Unusual dermatomycoses caused by Nannizzia nana: the geophilic origin of human infections

Dermatophytes are keratinophilic fungi that affect nails, hairs, and skin of humans, warm-blooded animals, and others [1, 2]. Approximately, 20–25% of the global human population is infected with a dermatophyte at least once per lifetime [3, 4]. About 30 clinically relevant dermatophyte species are known, but their taxonomy has been controversial because of the incongruence of phenotypic and molecular characters [5, 6].

The genus Nannizzia was introduced by Stockdale [7] with Nannizzia incurvata Stockdale 1961 as a type species to accommodate Microsporum-like species producing gymnothecia, which were discovered in 1927 by Nannizzi. Most of the species classified in the genus Nannizzia were described with double nomenclature after finding their heterothallic sexual form [1, 8]. Currently, for practical reasons and necessity, de Hoog et al. [6] proposed the sexual names as nomenclatural reference with molecular differentiation as the leading classificatory principle. Nannizzia (likewise the genus Arthroderma) is separated as an independent, holomorphic genus located between Trichophyton and the preponderantly zoophilic genus Microsporum [6]. Several species were found to cluster in the well-demarcated Nannizzia group, e.g. N. nana (C.A. Fuentes) Y. Gräser and de Hoog (2016) [6], formerly known as Microsporum nanum C.A. Fuentes (1956)[9].

Nannizzia nana is the common cause of ringworm in the pig [4, 10]. In the 80s, approximately 27% of pigs were infected by N. nana, since these infections were easily transmitted between animals in the same flock [10]. In recent years, there are scarce reports on the occurrence of infections caused by this dermatophyte species. In 2009, an outbreak of ringworm caused by N. nana was reported in sows in Spain [11]. In exceptional cases, infections have been reported in other animals such as dogs, cats, and mice [12‐14]. It is also important to underline that this dermatophyte species is usually acquired directly from the soil, rather than from other animal hosts [4, 14]. N. nana is distributed worldwide, but only few cases have been described in humans [11, 12, 15].

The direct analysis of the material revealed the presence of arthrospores in the samples collected from the skin lesions. The macro- and micromorphology of the colonies and mycelial structures obtained from the clinical material of the three patients were almost identical and suggested that all the strains belong to one specific species (Fig. 2). After 3 weeks of incubation, the colonies were gently fluffy and cream to beige in colour with a suede-like texture; reddish-brown pigmentation was present on the reverse side of the colony. Microscopic examination of the culture-derived preparations stained with lactophenol cotton blue (LPCB) revealed short pyriform thick-walled macroconidia with one to three (mostly two) cells. Based on the morphology and classical phenotypic methods, all the three dermatophyte isolates were identified as Nannizzia nana. A comparative analysis of ITS sequences (PCR products obtained with ITS1 and ITS4 primers) of the isolated strains with the sequences of reference strains available in the NCBI (National Center for Biotechnology Information) database revealed a 99% similarity to Nannizzia nana CBS 314.54 (Table 1). The MP-PCR showed genomic diversity of the examined dermatophyte strains. The examination based on agarose gel electrophoregram indicated two different types of profile: one characteristic for the strain isolated from the man and the other one specific for the clinical strains obtained from both women (Fig. 3). Screening of the material taken from animals with which the women had contact showed no presence of the dermatophytes.

In vitro activities of antifungal agents that can potentially be used either orally or topically showed that all the isolates tested were susceptible to itraconazole, ketoconazole, terbinafine, and naftifine hydrochloride (Table 1). Minimal inhibitory concentration (MIC) values among the isolates varied from 0.125 to 0.5 µg/ml. Geometric means of the minimal inhibitory concentration of ketoconazole (0.167 µg/ml) and terbinafine (0.208 µg/ml) were the lowest for the examined isolates, indicating that these drugs were the most potent in the therapy of each of the cases described in this work. Nonetheless, the mean MICs of the antifungal drugs did not exhibit statistically significant differences between each other (p > 0.05).

Nannizzia nana, reported previously as Microsporum nanum, was first described in 1954 by Fuentes et al. [9] in Cuba as the aetiological agent of tinea capitis in a child, identified as a dwarf form of Microsporum gypseum [22]. Swine are the natural host for this dermatophyte [22]. N. nana is regarded as a low-virulence aetiological agent; it was even thought to be part of pigs’ skin microbiota [23]. The majority of human patients with dermatomycosis caused by N. nana was linked with direct daily or long-lasting contact with these animals on pig farms [15, 22, 24, 25].

Clinical pictures of N. nana infections in humans are related mainly to tinea corporis with a characteristic ring shape with an erythematous, scaly, circinate plaque and, rarely, dry or inflammatory tinea capitis [11]. Occasionally, lesions on the skin may suggest Microsporum canis infection [15]. Bonifaz et al. [22] described two severe cases of N. nana infection in siblings in 2019, in which an 8-year-old boy was affected by mycosis of the scalp developing as a pseudoalopecic tumour lesion and a 6-year old girl, his sister, presented with dermatomycosis characterized by multiple erythematous-scaly plaques on her face, trunk, and arms. Dermatomycosis among children in Mexico is closely related to the coexistence with pigs on the same farm [22]. The authors of these publications do not consider other sources of N. nana infection in these two cases.

Our study reveals, however, completely different characteristics of this dermatophyte species. The three reported cases occurred in Poland in the summer months of 2018 in patients living at a distance of about 150 kms from each other. None of them reported contact with pigs. In addition, the location of the clinical lesions, which covered the edge of the foot and toenails in two of the three described cases, did not indicate a zoonotic origin of the dermatomycosis and might suggest rather a geophilic source. In the literature, there are also reports on exceptional cases of tinea pedis and onychomycosis caused by N. nana [26, 27]. Furthermore, N. nana reservoirs should also be sought in soil, and the information on the dual nature of this dermatophyte, both zoophilic and geophilic, seems to be relevant [28].

Interestingly, genotyping of clinical isolates by MP-PCR showed no homology of the strains. Two different types of profile were obtained: the first one characteristic for the male patient and the other one determined for the two female patients. Finding causal links between this genomic similarity and infection epidemiology can only be speculative, but it is important that methods for determining genomic diversity can also be used for N. nana. There are no reports in the literature about genomic polymorphism in this species of dermatophyte. In many outbreaks of dermatomycoses of different aetiology, e.g. caused by Microsporum canis [17, 29, 30] or Trichophyton verrucosum [20], genotyping methods have been found greatly suitable in searching for the most probable sources of infection and determining pathogen transmission pathways. Comprehensive analysis on this issue for N. nana infections is necessary to reach definitive findings.

Unfortunately, in the literature, there are only few reports on the minimal inhibitory concentration (MIC) values of N. nana. Hence, antifungal therapy is usually chosen based on the response observed when treating other dermatomycoses caused by fungi of the genus Microsporum [22, 31]. In our study, the in vitro susceptibility to the main antifungals indicates that N. nana is highly sensitive to itraconazole, ketoconazole, terbinafine, and naftifine hydrochloride. Wildfeuer et al. [32] observed the following MICs for N. nana (Microsporum nanum in the original publication): griseofulvin 3.1 μg/ml; voriconazole and itraconazole 0.78 μg/ml, and ketoconazole 0.2 μg/ml. In turn, in their case report of dermatomycoses in the siblings, Bonifaz et al. [22] noted that N. nana was sensitive to miconazole, clotrimazole, and ketoconazole without specifying the inhibitory concentration range of these drugs. Furthermore, there are no precise data in the literature about N. nana sensitivity toward terbinafine and naftifine hydrochloride. Noteworthy, these studies include only few strains. This issue requires more extensive research and deeper discussion.