Epidemiology of infections due to zoophilic dermatophyte trichophyton simii, an update

INTRODUCTION

Dermatophytes are a morphologically related group of filamentous fungi that invade the keratinized layers (stratum corneum and stratum lucidum) of the skin hair, nails, feathers, horns, and hooves. The species of dermatophytes belong to three genera: Microsporum, Trichophyton and Epidermophyton. Dermatophytes, like many other fungi, may have two different species names. One name belongs to the asexual form (the anamorph state) that occurs in vertebrate hosts. The other name given to the sexual form of the organism is called the teleomorph or the “perfect state,” produced by mating between anamorphs.^[1] The teleomorph (perfect) states of both Microsporum and Trichophyton belong to the genus Arthroderma. The dermatophytes known to have sexual states are placed in family Arthrodermataceae of the phylum Ascomycota of the Kingdom Fungi. The dermatophytes with no known sexual state, like other medically important fungi with this feature are classified as Deuteromycota (Fungi Imperfecti).^[1] Dermatophytoses occur frequently in livestock, in other domestic animals, and in wild animals. Infections caused by zoophilic dermatophytes are generally mild and self-limiting. These respond well to treatment; they rarely manifest as serious and systemic infections in immunocompromised hosts.^[1] The prevalence of dermatophytosis in animals varies greatly according to geographic locale. Zoophilic Microsporum species include Microsporum canis, M. gallinae, and M. persicolor. Horse-adapted isolates of M. canis, which are negative for the in-vitro hair perforation test (unlike most M. canis) and produce few conidia, were formerly called M. equinum. Based on genetic relatedness, they are now considered to be M. canis. Members of Trichophyton include Trichophyton equinum, T. bullosum, members of the T. mentagrophytes complex, T. simii, and T. verrucosum.^[1] Most or all of zoophilic dermatophytes infect animals (thus termed zoonotic), although some are transmissible to humans more often than others.^[2] Trichophyton simii was first isolated from the vesicular cutaneous lesions of a monkey (imported from India) in France.^[3] Later it was recovered from skin lesions of a monkey in the USA;^[4] the monkey had been imported from India. Stockdale et al.^[5] studied 29 isolates received from Prof, N.F. Conant; these isolates had been recovered from chickens (27), dog (1) and a poultry attendant (1) by Prof C.M. Singh, College of Veterinary Medicine & A.H., Mathura (UP, India). One of the isolates from chickens formed cleistothecia (perfect state) characteristic of the genus Arthroderma.^[5] Phenotypic identification of Trichophyton simii (anamorph of Arthroderma simii) is based on the formation of finely granular colonies in culture with white-to-pale yellow color on reverse and distinctive to fusiform 3–7 septate macroconidia converting into chlamydospores in older cultures, and pyriform microconidia.^[6] It can be distinguished from other Trichophyton species by its faster growth on agar media, inability to perforate hair in-vitro utho, and produce the enzyme urease.^[6–8] The clinical lesions caused by T. simii in humans and animals are usually inflammatory and erythematous. Trichophyton simii is known to infect monkeys, chickens, dogs, and humans worldwide, though infections are sporadic and epidemic potential and zoonotic risk for humans is low.^[9] This dermatophyte is also known to occur as a geophilic species in India and Ivory Coast.^[10]

DISCUSSION

Trichophyton. simii infection until recently considered to be confined to specific endemic areas has been reported in several countries in different continents as shown in this study. T. simii appears to be primarily a zoophilic dermatophyte in view of its more frequent and consistent association with the fur of Tatera indica (The Indian Gerbil), a predominant field rodent and Suncus murinus, a common shrew found in the wild and rural areas of North India (Gugnani, 1970).^[31] In another survey, several other species of rodents, viz. Rattus rattus, R. norvegicus, Mus musculus commonly inhabiting houses were found to be carriers of T. simii (Gugnani et al.).^[33] Other animal hosts are poultry, and domestic animals including dogs, and bovines (Gugnani and Randhawa 1973;^[7] Ranganathan et al. 1997^[28]). All these animal hosts and soil can be the source of human infections. Trichophyton simii can infect several body sites and scalp in humans.^[8,17,23] Literature review has established that T.simii is primarily a zoophilic dermatophyte that also occurs frequently as a geophilic species; human infections can be acquired from animals^[7] and soil,^[9] and occasionally from fomites, e.g. pullover.^[10] Reverse transmission of T. simii infection from animals to man is also known.^[34] Prior to this report, reverse transmission of M. gypsum infection from man to dog was reported.^[35] The extent of reverse transmission of T. simii infection from humans to animals is not known and needs to be explored. It may be recorded here that T. simii infections in humans have been reported from some countries in Europe, viz. Belgium, France,^[10] and in the Middle east, viz. Iran^[16] and Iraq,^[17] Saudi Arabia,^[18] though there are no reports of animal infections due to T. simii in these countries, or of occurrence of this dermatophyte in soil of these countries. This emphasizes the need for investigating the epidemiology of T. simii infections in the countries.

A rapid and sensitive real-time PCR assay based on internal transcribed sequences developed by Bergman et al.^[36] allows the rapid detection and identification of eleven clinically relevant species within the three genera of dermatophyte, viz. Trichophyton, Microsporum, and Epidermophyton in nail, skin, and hair samples within a few hours as compared to conventional culture technique which takes 2–4 weeks. In view of the scarcity of reports of isolation of T. simii from soil, a PCR technique based on genomic sequence following the method of Bergman et al.^[36] should be developed to explore geophilic occurrence of this dermatophyte in countries from where human and animal infections caused by it have been reported. In conclusion, it can be said that T. simii infections in humans and animals though formerly known to be restricted to specific endemic regions, now appear to be frequent in non-endemic areas. Comprehensive surveys of infection in humans and animals employing mycological culture and latest molecular techniques, namely automated DNA extraction and real-time PCR,^[36] and sequencing of ITS1 and ITS2 of the ribosomal DNA^[17] are needed to estimate the true prevalence of T. simii infections in different countries.