Benhamiae trichophyton

The trichophyton species of Arthroderma benhamiae is (recently) a quite frequently encountered zoophilic dermatophyte in Germany. In some regions the pathogen is detected more frequently than Microsporum canis.

Trichophyton species of A. benhamiae as causative agent of human dermatophytosis were first isolated in Japan in 2002 and identified by sequencing the internal transcribed spacer (ITS) and non-transcribed spacer (NTS) gene regions of the ribosomal RNA (rRNA) genes.

Identification of isolates with yellow stained colonies is possible by macro- and microscopic features. The pathogen forms flat, radiating colonies with yellowish stained air mycelium and dense, velvety surface. The underside of the colonies is bright yellow in colour.

Other colony morphologies are rarer: white, granular, also powdery, possibly radiating (resemble Trichophyton interdigitale, whose older name was Trichophyton mentagrophytes var. granulosum [zoophilic subspecies] or var. interdigitale [anthrophilic subspecies]). The whitish phenotype shows a different molecular signature (Symoens F et al. 2013).

Microscopic: The microscopic image is diverse and therefore less specific; under good growth conditions, trichophyton species of Arthroderma benhamiae develop cigarette- and club-shaped macroconidia. These are usually chambered by 3-8 transverse septums. Microconidia grow both botrytis-shaped (berry-like) and acladium-shaped (corn ears-shaped), the latter can insert laterally and terminally into the hyphae of the mycelium. The spiral hyphae typical for T. Interdigtale also occur here.

A further differentiation, above all in demarcation to other Trichophyton species, is possible by means of molecular biological methods. With a PCR-ELISA or a real-time PCR the dermatophyte can be directly detected in clinical material. Sequencing of the internal transcribed spacer region (ITS) of the ribosomal DNA has proven to be a useful culture confirmation test. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry(MALDI-TOF MS) is also suitable for this advanced diagnosis (Kawasaki M et al. 2002/2011).

Reliable epidemiological frequency analyses can only be expected in the coming years, when the pathogen will be given more attention. However, it must be assumed that trichophyton species of Arthroderma benhamiae is the most frequent zoophilic pathogen of dermatophytoses preferably with children and adolescents in Germany. Source of infection for Trichphyton species of A. benhamiae are guinea pigs as well as other small rodents such as hamsters and rats; in individual cases also cats and dogs. A recent study on guinea pigs from pet shops showed a 93%! Infestation with this zoophilic dematophyte (Kupsch C et al. 2017)

Up to now T. species of A. benhamiae are rather rarely identified in Germany as cause of a dermatomycosis. However, this is mostly due to the fact that T. species of A. benhamiae are wrongly identified as the pathogen cannot be clearly delimited morphologically from the zoophilic Trichphyton interdigital and other morphologically similar dermatophytes (e.g. Microsporum canis) by conventional methods due to its thallus staining and the multitude of microconidia.

A. benhamiae, like other dermatophyte species, produces proteases and other hydrolytic enzymes responsible for keratin degradation. The infection is caused by trichophyton species of Arthroderma benhamiae, in contrast to anthropophilic dermatophytes, (e.g. trichophyton tonsurans), by the expression of a broad, proinflammatory cytokine and chemokine response(interleukin-1ß, interleukin -2, interleukin-4, interleukin-6, interleukin-10, interleukin -13, interleukin-15, interleukin-16, interleukin-17 as well as interferon gamma), mostly highly inflammatory local infections (Shiraki Y et al. 2006).

Trichophyton species of Arthroderma benhamiae cause inflammatory tinea (mostly) in children and adolescents (Nenoff P et al 2013). Tinea faciei, tinea capitis (Hiernickel C et al. 2016), tinea corporis and tinea manuum are caused by the pathogen.

The treatment of extensive dermatophytosis caused by T. species of Arthroderma benhamiae, especially of tinea capitis, is carried out with oral antimycotics, preferably terbinafine; alternatives are fluconazole and itraconazole.

Terbinafine is primarily recommended for systemic therapy because of the interdigital resistance to imidazoles and griseofulvin which is sometimes present in Trichophyton. Only if the parents should reject the "off label" use of Terbinafine in children in ½ to ¼ dosage Fluconazole, Itraconazole and Griseofulvin can be considered alternatively. “

For the local treatment of infections caused by T. species of A. benhamiae any antimycotic effective against dermatophytes can be used such as: imidazole(clotrimazole, bifonazole), ciclopirox or terbinafine.

Extensive infections by T. species of A. benhamiae, especially tinea capitis, should be treated immediately with oral antimycotics.

Note: In children, however, oral antimycotic treatment is only ever carried out as a so-called individual treatment attempt in accordance with the German Medicines Act with the written consent of the parent or guardian.

Hiernickel C et al (2016) Trichophyton species of Arthroderma benhamiae: Clinical therapeutic aspects of a new pathogen in dermatology. Dermatologist 67:706-711.

Kano R et al (2011) Arthroderma benhamiae (the teleomorph of Trichophyton mentagrophytes) mating type-specific genes. Mycopathologia 171:333-337.

Kawasaki M et al,(2002) Two arthroderma benhamiae isolates showing mitochondrial DNA type of Trichophyton verrucosum. Nihon Ishinkin Gakkai Zasshi 43:103-106.

Kawasaki M et al (2011) Multiple gene analyses are necessary to understand accurate phylogenetic relationships among Trichophyton species. Med Mycol J 52:245-254.

Kimura U et al.M (2015)Tinea faciei caused by Trichophyton mentagrophytes (molecular type Arthroderma benhamiae mimics impetigo: a case report and literature review of cases in Japan. Med Mycol J 56:E1-5.

Kupsch C et al (2017) Dermatophytes and guinea pigs. Dermatologist 68: 827-830

Nenoff P et al(2014) Trichophyton species of Arthroderma benhamiae - a new infectious agent in dermatology. J Dtsch Dermatol Ges 12:571-581.

Nenoff P et al.(2013) Kerion caused by the zoophilic dermatophyte Trichophyton species of Arthroderma benhamiae in a child. A new emerging pathogen of dermatomycoses in Germany. Dermatologist 64:846-849.

Shiraki Y et al (2006) Cytokine secretion profiles of human keratinocytes during Trichophyton tonsurans and Arthroderma benhamiae infections. J Med Microbiol 55:1175-85.https://www.ncbi.nlm.nih.gov/pubmed/16914646

Symoens F et al.(2013)The dermatophyte species Arthroderma benhamiae: intraspecies variability and mating behaviour. J Med Microbiol 62:377-385.

White TJ et al (1990) Analysis of phylogenetic relationships by amplificationand direct sequencing of ribosomal RNA genes. PCR protocols: A Guide to Methods and Applications, pp. 315-322, New York, Academic Press.