Antifungal Drug Susceptibility Testing of Dermatophytes: Laboratory Findings to Clinical Implications

Abstract

© 2019 Indian Dermatology Online Journal | Published by Wolters Kluwer Medknow Dermatophytes are the most commonly encountered fungi in humans and other vertebrates that spread through direct contact with infected humans, animals, and soil.[1] Infections due to these agents are usually restricted to the stratum corneum and are generally referred as ‘tinea’ or ‘ringworm’ (tinea capitis; tinea barbae; tinea corporis; tinea cruris; tinea manuum; tinea pedis and tinea unguium).[2,3] Dermatophytes belong to 3 closely related genera‐ Trichophyton, Microsporum and Epidermophyton.[4] Worldwide, several studies have documented a varied prevalence rate of dermatophytosis ranging from 14‐26.8% in North America, East Asia and Europe, and 5‐31.6% in Africa (Ethiopia, Kenya, Nigeria, and Tanzania).[5‐7] The regional variations are mainly due to differences in the lifestyle, socioeconomic conditions, underlying risk factors, and environmental factors of different geographic areas.[1] Epidemics of dermatophytosis have also been reported in the area of overcrowding and poor hygienic conditions.[4,8‐10] In 2005, World Health Organization (WHO) reported a prevalence of up to 19.7% for tinea capitis in the general population of developing countries.[11] High prevalence rates of tinea pedis and onychomycosis have been recognized in certain occupational groups like a marathon runner (22‐31%), miners (21‐72.9%), and soldiers (16.4‐58%).[12,13] Trichophyton species are the major causative agents responsible for dermatophytosis with a prevalence rate of 70‐90% for onychomycosis and 53‐86% for rest of the tinea infections.[14,15] Of these, Trichophyton rubrum is the key etiological agent followed by T. mentagrophytes complex, Microsporum canis, and M. gypseum.[16‐18] In India, we are presently noticing a significant rise in number of dermatophytosis cases with chronic recalcitrant disease, atypical presentations, frequent relapses, and treatment failures.[19‐22] Though the reason for this phenomenon is not yet clear, it is assumed that unchecked availability of cheap and irrational fixed‐dose corticosteroid–antifungal–antibacterial combinations sold over the counter in India and in‐vitro resistance to common antifungals (to some extent) is playing a pivotal role. Due to recent increase in the reports of antifungal drug resistance in dermatophytes, many groups have suggested to perform the antifungal drug susceptibility testing especially for the dermatophytes isolated from chronic/recurrent/recalcitrant cases or those with atypical presentations. Clinical successful treatment does not always correlate with the MIC (minimum inhibitory concentration) value of antifungals (in‐vitro) [Table 1]. The discordance between the in‐vivo and in‐vitro resistance in fungi has been illustrated by the “90–60 rule,” which states that Antifungal Drug Susceptibility Testing of Dermatophytes: Laboratory Findings to Clinical Implications Editorial