Fungi causing superficial mycoses

This section includes cutaneous and subcutaneous mycoses. Cutaneous mycoses include fungal infections of the skin, hair and/or nails.

Dermatophytes

They can be classified as:

1. Trichophyton: infect skin, hair and nails
2. Microsporum: infect only skin and hair
3. Epidermophyton: infect only skin and nails

Clinical features of Dermatophytosis: They infect dead, keratinised tissue and secrete keratolytic enzymes which helps in pathogenicity. Remember erythematous, itchy rash in ringworm or tinea infections is due to the above mechanism. The lesions are typically scaly, itchy, erythematous and well demarcated. Following types are seen:

1. Tinea corporis: on non hairy skin.
2. Tinea pedis/ Athlete’s foot: interdigital area, foot.
3. Tinea cruris: groin area.
4. Tinea barbae: on bearded areas of the face.
5. Tinea faciei: on skin of face excluding bearded area.
6. Tinea capitis: on scalp,may become inflammatory, hair loss, prominent scales seen.
7. Tinea imbricata: concentric rings of scaling and rash seen, caused by Tinea concentricum.
8. Tinea unguium/ Onychomycosis: nail plates involved, nail looks white, opaque, thick and brittle.

Laboratory diagnosis of Dermatophytoses: It is done by KOH preparation of skin, nail or hair. KOH dissolves the keratin so that fungal elements can be clearly seen. The smear will show branching, septate hyphae. Fungal culture is done on Sabouraud’s agar or Dermatophyte Test medium (DTM). Growth on DTM shows red colonies. Wood’s lamp examination may help in a few cases. Below table shows results of Wood’s lamp examination in superficial mycoses.

Tinea versicolor

Also known as Pityriasis versicolor is a chronic infection of the skin caused by Malassezia furfur / Pityrosporum orbiculare. It presents as hypopigmented, rarely hyperpigmented, well demarcated, non inflammatory, sometimes itchy patches commonly seen on the face, neck, torso and upper body. Laboratory diagnosis is done by Wood’s lamp examination showing yellow fluorescence, characteristic “banana and grape” or “spaghetti and meatballs” appearance on KOH films from skin scrapings.

Sporothrix schenckii

It causes granulomatous infection of skin and subcutaneous tissue called Sporotrichosis or Rose gardener’s disease as infection often follows a thorn prick. It is a dimorphic fungus. It presents as a small, firm , violaceous nodule on the skin followed by nodules along the draining lymphatics or as ulcerative, suppurative lesions. Rarely, in immunocompromised individuals it may cause pneumonitis from inhalation of conidia (spores). KOH smear is usually negative. Histopath examination of skin biopsy samples will show yeast forms and cigar shaped budding with eosinophilic rays surrounding it called asteroid bodies. Culture shows cream or tan colonies which show thin hyphae and flower-like clusters of conidia resembling daisy flowers on microscopy.