children; scaly alopecia, "black dot" broken hairs, occipital nodes, kerion | Oral griseofulvin or terbinafine, plus antifungal shampoo |
| Corporis | Annular plaque, scaly advancing border, central clearing; pets, wrestlers | Topical azole or terbinafine for 2–4 weeks |
| Cruris | Groin and inner thighs, spares the scrotum; often with tinea pedis | Topical azole or terbinafine |
| Pedis | Most common; macerated web spaces, moccasin scaling, instep vesicles | Topical azole or terbinafine |
| Unguium | Thick, yellow-white, crumbly nails with subungual debris | Oral terbinafine (12 weeks for toenails) or itraconazole |
Tinea capitis. Mainly school-age children, from T. tonsurans (person to person) or M. canis (pets). A kerion — a boggy, tender, pustular mass — is an intense inflammatory reaction that can leave scarring alopecia; it needs oral antifungals, not incision. Topicals cannot reach the hair follicle, so therapy is oral griseofulvin or terbinafine, with selenium sulfide or ketoconazole shampoo to limit spore shedding.
Griseofulvin. Binds tubulin to disrupt the fungal mitotic spindle and deposits in newly formed keratin. It is absorbed best with a fatty meal, induces cytochrome P450 (weakening warfarin and oral contraceptives), can cause a disulfiram-like reaction, and is teratogenic.
Tinea corporis, cruris, and pedis. Corporis spreads from pets, wrestling mats (tinea gladiatorum), or the patient's own feet; extensive disease needs oral therapy. Cruris affects sweaty or obese men and characteristically spares the scrotum. Tinea pedis, usually from T. rubrum (the most common dermatophyte), is spread in locker rooms and occlusive shoes; its fissured web spaces between the 4th and 5th toes are a portal for leg cellulitis. An "id" reaction — itchy, sterile vesicles on the hands — is a hypersensitivity response to active tinea pedis, not a new infection.
Onychomycosis. Toenails are affected more than fingernails. Because psoriasis and trauma look similar, confirm with KOH, a PAS-stained nail clipping, or culture before months of therapy. Terbinafine inhibits squalene epoxidase, depleting ergosterol while toxic squalene builds up, and concentrates in keratin; check liver enzymes, as it can be hepatotoxic.
Malassezia: pityriasis versicolor and seborrheic dermatitis
Pityriasis versicolor. Malassezia is a lipophilic yeast of normal skin flora that overgrows with heat, humidity, sweating, and oily skin. It causes hypopigmented, hyperpigmented, or pink macules with fine scale that coalesce on the upper trunk and shoulders, often noticed when surrounding skin tans. Malassezia produces azelaic acid, which inhibits melanocyte tyrosinase, so pale spots contain melanocytes that make less pigment. KOH shows "spaghetti and meatballs" — short hyphae with round yeast.
Treatment. Topical selenium sulfide, ketoconazole shampoo, or another topical azole is first-line; oral fluconazole or itraconazole is used for extensive or relapsing disease. Griseofulvin and oral terbinafine do not work. Pigment evens out only over months, and recurrence is common.
Seborrheic dermatitis. Malassezia also drives greasy yellow scaling on the scalp, eyebrows, and nasolabial folds (cradle cap in infants), which is often severe in HIV and Parkinson disease; ketoconazole or selenium sulfide shampoo and low-potency topical steroids control it.
Sporotrichosis
Organism and exposure. Sporothrix schenckii is dimorphic: a mold on plants, in soil, and in culture at 25 °C, and a cigar-shaped budding yeast in tissue. Thorns (classically roses), sphagnum moss, or hay implant it, so gardeners, florists, and farmers are at risk.
Lymphocutaneous disease. A papule at the puncture site ulcerates, and new nodules then appear one after another along the lymphatic channel draining the hand and forearm (nodular lymphangitis). Pulmonary or disseminated disease is rare and seen with alcohol use disorder or HIV.
Treatment. Oral itraconazole for 3–6 months is first-line; saturated solution of potassium iodide is a cheap alternative ("plant a rose in the pot"), and amphotericin B is reserved for disseminated disease.
Sporotrichoid spread has a short differential: Sporothrix, Mycobacterium marinum (fish tanks), Nocardia brasiliensis, and Leishmania.
Look-alikes and deeper implants
| Condition | Organism | Clue | Microscopy | Treatment |
|---|---|---|---|---|
| Candida intertrigo | Candida albicans | Beefy red, macerated, involves scrotum, satellite pustules | Pseudohyphae, budding yeast | Topical azole or nystatin |
| Tinea nigra | Hortaea werneckii | Painless brown-black palm or sole macule mimicking melanoma | Pigmented septate hyphae | Topical azole or keratolytic |
| Chromoblastomycosis | Fonsecaea and other dark molds | Slowly enlarging verrucous leg plaques after splinters | Copper-penny sclerotic (Medlar) bodies | Itraconazole plus excision or cryotherapy |
| Eumycetoma (Madura foot) | Madurella and other molds | Swollen foot, sinuses draining grains, bone invasion | Fungal grains | Itraconazole plus surgery |
Other groin and web-space mimics. Erythrasma (Corynebacterium minutissimum) produces brown patches that glow coral-red under a Wood lamp. Tinea nigra comes from tropical coastal soil and is purely superficial, but its acral site raises fear of acral lentiginous melanoma. Actinomycetoma looks like eumycetoma but is bacterial (Nocardia, Actinomadura) and needs antibiotics, so the grains must be identified.