Fungal & Parasitic Skin Infections
A boards-focused walkthrough of superficial fungal (dermatophyte, Malassezia, Candida) and parasitic (scabies, cutaneous larva migrans, lice) skin infections, pairing classic vignette buzzwords and KOH/scraping findings with next-best-step management for STEP 1 and STEP 2 CK.
Overview & Pathophysiology
Superficial skin infections split into fungal (dermatophytes, yeasts) and parasitic (mites, hookworm larvae, lice) causes. Dermatophytes (Trichophyton, Microsporum, Epidermophyton) secrete keratinase to digest keratin, confining them to the dead stratum corneum, hair, and nails — they cannot invade living tissue, so symptoms arise from the host inflammatory response. Malassezia, a lipophilic yeast of normal flora, overgrows in sebum-rich skin; its dicarboxylic acids (e.g., azelaic acid) inhibit tyrosinase, contributing to the pigment change of tinea versicolor (patches may be hypo- OR hyperpigmented — hence versicolor). Candida is an opportunist favoring warm, moist, macerated folds. On the parasitic side, the mite *Sarcoptes scabiei* burrows through the stratum corneum and triggers a delayed (type IV) hypersensitivity reaction — explaining why itching lags 3–6 weeks behind a first infestation but recurs within days on re-exposure. Boards test one pattern: buzzword morphology → KOH/scraping finding → next best step.
- Tinea corporis ("ringworm"): annular scaly plaque, central clearing, raised advancing active border
- Tinea capitis: scaly patches + alopecia, may form a boggy kerion; requires ORAL therapy — topical fails. Trichophyton tonsurans = most common in US (endothrix, non-fluorescent); Microsporum canis (cats/dogs) is ectothrix and fluoresces
- Tinea cruris ("jock itch"): groin, spares the scrotum (vs Candida, which involves it)
- Tinea pedis ("athlete's foot"): interdigital maceration or moccasin-distribution scale
- Onychomycosis (tinea unguium): thick, yellow, crumbling nails; confirm with KOH / PAS / culture before committing to oral terbinafine
- KOH prep → septate, branching hyphae
- Wood lamp: Microsporum (ectothrix) = blue-green; Trichophyton tonsurans (endothrix) does NOT fluoresce
- Rx: topical azole/terbinafine for skin; ORAL required for scalp (griseofulvin or terbinafine) and nails (terbinafine first-line)
Superficial Fungal Infections at a Glance
| Infection | Organism | KOH / buzzword | First-line Rx |
|---|---|---|---|
| Dermatophytosis (tinea) | Trichophyton, Microsporum, Epidermophyton | Septate branching hyphae; annular "ringworm" | Topical azole/terbinafine; oral for hair & nails |
| Tinea versicolor | Malassezia furfur / globosa | "Spaghetti & meatballs" (short hyphae + round yeast) | Topical selenium sulfide or ketoconazole |
| Cutaneous candidiasis | Candida albicans | Pseudohyphae + budding yeast; satellite pustules; involves folds & scrotum | Topical nystatin/azole; keep area dry |
Vignette: A 19-year-old lifeguard notes multiple well-demarcated hypopigmented macules with fine scale on his upper back and chest, now obvious in summer because the patches fail to tan. Gentle scraping evokes fine scale (evoked-scale sign). KOH shows short hyphae plus round yeast — "spaghetti and meatballs."
Diagnosis: Tinea (pityriasis) versicolor — Malassezia.
Next best step / Rx: Topical selenium sulfide or ketoconazole shampoo/cream (oral itraconazole for extensive or recurrent disease). Note oral terbinafine does NOT work here — it fails to reach the sebum-rich stratum corneum. Counsel that dyspigmentation normalizes over weeks to months after the yeast clears — persistent color change is NOT treatment failure.
Wood-lamp caveat: any fluorescence (pale yellow-gold / copper) is inconsistent and often absent, so a negative lamp does not exclude it — KOH is the confirmatory test.
- Scabies (Sarcoptes scabiei): intense pruritus worse at night; thread-like burrows in finger web spaces, wrists, axillae, waistline, genitalia/areolae; spares the head in adults (but involves face/scalp/palms/soles in infants and the elderly)
- Diagnosis: skin scraping → mite, eggs, and scybala (feces) on microscopy; dermoscopy shows the "delta-wing jet" sign (mite's head end)
- Crusted (Norwegian) scabies: immunocompromised/HIV/elderly; hyperkeratotic crusts teeming with thousands of mites — extremely contagious
- Rx scabies: permethrin 5% cream neck-to-toe, wash off in 8–14 h, repeat in 1 week; oral ivermectin for crusted disease/institutional outbreaks; treat all close contacts + machine-wash linens hot
- Cutaneous larva migrans (Ancylostoma braziliense, dog/cat hookworm): intensely pruritic serpiginous, migrating track; barefoot on beach/sandbox; Rx ivermectin or albendazole
- Pediculosis (lice): nits cemented to hair shafts; topical permethrin
Vignette: A mother and both children develop intractable itching, worse at night, over 3 weeks. Exam shows excoriated papules and thin linear burrows in the finger webs and wrists, plus pruritic nodules on the boy's genitalia. Scraping reveals a mite with eggs and scybala.
Diagnosis: Scabies.
Next best step: Apply permethrin 5% from the neck down (all skin, including under the nails), wash off after 8–14 h, and repeat in 7 days. Critically, treat every household member/close contact simultaneously — even if asymptomatic — and machine-wash and hot-dry bedding and clothing (or bag items ×72 h). Warn that post-scabetic pruritus persists 2–4 weeks from residual hypersensitivity and does not by itself signal failure.
Dermatophyte tissue tropism (hair vs skin vs nails):
- *Trichophyton* → Takes all: hair + skin + nails (most common tinea in the US)
- *Microsporum* → Misses nails (hair + skin only)
- *Epidermophyton* → Excludes hair (skin + nails only)
Quick recall: "Microsporum Misses nails; Epidermophyton Excludes hair." And Microsporum is the classic Wood-lamp–fluorescent dermatophyte (blue-green, ectothrix), whereas Trichophyton tonsurans (endothrix) stays dark.
Parasitic Skin Infections — Comparison
| Feature | Scabies | Cutaneous larva migrans | Head/pubic lice |
|---|---|---|---|
| Organism | Sarcoptes scabiei (mite) | Ancylostoma braziliense (hookworm larva) | Pediculus / Pthirus |
| Buzzword lesion | Burrows in web spaces; nocturnal itch | Serpiginous creeping track | Nits on hair shafts |
| Key exposure | Skin-to-skin contact | Barefoot on sand/soil | Close contact / sharing combs, hats |
| First-line Rx | Permethrin 5% (+ all contacts) | Ivermectin / albendazole | Permethrin |
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