2.3.1 Dermatophyte Infections

Tinea corporis is more common in children and adolescents than in adults. Tinea capitis is most commonly found in children aged 3-14 years old. The incidence of favus (more severe, chronic form of tinea capitis) has dramatically declined, although it may be endemic in some geographic areas. Tinea barbae is found exclusively in males, most commonly among farmers. Tinea pedis is one of the most common dermatological diseases, showing 20% prevalence among adults. Tinea manuum almost always occurs following tinea pedis. Onychomycosis is found in about 20% of adults; more than 80% of all nail infections are caused by dermatophytes.

Dermatophytes are keratinophilic fungi which infect the stratum corneum, hair follicles and nails, and include three genera (Table I):

• Microsporum

• Trichophyton

• Epidermophyton

Anthropophilic dermatophytes Trichophyton (T.) rubrum, and T. interdigitale are adapted to humans and cause chronic infections with minimal inflammation.

Zoophilic dermatophytes (Microsporum (M.) canis, T. mentagrophytes) are found on warm-blooded animals; these are highly transmissible through contact into humans and usually cause intense inflammation. The animals (e.g. pets) are often asymptomatic carriers.

Geophilic dermatophytes (M. gypseum) are found in the ground; these rarely cause mycoses in humans. Transmission is usually human to human and animal to human; rarely soil to human or animal. In addition to agent-dependent factors, the host response influences the clinical manifestations of the disease. The host factors which encourage dermatophyte invasion include decreased sebum production, immunosuppression, macerated skin and minor skin trauma.

• Tinea corporis

Tinea corporis usually presents with annular, erythematous, circumscribed plaques with peripheral scales (caused by the spread of the fungus within the stratum corneum) and occasionally pustules. The plaques tend to heal centrally as the periphery advances, giving the lesions a typical ring shape. Itching is variable. Any dermatophyte can cause tinea corporis, but the most common pathogen is T. rubrum, followed by T. mentygrophytes and M. canis.

• Tinea capitis

Tinea capitis is a dermatophyte infection of the scalp caused by members of two genera: Trichophyton and Microsporum. Trichophyton spp. produce both ectothrix and endothrix infections, while Microsporum spp. always cause ectothrix infections. In an ectothrix infection the fungi affect the follicle until they reach the zone of keratinizitation, without invading non-keratinized parts of the hairs; whereas in an endothrix pattern the fungi reproduce within the hair shaft, which results with its destruction.

Clinically, one should distinguish between classic tinea capitis (scalp ringworm), kerion and favus.

Classic tinea capitis is the most common dermatophyte infection in childhood. In anthropophilic endothrix infections (T. tonsurans), the hair is filled with spores, visible as small black dots in the follicle opening (“black dot ringworm”). Microsporum infections caused by anthropophilic M. audouinii or by zoophilic M. canis often lead to bland deep folliculitis without abscess formation. Hairs break off shortly after emerging above the scalp surface. These hairs fluoresce yellow-green on Wood’s light examination. However, when M. canis is responsible, transitions to inflammatory tinea capitis may be expected.

Zoophilic dermatophytes (T. verrucosum, T. mentagrophytes, M. canis) extend deep into the follicles and induce an intense inflammation, pustules and abscesses with massive purulent secretion (“kerion”).

Favus is caused by T. schoenleinii. Typical clinical finding are exuberant masses of fungal elements, scales and secretions producing scutula (Latin for small shield). They heal with scarring (cicatricial alopecia or pseudopelade).

• Tinea barbae

Tinea barbae affects men and presents as a deep abscess-forming folliculitis of the beard hair. It is most often acquired from animals and is typically caused by T. mentagrophytes or T. verrucosum (common dermatophyte in cows).

• Tinea pedis

Tinea pedis is one of the most common dermatological diseases. The most common causative dermatophytes are T. rubrum, T. interdigitale, T. mentagrophytes and Epidemophyton (E.) floccosum. There are 3 clinical forms: interdigital, hyperkeratotic and vesicular/dyshidrotic. The interdigital form is characterized by maceration and coarse scaling, most commonly between closely approximated 3rd and 4th toes. Shedding of the macerated skin produces erosions and fissures. The hyperkeratotic form presents as non-inflammatory diffuse scaling of the entire plantar surface of soles and is often mistaken for dry skin. Vesicular/dyshidrotic form is most often present on the arch of the foot, as well as the tips of the small and great toes. Typical lesions are scattered, intensely pruritic vesicles which may be cloudy.

• Tinea manuum

Tinea manuum usually affects only one hand, but in chronic cases may be bilateral (tinea manuum). It is most often caused by T. rubrum and T. interdigitale. Clinically, there may be fine collarette scaling on the cup of the palm, especially along the palmar creases. Dyshidrosiform type is characterized by itchy vesicles on palm and sides of fingers. A special type is “one-hand/two-feet mycosis” which always affects both soles as well as one palm. It is usually caused by T. rubrum.

• Tinea cruris

Tinea cruris (tinea inguinalis; “jock itch”) is a dermatophyte infection involving the groin. It is most commonly caused by E. floccosum, T. rubrum and T. interdigitale. Men are more commonly affected than women. Tinea cruris is commonly seen in patients with tinea pedis. It is clinically presented as ring- shaped erythematous lesion with scaly borders on the groins.

• Onychomycosis

Onychomycosis (tinea ungium) is a dermatophyte infection of the nail unit. More than 80% of all nail infections are caused by dermatophytes (usually T. rubrum and T. interdigitale). Impaired nail growth is a predisposing factor. More frequently affects the feet (starting from tinea pedis) rather than the hands. Several forms of fungal nail infection can be clinically distinguished: a) distal subungual onychomycosis is the most common form; b) white superficial onychomycosis (whitish surface of the nail; most often caused by T. interdigitale); c) proximal subungual onychomycosis (the least common type), d) complete dystrophic onychomycosis with marked destruction of the nail plate as final state of the first three forms.

Dermatophytoses are diagnosed via KOH (potassium hydroxide) examination and fungal culture. Hyphae are identified by KOH examination from scales of the skin lesion or nail. The type of dermatophyte can be identified only with cultivation using Sabouraud agar as medium. Cultivation often takes 3 weeks. PCR studies allow detection of dermatophytes at species level within 24 hours.

General measures include correction of predisposing factors (sweating, improper shoes) and treatment of (asymptomatic) carriers. The choice of treatment depends on the location and severity of infection. Topical antifungals are the first-line treatment for the treatment of localized dermatophyte infections such as tinea corporis, tinea pedis and tinea cruris. There is a wide spectrum of topical antifungals, including azoles, amorolfine, ciclopirox olamine and terbinafine. Polyenes (amphotericin B, nystatin) are ineffective against dermatophytes. For hyperkeratotic forms of dermatophytoses (e.g. tinea pedis) adjunctive topical products such as urea and salicylic acid may be used. Systemic antifungal treatment is required for tinea capitis, for the hairy sites other than the scalp such as tinea barbae, for widespread disease and for most cases of tinea unguium. Namely, for distal subungual onychomycosis involving less than 50% of nail plate, topical treatment may be employed, but with less effectiveness than with systemic treatment. Oral antifungals for the treatment of dermatophytosis include terbinafine as the gold standard; alternatively, triazoles such as itraconazole and fluconazole can be used. Griseofulvin is restricted to Microsporum infections in childhood.