ABSTRACT
Dermatophytes are fungi that infect skin, scalp and nails and uniquely require keratin for their growth. There are five genera including Arthroderma, and Nannizzia and the three major genera affecting humans: Trichophyton, Epidermophyton and Microsporum. Fungal spores attach initially to the stratum corneum, and the hyphae following germination use various adhesins to bind to ligands on the surface of the host keratinocytes and secrete a proteolytic enzyme keratinase. The fungus uses the metabolic product of keratin for catabolism and growth. Infection of humans can be from another human (Anthropophilic), an animal (Zoophilic) or the environment (Geophilic), e.g. soil. Dermatophytes colonize 30–70% of the human population without causing clinical disease. Glycans, glycolipids and glycoproteins act as microbe-associated molecular patterns (MAMPS) and are recognized by a variety of pattern-recognition receptor (PRR) families. These induce production of cytokines such as IL1, IL6 and IL17 and antimicrobial peptides such as defensins and cathelicidins that have anti-fungal effects. Infection can be contained in the epidermis but may also penetrate the dermis and beyond. Resident macrophages and Langerhans cells (LCs) in the dermis also produce cytokines and can kill dermatophytes probably through reactive oxygen species. LCs also migrate to the draining lymph nodes where T-cell immunity is stimulated (Th-1 and Th-17 cells) leading to production of IFN. However, data from animal models and mutations such as CARD9 indicate that host defense is mainly through the innate immune system. Patients can present with infections of the skin, hair (scalp and beard) or nails (finger and toe). The disease is named ringworm or tinea followed by location on the body, e.g. tinea corporis (body), tinea pedis (foot), tinea cruris (jock itch – groin) and tinea capitis (head). Infection of the nails is called onychomycosis. Diagnosis in most cases is made clinically. It can be aided by microscopic examination of skin scrapings, nail scrapings, hair or scales and culture and specific PCR. A skin biopsy may also be taken if the diagnosis is uncertain. Infected patients should be encouraged to wear loose-fitting clothes and more frequently: should avoid sharing garments or towels. Topical treatment for mild cutaneous infections consists of an azole, e.g. clorimazole or allylamine, e.g. terbinafine for 2–4 weeks. Onychomycosis treatment is usually topical ciclopirox which is a broad-spectrum anti-fungal with some anti-inflammatory properties, together with oral antifungal agents including terbinafine (currently drug of choice), itraconazole and fluconazole. Prevention includes good personal hygiene, keeping the skin dry, wearing loose clothing, not sharing towels, hairbrushes or combs, etc.
