Molecular identification of fungal pathogens in nodular skin lesions of cats

In a retrospective study, we investigated 52 formalin-fixed, paraffin-embedded (FFPE) samples from cats with histologically confirmed cutaneous and subcutaneous mycoses to determine if the pathogens could be identified by molecular methods. Aim of the study was to obtain a deep understanding of the spectrum of infectious agents, which, as we hypothesized, was not available by histopathology alone. Detection of feline and fungal DNA was achieved in 92.3% and 94.2% of the samples, respectively. Most of the subcutaneous infections in cats were caused by Alternaria spp. (63.5%), followed by Cryptococcus neoformans (7.7%), Histoplasma capsulatum (5.8%), Sporothrix spp. (3.8%), Aspergillus vitricola, Aureobasidium pullulans, Exophiala attenuata, Fusarium oxysporum, Lecythophora cateniformis, Microsporum canis, and Phialophora sp. (1.9% each). The results from molecular identification indicate that correct identifications of the fungal pathogens by histology alone were rarely possible. The spectrum of fungal pathogens identified after DNA extraction from FFPE samples was much broader than that expected by classical histopathology. This was especially noted in alternariosis in that the micromorphological pattern in tissue was misleading and could be confused with that of cryptococcosis. Due to different susceptibilities to antifungal agents, it is important to arrive at a definitive diagnosis, which might be possible by examination of the fungus recovered in culture and/or molecular methods, in addition to the histopathologic techniques.