Fungal Skin Infection — Symptoms, Causes & Treatment | MyMedicPlus

Fungal skin infections (dermatomycoses) are among the most prevalent infectious diseases in the world, affecting an estimated 20–25% of the global population at any given time — making superficial fungal infections the most common infectious skin diseases globally. They are caused by three main groups of fungi: dermatophytes (Trichophyton, Microsporum, and Epidermophyton species — causing tinea infections that invade keratinised tissue such as skin, hair, and nails), Candida species (yeast that cause candidiasis in moist mucosal and intertriginous areas), and Malassezia furfur (a lipophilic commensal yeast causing pityriasis versicolor). Common clinical presentations include tinea pedis (athlete's foot — most prevalent, affecting 15% of the population), tinea corporis (ringworm), tinea cruris (jock itch), tinea capitis (scalp ringworm — predominantly in children), onychomycosis (fungal nail infection — affects 10% of the general population and up to 50% of those over 70), oral candidiasis (thrush), and vulvovaginal candidiasis (affecting 75% of women at least once in their lifetime). While rarely life-threatening in immunocompetent individuals, superficial fungal infections cause significant physical discomfort, social embarrassment, and, if untreated, can serve as portals of entry for bacterial superinfection. In immunocompromised patients (HIV, organ transplant recipients, haematological malignancies), superficial infections can herald invasive life-threatening disseminated fungal disease.

Dermatophyte fungi — Trichophyton (the most common genus, causing tinea pedis, tinea corporis, tinea unguium, tinea capitis, and tinea cruris), Microsporum (predominantly tinea capitis in children, causing ectothrix hair shaft invasion), and Epidermophyton floccosum (tinea pedis, tinea cruris, onychomycosis) — are highly adapted keratinophilic organisms that secrete keratinases digesting the keratin of the stratum corneum, hair shaft, and nail plate. Transmission is anthropophilic (person-to-person — most dermatophyte species), zoophilic (from animals — Microsporum canis from cats and dogs; Trichophyton verrucosum from cattle), or geophilic (from soil). Candida albicans and related species (C. glabrata, C. tropicalis, C. parapsilosis) are dimorphic yeasts — commensal organisms of the gastrointestinal tract and mucous membranes that cause superficial infection when ecological balance is disrupted; overgrowth is facilitated by: broad-spectrum antibiotic use (eliminating competing bacterial flora), corticosteroid therapy (impairing cell-mediated immunity and skin defence), diabetes mellitus (elevated glucose in sweat and impaired neutrophil function), immunosuppression (HIV — CD4 below 200 cells/μL; transplant recipients; haematological malignancies; high-dose corticosteroids), and pregnancy (altered vaginal pH and glycogen availability). Malassezia furfur and related Malassezia species (M. globosa, M. restricta) are obligate lipophilic yeasts colonising the sebaceous-rich skin of virtually all adults (scalp, chest, upper back, face) — pityriasis versicolor develops when Malassezia transitions to its pathogenic filamentous form in susceptible individuals (hot humid climate, increased skin surface lipids, immunosuppression, genetic predisposition, topical oil application). Risk factors include: warm, humid environments; communal bathing facilities (swimming pools, gyms, military barracks); hyperhidrosis; tight, non-breathable synthetic clothing; prolonged antibiotic courses; diabetes; immunosuppression; and barefoot walking on contaminated surfaces.

Tinea pedis (athlete's foot) has three clinical patterns: interdigital (most common) — maceration, fissuring, and scaling between the toes (particularly 4th–5th web space) with intense pruritus and unpleasant odour; hyperkeratotic (moccasin) — diffuse, dry scaling and hyperkeratosis of the plantar sole and sides of the foot in a moccasin pattern, often bilateral, associated with Trichophyton rubrum; vesiculobullous — acute blistering reaction with intensely pruritic vesicles and bullae, often on the instep, associated with T. mentagrophytes. Tinea corporis (ringworm) presents as one or more circular or polycyclic, erythematous, scaly plaques with central clearing (central healing with an active, slightly raised, scaly, pruritic advancing border) — enlarging centrifugally over days to weeks. Tinea cruris (jock itch) — erythematous, scaly rash in the groin crease with a well-demarcated, serpiginous border extending toward the inner thigh; spares the scrotum (unlike candidal intertrigo, which typically involves the scrotum and scrotal folds). Tinea capitis — patchy areas of scalp hair loss with associated scaling, erythema, and broken-off hair stubs ('black dots' — endothrix infection with intrafollicular hair shaft invasion); kerion formation (boggy, tender, inflamed, pustular mass with hair loss and sinus formation — an aggressive inflammatory response, risk of permanent scarring alopecia if not treated urgently with systemic antifungals). Onychomycosis (tinea unguium): subungual hyperkeratosis (thickened, friable white-yellow-brown subungual debris), onycholysis (separation of nail plate from nail bed), nail plate thickening and dystrophy, and discolouration — distal and lateral subungual onychomycosis (DLSO) is most common. Candidal intertrigo: bright red, moist, sharply marginated rash in skin folds (submammary, inguinal, intergluteal, abdominal pannus) with satellite papules and pustules. Pityriasis versicolor: hypopigmented or hyperpigmented, finely scaly macules and patches on the upper trunk, shoulders, and neck — hypopigmentation results from azelaic acid produced by Malassezia inhibiting melanin synthesis.

Most superficial fungal infections are diagnosed clinically based on characteristic morphology and anatomical distribution — experienced clinicians achieve high accuracy without laboratory confirmation for typical presentations. However, laboratory confirmation is recommended before initiating oral systemic antifungal therapy and is essential for onychomycosis (to distinguish from psoriatic nail, yellow nail syndrome, or trauma-related nail changes) and treatment-resistant cases. Wood's lamp examination (365 nm UV light): Microsporum canis and M. audouinii fluoresce bright green-yellow (ectothrix tinea capitis); Trichophyton schoenleinii fluoresces dull green; pityriasis versicolor fluoresces yellow-gold or copper-orange under Wood's lamp; Corynebacterium minutissimum erythrasma fluoresces coral-red (distinguishing it from tinea cruris). Direct microscopy with 20% potassium hydroxide (KOH) preparation: skin scrapings or nail clippings are dissolved in KOH on a glass slide — KOH digests keratinocytes, leaving fungal hyphae (branching, septate — dermatophytes), yeast cells with pseudohyphae (Candida), or spores and short hyphae ('spaghetti and meatballs' pattern — Malassezia) visible under light microscopy; sensitivity 50–85%, performed within 20 minutes, provides rapid presumptive diagnosis in clinic. Fungal culture (Sabouraud dextrose agar, dermatophyte test medium): taken from skin scrapings, nail clippings, plucked hairs (tinea capitis — pluck rather than scrape), or swabs; grows in 2–4 weeks; allows species identification (important for tinea capitis where Microsporum versus Trichophyton species dictates treatment choice — griseofulvin superior for Microsporum; terbinafine for Trichophyton); sensitivity 65–90%. Nail biopsy with PAS staining: periodic acid-Schiff (PAS) staining of nail clipping cross-sections — stains fungal cell walls magenta; excellent sensitivity (80–90%) when culture is negative. PCR-based fungal DNA detection: rapid, highly sensitive and specific; used in specialist centres for difficult cases.

Topical antifungal agents are first-line for most uncomplicated superficial dermatophyte and Candida skin infections: allylamine class — terbinafine 1% cream or gel (the most effective topical agent for dermatophytes, particularly tinea pedis and corporis; fungicidal — kills rather than merely inhibits fungal growth; applied once or twice daily for 1–2 weeks; 70–80% cure rate); imidazole class — clotrimazole 1%, miconazole 2%, econazole 1%, ketoconazole 2% (fungistatic against dermatophytes; require 2–4 weeks twice-daily application; broadly effective against dermatophytes and Candida, making them first choice for mixed infections and vulvovaginal candidiasis). Fluconazole 1% and 2% sprays are convenient for tinea pedis. Nystatin (polyene antibiotic): effective against Candida but not dermatophytes — appropriate for cutaneous candidiasis in neonates and skin folds. Selenium sulfide 2.5% shampoo applied to affected areas for 15–30 minutes daily for 2 weeks treats pityriasis versicolor. Systemic oral antifungal therapy is required for: onychomycosis (oral terbinafine 250 mg/day for 6 weeks for fingernails, 12–16 weeks for toenails — mycological cure rate 70–80%; or itraconazole 200 mg twice daily for 7 days/month — pulse therapy — for 2–3 months); tinea capitis (always requires oral treatment — griseofulvin 10–20 mg/kg/day for 6–12 weeks for Microsporum species; terbinafine 125–250 mg/day for 4 weeks for Trichophyton species; adjuvant antifungal shampoo reduces environmental shedding and household spread); extensive or recalcitrant tinea corporis or pedis; immunocompromised patients with any superficial fungal infection (risk of invasive disease — fluconazole or itraconazole depending on species); recurrent vulvovaginal candidiasis (4+ episodes per year — maintenance fluconazole 150 mg weekly for 6 months — 90% effective in preventing recurrence; partner treatment if recurrently reinfected). Ciclopirox 8% nail lacquer: an alternative topical treatment for superficial and distal onychomycosis (applied daily for 48 weeks) — inferior to oral terbinafine but avoids systemic drug exposure.

Untreated or recurrent tinea pedis increases risk of bacterial cellulitis of the lower leg by providing a portal of entry through cracked skin. Onychomycosis may cause chronic pain, difficulty walking in footwear, and secondary bacterial nail infection. Invasive fungal infections (aspergillosis, candidemia) are life-threatening in immunocompromised patients — immunocompromised individuals with any fungal skin infection should be evaluated for systemic spread. Tinea capitis in children can cause kerion (inflammatory scalp abscess) leading to scarring alopecia if not treated promptly with systemic antifungals.

Keep skin clean and dry, especially in skin folds and between toes. Wear moisture-wicking synthetic or cotton socks and change them daily. Use antifungal foot powder in shoes if prone to tinea pedis. Wear shower sandals in communal changing rooms, pool areas, and gyms. Avoid sharing towels, clothing, combs, or nail scissors. Change out of sweaty clothing promptly after exercise. Treat athlete's foot promptly before it spreads to nails. Diabetic patients should inspect feet daily for early signs of fungal infection given impaired healing and infection risk.

See your GP for: a skin rash that has not improved after 2 weeks of over-the-counter antifungal cream — to confirm the diagnosis (psoriasis, eczema, and bacterial infections can mimic fungal infections) and prescribe stronger treatment; suspected onychomycosis (thickened, discoloured nails) — requires oral antifungal therapy (nail clippings for laboratory confirmation before systemic treatment); tinea in a child's scalp (tinea capitis) — always requires oral antifungal (griseofulvin 15-20 mg/kg/day or terbinafine) as topical agents do not penetrate the hair follicle; recurrent vaginal candidiasis (4 or more episodes/year) — assess for diabetes (blood glucose), predisposing factors, and consider maintenance fluconazole 150 mg weekly for 6 months (BASHH guidelines). Seek urgent medical care for: signs of bacterial cellulitis complicating a fungal infection — increasing redness spreading beyond the original rash, warmth, swelling, fever, and pain — requires systemic antibiotics; fever and systemic illness in an immunocompromised person with skin fungal infection — risk of invasive systemic fungal infection requiring urgent investigation (blood cultures, CT, systemic antifungal therapy).