Q2 LONG ANSWER 10M ★ High-yield

Describe the dermatophytes and the dermatophytoses with their laboratory diagnosis and treatment.

Microbiology · 2nd Professional — Para-Clinical

The Dermatophytes

Dermatophytes are keratinophilic fungi that invade only the keratinised tissues — skin, hair and nail — and do not penetrate living tissue. The infections they cause are called dermatophytoses or tinea.

Genus	Tissues invaded	Notes
Trichophyton	Skin, hair and nail — all three	The commonest genus; produces abundant pencil-shaped MICROconidia and few macroconidia. T. Rubrum and T. Mentagrophytes are the usual isolates
Microsporum	Skin and hair — not nail	Produces abundant thick-walled spindle-shaped MACROconidia; M. Canis is acquired from cats and dogs and fluoresces under Wood light
Epidermophyton	Skin and nail — never hair	Only E. Floccosum; club-shaped macroconidia in clusters and NO microconidia

Ecological group	Source	Host response
Anthropophilic	Man to man	Mild inflammation but chronic and relapsing — the fungus is well adapted to its host, so it is tolerated and persists
Zoophilic	Animals — cats, dogs, cattle	Intense inflammation, sometimes with a boggy kerion, but it tends to resolve, because the poorly adapted fungus provokes a vigorous response
Geophilic	Soil	Marked inflammation; uncommon

CLINICAL PEARL

The more inflamed a ringworm looks, the better the prognosis. A zoophilic fungus is poorly adapted to man, so it provokes a violent inflammatory response — and that response tends to clear it, sometimes without treatment. An anthropophilic fungus has co-evolved with us, provokes little reaction, and therefore persists for years. The angry kerion usually resolves; the quiet scaly patch is the one that recurs.

Clinical Types

Type	Site and features
Tinea capitis	Scalp; almost confined to children. Scaly patches with broken hairs and alopecia; a kerion is a boggy, tender, inflammatory swelling that may scar. Oral treatment is essential — topical agents cannot reach the hair follicle
Tinea corporis	Glabrous skin; annular lesions with an active, scaly, raised advancing edge and central clearing — hence "ringworm"
Tinea cruris	Groin ("jock itch"); commoner in men, favoured by heat, sweating and occlusive clothing; characteristically spares the scrotum, which distinguishes it from candidiasis
Tinea pedis	Feet ("athlete's foot"); interdigital maceration, scaling, or a "moccasin" pattern; associated with occlusive footwear and communal bathing
Tinea unguium (onychomycosis)	Nail; thickening, yellow discolouration, subungual hyperkeratosis and onycholysis. Requires prolonged oral treatment — weeks to months — and relapse is common
Tinea barbae, faciei, manuum	Beard area, face and hand; tinea faciei is frequently misdiagnosed and treated with steroids
Tinea incognito	Dermatophytosis modified by topical steroids — the inflammation and the scaly edge are suppressed, so it looks better while spreading widely and atypically
ID reaction (dermatophytid)	A sterile hypersensitivity eruption at a distant site, containing NO fungus; it resolves when the primary infection is treated

Laboratory Diagnosis

Specimen — skin scrapings from the active advancing edge (not the cleared centre, where the fungus is scanty); plucked hairs with their roots, not cut hairs; and nail clippings with subungual debris from as proximal a point as possible
KOH mount — 10 to 20% potassium hydroxide (40% with dimethyl sulphoxide for nails) digests the keratin and leaves the fungus visible as branching septate hyphae and arthrospores. It is cheap, takes minutes, and is the single most useful test
Calcofluor white binds chitin and fluoresces, making the hyphae far easier to see; used with a fluorescence microscope
Hair invasion patterns — ectothrix (spores outside the hair shaft, seen with Microsporum, and fluorescent) and endothrix (spores within the shaft, with T. Tonsurans and T. Violaceum, and non-fluorescent)
Wood lamp — ultraviolet light at 365 nm; Microsporum species fluoresce a bright green, whereas Trichophyton species do not fluoresce, so a negative Wood lamp does not exclude tinea capitis
Culture on sabouraud dextrose agar with cycloheximide (to suppress saprophytic moulds) and chloramphenicol (to suppress bacteria), at 25 to 28°C for up to 4 weeks. Identification is by colony colour and texture, reverse pigment, and the microscopic appearance of macro- and microconidia in a lactophenol cotton blue mount
Dermatophyte test medium turns red on alkali production, giving a presumptive result; PCR and MALDI-TOF where available

Treatment

Topical treatment suffices for limited glabrous skin disease — terbinafine, clotrimazole, ketoconazole or luliconazole, continued for 1 to 2 weeks beyond clinical clearance, which is the step most often omitted
Oral treatment is essential for — tinea capitis (always), tinea unguium, extensive or recurrent disease, tinea barbae, and infection in the immunosuppressed. Topical agents cannot penetrate hair follicles or the nail plate
Oral agents — terbinafine (fungicidal, concentrated in keratin, the usual first choice); itraconazole (broad, but with many interactions and negative inotropic effects); griseofulvin (older, fungistatic, requires a fatty meal for absorption, and remains preferred for Microsporum tinea capitis in children); fluconazole
General measures — keep the area dry, wear loose cotton clothing, avoid sharing towels and clothes, treat family members and pets, disinfect or discard footwear, and continue for the full course
Do not use topical or oral corticosteroids, and in particular do not use combination creams containing them

Differential Diagnosis of Annular Skin Lesions

Condition	Distinguishing features
Tinea corporis	Active scaly raised edge with central clearing; itchy; KOH positive
Pityriasis rosea	A herald patch followed by a "Christmas tree" distribution along skin lines; collarette of scale inside the edge; self-limiting
Nummular (discoid) eczema	Coin-shaped, uniformly scaly and vesicular without central clearing; very itchy
Psoriasis	Thick silvery scale, well-defined, on extensor surfaces; nail pitting; Auspitz sign
Granuloma annulare	An annular ring of firm papules with NO scale — the absence of scale is the key
Leprosy (borderline tuberculoid)	Hypopigmented or erythematous plaque with definite loss OF sensation and a thickened nerve — always test sensation in an annular patch
Erythema migrans	Expanding "bull’s-eye" after a tick bite; Lyme disease, rare in India

Two bedside tests separate most of these — a KOH mount and a test of sensation. Both take minutes and cost nothing, and omitting them is how leprosy and tinea are both missed

Applied Aspects — the Indian Epidemic

India is experiencing an epidemic of chronic recalcitrant dermatophytosis — extensive, atypical, steroid-modified and treatment-resistant tinea, which has changed the disease from a trivial complaint into a major dermatological problem
The dominant cause is the over-the-counter steroid-antifungal-antibacterial combination cream. The steroid suppresses inflammation so the rash looks better while the fungus spreads unchecked, producing extensive tinea incognito and selecting resistant organisms
A new species, Trichophyton indotineae, has emerged in India and spread internationally; it carries squalene epoxidase mutations conferring terbinafine resistance, so the standard first-line drug fails
Practical consequences — longer courses at higher doses, itraconazole as first-line in many centres, treatment of the whole household simultaneously, and attention to clothing and environment
Confirm the diagnosis with a KOH mount before treating; the steroid-modified lesion no longer looks like ringworm, and an unconfirmed diagnosis leads to months of wrong treatment
Counsel patients explicitly against combination creams, which are cheap, widely available and give rapid symptomatic relief — which is precisely why they are so damaging; several have now been banned in India but remain in circulation
Treat for 1 to 2 weeks beyond clinical clearance; stopping when the rash disappears is the commonest reason for relapse, since viable fungus persists in apparently normal skin
Treat the whole household and any pets in recurrent tinea; reinfection from an untreated family member is otherwise inevitable
Scrape from the advancing edge, not the centre; the cleared centre contains few organisms and yields a falsely negative KOH mount
Tinea capitis always requires oral treatment, and a child treated with creams alone will not clear; this is among the commonest errors in general practice
A negative Wood lamp does not exclude tinea capitis, since only Microsporum fluoresces and most Indian cases are caused by Trichophyton

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