Dermatophytosis

Definition and Causative Organisms

• Trichophyton rubrum - the most common cause of tinea pedis, cruris, and corporis
• Trichophyton mentagrophytes
• Epidermophyton floccosum - infects skin and nails but NOT hair
• Trichophyton tonsurans - predominant cause of tinea capitis in children (endothrix)
• Microsporum canis - zoophilic (dogs and cats); causes tinea capitis and corporis

Classification by Habitat

Pathogenesis and Immunity

• Moisture and warmth (shoes, occlusive clothing)
• Hot, humid climates and crowded conditions
• Youth and genetic predisposition
• Immunocompromise (may allow invasive infection in rare cases)

Clinical Forms

1. Tinea Corporis (Ringworm)

• Affects non-hairy (glabrous) skin of trunk and extremities
• Classic lesion: annular, erythematous, scaly plaques with central clearing and advancing border
• Can be pruritic; vesiculation at the active border possible
• Deep inflammatory nodules or granulomas may form when inappropriately treated with mid- to high-potency topical corticosteroids
• Caused most often by T. rubrum and E. floccosum
• Harrison's Principles of Internal Medicine 22E, p. 432

2. Tinea Pedis (Athlete's Foot)

• The most prevalent of all dermatophytoses
• Typically caused by T. rubrum
• Three patterns:
  • Interdigital (most characteristic): erythema, maceration, fissuring between toes (especially 4th-5th web space), intense pruritus
  • Chronic hyperkeratotic (moccasin): plantar erythema and scaling, diffuse involvement, may be asymptomatic
  • Inflammatory/vesicular: painful vesicles on the foot
• More common in men; rare in children
• Often chronic and frequently relapses - Textbook of Family Medicine 9e, p. 939

3. Tinea Cruris (Jock Itch)

• Dermatophyte infection of the groin, more common in males
• Presents as erythematous, scaling eruption of the proximal medial thighs, may extend to buttocks and lower abdomen
• Scrotum is characteristically spared (unlike Candida)
• Associated with tinea pedis (feet are often the source)
• Triggered by heat, humidity, and occlusive clothing - Harrison's, p. 432

4. Tinea Capitis

• The most common dermatophytosis in children
• Predominant organism: T. tonsurans (endothrix - spores inside hair shaft; does NOT fluoresce under Wood's lamp; "black dot" pattern with hair breakage at follicular opening)
• Microsporum species: ectothrix (spore sheath around hair shaft); fluoresces green-silver under Wood's lamp (365 nm)
• Features: irregular or well-demarcated alopecia, scaling, possible cervical/occipital lymphadenopathy
• Kerion: boggy, sterile, inflammatory scalp mass - a cell-mediated hypersensitivity reaction, more common with zoophilic species; does NOT require antibiotics or incision/drainage
• Favus: caused by T. schoenleinii; scutula (crusts) form around follicles

5. Tinea Unguium (Onychomycosis)

• Fungal infection of nails; often coexists with tinea pedis
• Features: opacified, thickened nails with subungual debris
• Distal-lateral subungual variant is most common
• Proximal subungual onychomycosis may indicate HIV or other immunocompromised states
• Caused mostly by T. rubrum; E. floccosum does not infect hair

6. Tinea Barbae

• Affects the bearded region
• Especially when a zoophilic dermatophyte is involved, can produce a highly inflammatory reaction resembling pyogenic infection

Diagnosis

Treatment

Topical Agents (first-line for uncomplicated tinea corporis, cruris, limited tinea pedis)

• Allylamines (terbinafine, naftifine, butenafine): fungicidal; superior efficacy, shorter treatment duration vs. azoles
  • Terbinafine 1% cream once daily x 7 days is highly effective for tinea corporis/cruris
• Azoles (clotrimazole, miconazole, econazole, ketoconazole, oxiconazole): fungistatic; effective but require longer courses
• Apply twice daily; continue for 1 week beyond clinical resolution; tinea pedis needs longer courses
• Nystatin is NOT active against dermatophytes

Systemic Agents (required for: tinea capitis, onychomycosis, recalcitrant infections)

• Nail infections are hardest to treat; often require months of oral terbinafine or itraconazole; surgical nail removal sometimes needed; relapses common
• Luliconazole: newer topical imidazole formulated to penetrate nail plate
• Markedly inflammatory tinea capitis: systemic antifungal + short course systemic or topical glucocorticoids to prevent scarring/hair loss
• All oral agents carry hepatotoxicity risk; avoid in pregnancy/breastfeeding - Harrison's 22E, p. 433

Key Treatment Principles (Evidence Summary)

• Allylamines cure slightly more infections than azoles but are more expensive (SOR: A) - Textbook of Family Medicine 9e
• Terbinafine is more effective than griseofulvin for onychomycosis (Cochrane review, SOR: A)
• Topical antifungals (excluding ciclopirox) show no benefit over placebo for nail infections (SOR: A)
• Fungal etiology must be confirmed before prescribing oral antifungals

Special Situations

• Invasive/disseminated dermatophytosis: very rare; seen in severely immunocompromised patients
• Tinea incognito: dermatophytosis modified and masked by inappropriate corticosteroid use; may present as deep granulomas
• Differential diagnosis of tinea cruris: inverse psoriasis (does not respond to antifungals; check for nail pitting), erythrasma (coral-pink fluorescence under Wood's lamp; diffuse scale without central clearing), candidiasis (involves scrotum; satellite pustules)

Key Concepts Summary

1. Dermatophytes infect only non-viable keratin; growth is inhibited by serum and body temperature, so they rarely become invasive
2. Geophilic and zoophilic species cause acute, inflammatory, treatment-responsive infections; anthropophilic species cause mild, chronic, recurrent infections
3. KOH preparation is the most important bedside diagnostic test
4. Topical allylamines are preferred for skin infections; systemic therapy is mandatory for hair and nail infections
5. Tinea capitis and kerion are clinical emergencies in children that require oral treatment (not topical)
6. Proximal subungual onychomycosis is a marker for immunocompromise including HIV

Dermatophytosis

Definition

Causative Organisms and Classification

• Trichophyton spp. - infect hair, skin, and nails
• Microsporum spp. - infect only hair and skin (not nails)
• Epidermophyton floccosum - infects skin and nails only (NOT hair)

Morphology and Identification in Culture

• Trichophyton: cylindric, smooth-walled macroconidia and characteristic microconidia
  • T. rubrum: white cottony colony; deep red pigment on reverse; small piriform microconidia
  • T. mentagrophytes: cottony to granular; grape-like clusters of spherical microconidia; coiled/spiral hyphae
  • T. tonsurans: flat, powdery to velvety; elongate microconidia
• Microsporum: multicellular macroconidia with echinulate (rough) walls; both types borne singly
  • M. canis: white cottony, deep yellow reverse; 8-15 celled macroconidia with curved/hooked tips
  • M. gypseum: tan, powdery; abundant thin-walled, 4-6 celled macroconidia
• Epidermophyton floccosum: smooth-walled, clavate (club-shaped), 2-4 celled macroconidia in small clusters; flat, velvety, tan to olive-green; produces only macroconidia (no microconidia)

Pathogenesis and Immunity

• Infection begins after trauma and contact with contaminated material (soil, animal, human, fomite)
• Risk factors: moisture, warmth, occlusive shoes/clothing, hot humid climate, crowding, youth, genetic predisposition
• Dermatophytes produce keratinases, elastases, and other enzymes enabling tissue invasion
• Transmission via shed skin scales, hairs, or nails on fomites (can remain viable for long periods)
• Anthropophilic species spread by direct contact or contaminated towels, clothing, shower stalls

• Patients with chronic, non-inflammatory infections often have poor cell-mediated immunity to dermatophyte antigens
• These patients tend to be atopic with elevated IgE (immediate-type hypersensitivity pattern)
• Normal host immunity varies by site, host, and fungal species

Clinical Forms

Clinical Features Table (Jawetz, p. 689)

Tinea Corporis (Ringworm)

• Superficial infection of glabrous (non-hairy) skin of trunk and extremities
• Classic: annular, erythematous, scaly plaque with central clearing and advancing scaling border
• Pruritic; vesiculation at active border possible
• Tinea incognito: deep inflammatory nodules or granulomas when inappropriately treated with mid-to-high potency topical corticosteroids - Harrison's 22E, p. 432

Tinea Pedis (Athlete's Foot)

• The most prevalent of all dermatophytoses
• Most commonly caused by T. rubrum
• Three patterns:
  1. Interdigital (most common): erythema, maceration, fissuring between toes (4th-5th web space most often); intense pruritus
  2. Moccasin (chronic hyperkeratotic): diffuse plantar erythema and hyperkeratosis; may be asymptomatic
  3. Vesicular/inflammatory: painful vesicles on the foot
• More common in men; rare in children
• Occlusive footwear creates a warm, humid, macerating environment promoting infection
• Often chronic; frequently relapses - Textbook of Family Medicine 9e, p. 939

Tinea Cruris (Jock Itch)

• Groin infection; more common in males
• Erythematous, scaling eruption of proximal medial thighs, may extend to buttocks and lower abdomen
• Scrotum characteristically spared (key distinguishing feature from Candida)
• Frequently co-exists with tinea pedis - feet are often the source
• Triggered by heat, humidity, and occlusive clothing
• Burning and pruritus are the main complaints - Family Medicine, p. 940

Tinea Capitis

• Most common dermatophytosis in children; also affects immunocompromised adults
• Transmission via contaminated hats, brushes, pillowcases; shed hairs remain viable >1 year
• Features: alopecia, scaling; cervical and occipital lymphadenopathy may be prominent

• "Black dot" tinea capitis: hairs weaken and break at follicular opening due to endothrix invasion
• Kerion: boggy, sterile, inflammatory scalp mass; cell-mediated hypersensitivity reaction; more common with zoophilic species; does NOT require antibiotics or incision and drainage
• Favus: caused by T. schoenleinii; scutula (crusts) form around hair follicles; hyphae do not form spores within the shaft
• In prepubescent children, epidemic tinea capitis is often self-limiting - Jawetz, p. 690

Tinea Unguium (Onychomycosis)

• Opacified, thickened nails with subungual debris
• Distal-lateral subungual variant: most common
• Proximal subungual variant: marker for HIV or other immunocompromised state
• Most common cause: T. rubrum
• E. floccosum does NOT infect hair (but infects nails)
• Hardest of all dermatophytoses to treat; relapses common - Harrison's, p. 432

Tinea Barbae

• Affects bearded area; when caused by zoophilic dermatophytes, may produce a highly inflammatory reaction closely resembling pyogenic (bacterial) infection

Trichophytid (Dermatophytid) Reaction

• Distant hypersensitivity reaction to fungal antigens or products
• Manifests as vesicles, most often on the hands
• Trichophythin skin test is markedly positive
• Separate from the primary infection site - Jawetz, p. 690

Diagnosis

Treatment

Principles

• Topical therapy: sufficient for uncomplicated tinea corporis, cruris, and limited tinea pedis
• Systemic therapy: mandatory for tinea capitis, onychomycosis, and infections recalcitrant to topical agents
• Nystatin is NOT active against dermatophytes
• Apply topicals twice daily; continue for 1 week beyond clinical resolution; tinea pedis requires longer courses
• All oral antifungal agents carry hepatotoxicity risk; avoid in pregnancy and breastfeeding

Topical Agents

• Terbinafine 1% cream once daily x 7 days: highly effective for tinea corporis/cruris
• Allylamines cure slightly more infections than azoles but cost more (SOR: A)
• Luliconazole: newer topical imidazole formulated to penetrate the nail plate for onychomycosis

Systemic Agents

Site-Specific Treatment Summary

Key Summary Points

1. Dermatophytes infect only non-viable keratin; growth is inhibited by serum and body temperature - they rarely invade deeper tissues
2. Geophilic and zoophilic species produce acute, inflammatory, treatment-responsive infections; anthropophilic species cause mild, chronic, recurrent infections
3. KOH preparation is the most important bedside test
4. Wood's lamp fluorescence only occurs with ectothrix Microsporum infections - not with T. tonsurans
5. Tinea capitis and onychomycosis always require systemic therapy; topicals alone fail
6. Proximal subungual onychomycosis is a clinical marker for HIV or immunocompromise
7. Kerion is a sterile inflammatory mass - do NOT use antibiotics or drain; treat with antifungals ± oral steroids
8. Tinea incognito occurs when dermatophytosis is masked and worsened by inappropriate topical steroid use