Dermatophytes are a group of phylogenetically related fungi that cause dermatophytosis. Until a couple of years ago these fungi were included in the genera Epidermophyton, Microsporum and Trichophyton, which were not monophyletic. For this reason, and coinciding with the abandonment of the dual nomenclature of fungi in 2013 by the International Code of Nomenclature for algae, fungi, and plants, there has been an increase in the number of genera that group these species. According to the new proposal of phylogenetic taxonomy of dermatophytes,1 some of these species are currently included in the genera Lophophyton or Nannizzia.

Along with evolution, some of these fungi became adapted to certain animal species. Anthropophilic species, such as Trichophyton rubrum or Epidermophyton floccosum, are those that only cause dermatophytosis in humans and rarely infect other animals. However, humans may be infected by zoophilic species, such as Microsporum canis (Fig. 1) and Trichophyton verrucosum, which characteristically cause dermatophytosis in cats and cows, respectively, and are the cause of frequent zoonoses.

Fig. 1.

Macroconidium of Microsporum canis. This species is the main causal agent of ringworm in cats and dogs.

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Other dermatophyte species that infect animals have had their names changed. Lophophyton gallinae (formerly Microsporum gallinae) in the case of poultry ringworm, and Nannizia nana (formerly Microsporum nanum) in the ringworm of pigs, are two examples. The presence of this mycosis is usually very small in the current livestock production systems of these animal species. However, it can increase when farm hygiene is poor. Although the cases are not frequent, they can appear in the form of outbreaks, such as the one that affected not many years ago 100% of lactating sows in an extensive Iberian pig farm.2

There are also geophilic species such as Nannizzia gypsea (formerly Microsporum gypseum), whose normal habitat is the soil, that can cause dermatophytoses in both humans and animals. On the other hand, there are other species that share a common morphology and phylogeny with dermatophytes, but that have not been described as etiological agents of dermatophytosis, or their infections are cited as questionable. Two examples are the typically geophilic species Arthroderma uncinatum (formerly Trichophyton ajelloi) or Paraphyton cookei (formerly Microsporum cookei) (Fig. 2). Little by little we will have to get used to these changes.

Fig. 2.

Macroconidium of Paraphyton cookei (formerly Microsporum cookei). This species does not produce ringworm, but it can be isolated from the fur of animals.

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