Microsporum ferrugineum: The Renaissance of a Forgotten Dermatophyte

In Germany, the anthropophilic dermatophyte Microsporum ferrugineum has been rarely isolated for the last 50 years. Currently, started in 2016, M. ferrugineum strains were occasionally identified although this dermatophyte is difficult to differentiate from Microsporum audouinii or Microsporum canis. Furthermore, a small outbreak of infections was observed in Germany. From July 2016 until April 2019, 19 patients with M. ferrugineum infections were diagnosed all over Germany. This included 15 both culture and molecular confirmed cases, and four cases exclusively diagnosed based on sequencing of fungal DNA. Cultural analysis revealed slowly growing colonies with white thallus and peripheral yellow-brownish submerged hyphae bundles. The reverse side of the furrowed colonies showed cream color to yellow staining. Microscopically, big spherical and oval double-walled intercalary-localized chlamydospores, typical “bamboo” hyphae, and acute-angled branched hyphae were observed. Fungal culture material from all isolates was identified by polymerase chain reaction (PCR), Sanger sequencing of the internal transcribed spacer (ITS) region, and/or the translation elongation factor (TEF)-1α gene. Results were referred to the M. ferrugineum CBS 497.48 strain (Centraalbureau voor Schimmelcultures CBS, Utrecht, The Netherlands, www.westerdijkinstitute.nl). Patients were children and adolescents under 18 years, mainly males. Suggested source of infection was martial sports, e.g., wrestling, judo, and boxing. Surprisingly, a significant part of affected patients were Germans of Russian descent. A migrant 3-year-old boy from Afghanistan suffering from Tinea capitis was also among the patients. Another strain was isolated from a 10-year-old wrestler with suspected Tinea corporis. There was no migration background or contact to foreigners, the boy did not stay abroad. The mycological challenge is the cultural identification of M. ferrugineum due to the morphological similarity not only to M. canis, but also to M. audouinii, and Trichophyton verrucosum. Phylogenetic analysis of ITS region of ribosomal DNA and the TEF-1α gene was performed using MEGAX, the statistical maximum likelihood method, and the Tamura-Nei substitution model. Bootstrapping was performed with 1000 replicates. The alignment of the ITS sequences (partial 18.S, ITS1, 5.8S, ITS2, partial 28.S) with known sequences deposited at the Genbank database available at the National Center for Biotechnology Information (NCBI), Bethesda, Maryland, U.S., clearly identified the M. ferrugineum strains after cultural analysis. The phylogenetic analysis of the dermatophytes—the dendrogram of fungal strains—demonstrated the genetic differences between M. ferrugineum strains and M. audouinii or M. canis. The three species could be clearly distinguished from each other. In particular, sequencing of the TEF-1α gene allowed a better differentiation between M. ferrugineum and M. audouinii or M. canis than sequencing of the ITS 2 region.