Decreasing Incidence of Trichophyton mentagrophytes in Korea: Analysis of 6,250 Cases during the Last 21-Year-Period (1992-2012)

Trichophyton mentagrophytes is the second most common dermatophyte in Korea (1-3). Its subtypes include T. mentagrophytes var. interdigitale, T. mentagrophytes var. mentagrophytes, T. mentagrophytes var. nodulare, T. mentagrophytes var. goetzii, T. mentagrophytes var. granulosum and T. nentagrophytes var, erinacei (4, 5). T. mentagrophytes var. interdigitale, an anthropophilic isolate, is a frequent cause of chronic dermatophyte infection of feet, nails, and the groin (6). T. mentagrophytes var. mentagrophytes, a zoophilic isolate, is more often associated with inflammatory lesions of the scalp, the glabrous skin, the nails, and the beard region (6). Although T. mentagrophytes is a common dermatophyte in Korea, few reports have been issued on its epidemiological and mycological characteristics based on long-term large-scale study. Accordingly, this study describes the annual incidence and the distribution of subjects by age, sex, season, involved sites, and place of residence, and the fungal colony appearance of T. mentagrophytes in Korea.

The fungal universe is composed of more than 1.5 million species (6). Dermatophytes are represented by approximately 40 species divided into three genera: Epidermopyton, Microsporum, and Trichophyton (6). In Korea, Trichophyton species, such as T. rubrum and T. mentagrophytes are mostly commonly isolated. However, although T. mentagrophytes is a common dermatophyte in Korea, few investigative reports have been issued on its epidemiological and mycological characteristics based on long-term large-scale analysis.

The prevalence of dermatophytosis in skin has changed over the last 50 yr. The incidence of dermatophytoses, with the exception T. rubrum infections, are decreasing, and the present study also shows that the incidence of T. mentagrophytes had decreased in Korea since 2005. Choe et al. (7) reported that the incidence of T. verrucosum infection decreased in the 2000s much more so than in the late 1900s. Lee et al. (8) also reported that the prevalence of Microsporum canis infection decreased markedly in the 2000s. In addition, the incidences of T. schoenleinii, T, violaceum, M. gypseum, M. ferrugineum, and Epidermophyton floccosum are also decreasing, and that of T. tonsurans infection has shown a recent decrease. The first report on T. tonsurans infection in Korea was issued by Suh et al. (9) in 1995. Since T. tonsurans was isolated from a middle school wrestler, it spread to amateur wrestlers and judoists as it was referred to as trichophytosis gladiatorum and to the general population (10). The observed decrease in T. mentagrophytes infection may have been caused by public health education, improved hospital accessibility, environmental improvements, and increased lifestyle diversity.

T. mentagrophytes is a causative fungus of tinea facieii, tinea capitis, tinea cruris, tinea pedis, tinea unguium, and tinea manus (11). In this study, tinea pedis, especially the interdigital type, was the most common T. mentagrophytes infection. The spreading of tinea pedis was accompanied by a parallel increase in the frequency of onychomycosis - a high proportion of onychomycosis was also revealed by the present study.

T. mentagrophytes is further classified as anthropophilic or zoophilic (6). Anthropophilic isolates, such as, T. mentagrophytes var. interdigitale, infect humans, and include the powdery, persicolor, and downy colony appearance types of T. mentagrophytes in fungal culture. Powdery colonies are composed of fine powder, whereas persicolor colonies are composed of thin, white, cottony strands, and the downy colonies exhibit white aerial mycelium. Zoophilic isolates of T. mentagrophytes, such as, T. mentagrophytes var. mentagrophytes also exhibit a granular colony appearance (12, 13), which is observed under the microscope as conspicuous sandy-brown clumps of conidiation, which give the colony surface a coarsely granular appearance.

In this study, powdery and persicolor colony appearances were much more common than the granular colony appearance of T. mentagrophytes, which supports the notion that occupational diversity has driven the observed marked decrease in the incidence of zoophilic T. mentagrophytes infections. However, in the present study, patients aged less than 20 yr old were most commonly infected with zoophilic T. mentagrophytes, indicating that pets should be considered as a major source of zoophilic T. mentagrophytes. Zoophilic T. mentagrophytes is associated with a wide range of rodents, such as lagomorphs (rabbits and relatives), hedgehogs, and other small mammals (14). Although these smaller mammals are probably the primary reservoir, zoophilic T. mentagrophytes may also cause infection in horses and other large mammals (14).

In the present study, the incidence of T. mentagrophytes was highest in the fourth decade till 2001 and highest in the fifth from 2001. Contrary to a previous report, which found that the prevalence of T. mentagrophytes increases with age (15), we found that T. mentagrophytes infection was greatest among the middle-aged, which suggests that social interactions may be an important factor of T. mentagrophytes infection in Korea. Furthermore, the incidence rate in men was much higher than that in women. In addition, as was expected based on the relation between fungal growth, humidity, and temperature, its incidence was highest in summer (16).

This study provides considerable information on T. mentagrophytes infection in Korea. In particular, it confirms a continuing reduction in its annual incidence and shows that a powdery colony phenotype is the most common.