Fungal Skin Infection — Causes, Symptoms & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Fungal skin infections are among the most common skin conditions worldwide, affecting approximately 1 billion people globally. They are caused by three main groups of fungi: dermatophytes (fungi that infect keratinised tissues — skin, hair, and nails — Trichophyton, Microsporum, and Epidermophyton species), Candida (a yeast causing candidiasis in moist skin areas and mucous membranes), and Malassezia (a lipophilic yeast causing pityriasis versicolor and seborrhoeic dermatitis). Dermatophyte infections are collectively called tinea (ringworm), named by body site: tinea pedis (athlete's foot — feet), tinea unguium or onychomycosis (nails), tinea capitis (scalp — predominantly in children), tinea corporis (body), tinea cruris (groin — jock itch), tinea manuum (hands), tinea barbae (beard area). Most superficial fungal infections are not life-threatening but cause significant discomfort, itching, and cosmetic concern. Invasive fungal infections (aspergillosis, candida bloodstream infections) in immunocompromised patients are a separate, serious medical emergency not covered here.

Dermatophytes (Trichophyton rubrum — most common cause of tinea pedis, onychomycosis, and tinea cruris; T. tonsurans — most common cause of tinea capitis in the UK and USA; T. interdigitale — tinea pedis; Microsporum canis — tinea capitis from cats and dogs; Epidermophyton floccosum — tinea cruris and tinea pedis) infect dead keratinised tissue (outer epidermis, hair shafts, nails). Transmission: direct skin-to-skin contact (tinea corporis from wrestling — tinea gladiatorum), indirect contact via contaminated surfaces (shower floors, swimming pools — tinea pedis), infected animals (M. canis), or soil (geophilic dermatophytes). Candida: Candida albicans (normal commensal of gut, mouth, vagina) proliferates when immune defence or local ecology is disturbed — antibiotics, corticosteroids, immunosuppression, diabetes, obesity, moisture. Pityriasis versicolor: Malassezia furfur — hypopigmented or hyperpigmented macules on the trunk and upper arms; triggered by heat, humidity, oily skin, corticosteroid use. Risk factors for dermatophytosis: warm, humid environments (tropical climates), communal facilities (gyms, swimming pools, changing rooms), shared footwear, immunosuppression (HIV, organ transplant, chemotherapy, long-term corticosteroids — increases risk of severe tinea), diabetes (impaired immune defence), obesity (skin fold moist environment), occlusive footwear, nail trauma.

Tinea pedis (athlete's foot — most common dermatophyte infection): interdigital type — maceration, scaling, fissuring, and itching in the fourth interdigital space; moccasin type — diffuse scaling and mild itching affecting the sole and sides of the foot; vesicular type — blistering on the instep; acute ulcerative type — severe erosion and secondary bacterial infection. Tinea unguium (onychomycosis — nail fungus): distal and lateral subungual onychomycosis (most common) — yellow-white-brown discolouration, nail thickening (onychauxis), subungual debris, nail plate fragmentation and separation (onycholysis) — usually painless. Tinea corporis (ringworm): characteristic annular (ring-shaped) lesion — well-defined, raised erythematous border with central clearing and scaling; expands centrifugally; moderate pruritus. Tinea capitis (scalp ringworm — predominantly in children): scaling, broken hair shafts, alopecia; inflammatory form — kerion (boggy, painful, suppurative mass requiring systemic treatment). Tinea cruris (jock itch): pruritic erythematous patch in the groin with well-defined advancing scaly border; spares the scrotum (unlike Candida intertrigo). Cutaneous candidiasis: erythematous, glazed, moist patches in skin folds (inframammary, groin, axilla, interdigital) with satellite papulopustules — characteristic of Candida. Pityriasis versicolor: fine scaly hypopigmented macules on the trunk, upper arms — more visible after sun exposure; occasionally hyperpigmented in dark skin.

Diagnosis is usually clinical based on characteristic appearance and body site. Wood's lamp (ultraviolet light): some Microsporum species fluoresce green (M. canis, M. audouinii) — useful for tinea capitis screening; T. tonsurans (most common in UK/USA) does not fluoresce. Skin scraping with KOH preparation: skin, nail, or hair material dissolved in 10–20% potassium hydroxide (KOH) — reveals fungal hyphae (branching filaments) or spores on microscopy — rapid, inexpensive, 70–80% sensitivity for dermatophytes; confirms diagnosis before systemic treatment. Nail clipping culture: fungal culture on Sabouraud dextrose agar for definitive identification and antifungal sensitivity testing — takes 2–4 weeks; important before prescribing systemic treatment as it rules out non-fungal nail disease (psoriasis, lichen planus — which can mimic onychomycosis). PCR testing: faster and more sensitive than culture — available in specialist centres. Skin biopsy with PAS staining: for clinically atypical or treatment-resistant cases. Differential diagnoses: psoriasis (tinea pedis and onychomycosis), seborrhoeic dermatitis (tinea capitis), eczema (tinea corporis), erythrasma (Corynebacterium — fluoresces coral-red under Wood's lamp — in intertriginous areas; treated with erythromycin not antifungals).

Topical antifungals for most dermatophyte skin infections: terbinafine 1% cream (fungicidal — most effective — OD for 1–2 weeks); clotrimazole 1% cream or miconazole 2% cream (fungistatic — BD for 4 weeks); econazole, ketoconazole shampoo (for pityriasis versicolor and seborrhoeic dermatitis). Tinea pedis: topical terbinafine for 1–2 weeks (interdigital type); adjunctive foot hygiene (dry between toes, moisture-wicking socks, antifungal powder). Tinea cruris and tinea corporis: topical terbinafine or azole cream for 2–4 weeks; avoid combined antifungal-corticosteroid preparations (tinea incognito — steroid suppresses inflammation without curing infection, spreading disease). Tinea capitis: requires systemic antifungal — oral terbinafine 250 mg OD for 4–6 weeks (Trichophyton species); griseofulvin 10–25 mg/kg/day for 6–12 weeks (all species — particularly Microsporum — preferred agent for M. canis); itraconazole oral solution for children. Adjunctive antifungal shampoo (ketoconazole 2% or selenium sulfide) to reduce spore shedding during treatment. Onychomycosis: systemic treatment required (topicals penetrate poorly into nail). Oral terbinafine 250 mg OD for 6 weeks (fingernails) or 12 weeks (toenails) — most effective for Trichophyton. Itraconazole pulse therapy (200 mg BD for 1 week per month — 2 pulses for fingernails, 3 pulses for toenails) — effective and reduces liver exposure. Amorolfine nail lacquer 5% (weekly for 6–12 months) — for mild distal onychomycosis or as adjunct. Efinaconazole 10% or tavaborole 5% nail solutions — newer topicals with improved nail penetration. Nail removal (surgical or urea paste chemical avulsion) may be needed for severely thickened nails before treatment. Confirmation with KOH or culture before starting systemic treatment is recommended. Cutaneous candidiasis: treat underlying factors (dry skin folds, manage diabetes, stop antibiotics if possible); topical clotrimazole, miconazole, or nystatin; severe or recurrent — oral fluconazole 150 mg single dose or weekly.

Secondary bacterial superinfection (macerated and fissured tinea pedis provides a portal of entry for Staphylococcus aureus and beta-haemolytic Streptococcus — causing ascending erysipelas or cellulitis of the lower leg; a clinically significant complication particularly in elderly, diabetic, or immunocompromised patients requiring systemic antibiotics). Tinea incognito (widespread, atypical, and treatment-resistant infection caused by inappropriate topical corticosteroid application to unrecognised tinea — steroid suppresses the inflammatory response without eliminating the fungus, allowing extensive spreading that can be mistaken for eczema or psoriasis). Kerion (severe inflammatory boggy scalp mass from hypersensitivity reaction to dermatophyte in tinea capitis — causes permanent scarring alopecia if not treated urgently with systemic antifungal plus oral corticosteroids). Invasive or disseminated infection in immunocompromised patients (HIV, organ transplant, haematological malignancy — dermatophytes may cause deep tissue infection requiring specialist mycological management).

Keep skin clean and dry — fungi thrive in warm, moist environments. Dry thoroughly between the toes after bathing. Wear moisture-wicking synthetic or natural fibre socks and change them daily. Use antifungal powder (miconazole or tolnaftate) in shoes and between toes if prone to tinea pedis. Wear flip-flops or sandals in communal showers, changing rooms, and around swimming pools. Do not share personal items — towels, socks, shoes, combs, hairbrushes. Wash towels and bedlinen at 60°C or higher to kill dermatophyte spores. Treat household pets with skin disease (tinea corporis from M. canis) — veterinary antifungal treatment prevents re-infection. Tinea capitis control in schools: use antifungal shampoo on all household contacts during treatment of an index case; screen siblings. Manage diabetes meticulously — hyperglycaemia promotes Candida overgrowth. Wear loose, breathable underwear and clothing (cotton) to reduce tinea cruris recurrence. After successful onychomycosis treatment, discard and replace all old footwear (reservoir of re-infection).

See a GP or dermatologist if: an over-the-counter antifungal cream has been used correctly for 4 weeks without improvement (possible alternative diagnosis — psoriasis, eczema — or incorrect antifungal agent); if tinea capitis is suspected in a child (school-age alopecia with scalp scaling — requires systemic antifungal, not topical); if nail changes are present (onychomycosis requires prescription systemic treatment — confirm with KOH or culture first to avoid unnecessary treatment); or if you have diabetes, are immunosuppressed, or are on oral corticosteroids and develop any skin infection (higher risk of spreading, treatment-resistant infection). Seek urgent medical review for: rapidly spreading tinea with swelling, pain, and fever (kerion — inflammatory tinea capitis — or secondary bacterial infection of tinea pedis requiring antibiotics). People with HIV or on biological therapies who develop widespread, treatment-resistant tinea should be assessed by a specialist dermatologist.