Dermatophytosis caused by Nannizzia nana (Microsporum nanum): a comprehensive review on a novel pathogen

Abstract

Keratinophilic fungi are mostly soil-inhabiting organisms with occasional infections in humans and animals. Even though most dermatophytes are host-adapted, cross-species infections are common by zoophilic and geophilic dermatophytes. N. nana is considered an etiological agent of ringworm in pigs but has also been isolated from other animals, including humans. However, it also possesses many characteristics of geophilic dermatophytes including the ability to grow in soil. N. nana produces characteristic pear-shaped macroconidia and usually exhibits an ectothrix pattern of hair infection. It has been isolated from dermatitis lesions as well as from soil. N. nana infections in pigs are not of much concern as far as economy or health is concerned. But it has been associated with onychomycosis and gonathritis in humans, which are significant in human medicine. The shift in the predominance of dermatophytes in humans and the ability to evolve into a potential tinea pathogen necessitates more understanding of the physiology and genetics of N. nana. In this review, we have attempted a detailed analysis of the studies about N. nana, emphasizing growth and cultural characters, physiology, isolation, infection in humans and animals, molecular characterization and antifungal susceptibility.

Keywords: Cross-species infection; Dermatophytes; Microsporum nanum; Nannizzia nana; Ringworm; Skin disease.

Fig. 1

Micro- and macroscopic morphology of Nannizzia nana. (A) Obverse of N. nana culture. (B) Reverse of N. nana culture. (C) Reddish discoloration on Dermatophyte Test medium due to N. nana growth. (D) Micro-morphology of N. nana, arrows indicate characteristic macroconidia (× 400) (D’) Macroconidia stained with lactophenol blue (× 1000). Reproduced from Gnat et al. 2020 [15] under a Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/)

Fig. 2

Conidia produced by N. nana. (A–C) Macroconidia. (B) Microconidia. Scale bars = 10 μm. Reproduced from Dukik et al. 2020 [21] under a Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0)

Fig. 3

Schematic representation of cleistothecia. Image created using Biorender.com

Fig. 4

Conidia of Trichothecium roseum stained with lactophenol cotton blue

Fig. 5

Maximum likelihood concatenated tree of Nannizzia species based on ITS LSU, TUB2, TEF3, and RP 60S L1. AUT: authentic strain. T: type strains. A to E: different clades within genus Nannizzia. Reproduced from Dukik et al. 2020 [21] under the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/)