Epidemiology of superficial fungal infections

Abstract

Fungal infections of the skin, hair, and nails are common worldwide, and their incidence continues to increase. The principal causative agents are dermatophytes, and their geographic distribution is variable. This is reflected in the differing patterns of dermatophytoses seen in different parts of the world. The epidemiology of dermatophyte infection has changed as a result of migration, lifestyle, drug therapy, and socioeconomic conditions. This contribution discusses global patterns of dermatophyte infection and the changing epidemiology of the causative agents.

Introduction

Superficial mycoses are common worldwide. They are believed to affect 20% to 25% of the world's population, and the incidence continues to increase.¹ They are predominantly caused by dermatophytes, and the causative species vary with geographic region. Some species are distributed worldwide, such as Trichophyton rubrum, T. mentagrophytes var. interdigitale, Microsporum canis, and Epidermophyton floccosum. Others have partial geographic restriction, such as T schoenleinii (Eurasia, Africa), T soudanense (Africa), T violaceum (Africa, Asia, and Europe), and T concentricum (Pacific Islands, Far East, and India).

Most cases of tinea unguium, tinea cruris, tinea corporis, and tinea pedis are caused by T rubrum, which is the commonest dermatophyte in most developed countries2, 3 as well as in urban areas of some developing countries.⁴M canis is the predominant dermatophyte in Central and Southern Europe as a result of the high rate of tinea capitis. In Asia, T rubrum and T mentagrophytes are the most commonly isolated dermatophytes, and as in Europe, this is as a result of the high prevalence of tinea pedis and tinea unguium.5, 6 In Latin America, Africa, and the Middle East, the pattern of dermatophyte infection is a much more variable, and there are some strong regional associations. For example, M audouinii is the most prevalent pathogen in Africa overall; however, T violaceum infection is highly endemic in the horn of Africa as well as in North and South Africa, and T soudanense infection in northwestern central tropical Africa.7, 8, 9

The epidemiology of dermatophyte infection is likely to alter with changing patterns of migration, growth in tourism, and changes in socioeconomic conditions. Dermatophytes endemic to Asia and Africa, such as T soudanense, T violaceum, and M audouinii, have increased in frequency in Europe and North America as a result of migration. Changes to the epidemiology of causative agents are also a reflection of changing patterns of dermatophytosis. For example, a century ago in Europe, tinea capitis was the principal dermatophytosis, but tinea pedis became far more common towards the late 20th century, and the incidence of tinea capitis decreased.

Few studies have investigated the etiology of superficial fungal infections in the developing world, and consequently, there is less knowledge of any changes to their epidemiology. It is difficult to ascertain reliably the overall incidence and prevalence of the various skin diseases caused by superficial mycoses in different parts of the world because studies of one region of the country may not be a true representation of the overall disease pattern of that country; furthermore, incidence and prevalence figures may only be representative of the population sampled, which may have associated risk factors for infection.

Knowledge of the predominant causative species provides a clearer understanding of risk factors for superficial fungal infections and future epidemiologic trends. Improvements in living conditions have generally been associated with a decline in zoophilic dermatophyte and an increase in anthropophilic dermatophyte infections. The epidemiology of tinea capitis and tinea pedis (together with onychomycosis) has undergone the most radical changes and will therefore be discussed in detail. Epidemiologic changes in the prevalence of tinea corporis, tinea cruris, tinea manuum, and tinea faciei have been less extensively studied. Their prevalence and the dermatophytes responsible for causing them may reflect local trends in tinea capitis and tinea pedis, which may be the source of infection.

In Western Europe, T rubrum is the most frequently isolated dermatophyte in cases of tinea corporis and tinea cruris. T rubrum is also the most common cause of tinea pedis and tinea unguium, which may act as a reservoir of infection predisposing to tinea corporis or cruris through autoinoculation. Anthropophilic tinea capitis is often associated with tinea corporis, and T tonsurans may therefore become an important agent of tinea corporis in the future. An example of the association between tinea capitis and tinea corporis has occurred in Libya, where T violaceum is responsible for almost half of all cases of tinea corporis⁷ and is probably a reflection of the high prevalence of T violaceum tinea capitis in the population.¹⁰