EQUINE VETERINARY EDUCATION / AE / NOVEMBER 2020


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a)


Fig 2: Cytological image of Cryptococcus spp. from a transtracheal wash with its characteristic wide, nonstaining capsules and narrow-based budding (Modified Wright's stain 340). Image courtesy of Elizabeth Welles, Auburn University, Alabama, USA.


culture, immunohistochemical staining using monoclonal antibodies or PCR is required (Stewart 2019b). If fungal organisms are suspected, then it is advisable to


request specific fungal culture on Sabouraud and inhibitory mould agars. In this case, although yeast colonies were grown after 8 days, RNA sequencing was required to identify the organism as Cryptococcus terrestris (Daniel et al. 2020). Serological testing with latex agglutination to identify cryptococcal capsular antigen is also useful for diagnosis, with resolution of lesions correlating with declining serum titres (Begg et al. 2004; Stewart et al. 2009; Secombe et al. 2017). These titres are readily available and usually inexpensive. Due to disease recrudescence or incomplete resolution, 6-month monitoring after cessation of therapy is recommended (Stewart et al. 2009). Cryptococci are ubiquitous, saprophytic, basidiomycetous


yeast-like fungi with the most common varieties being Cryptococcus neoformans (var. gattii and var. neoformans). There is an epidemiologic relationship between C. neoformans var. gattii and the Australian river redgum tree (Eucalyptus camaldulensis), while C. neoformans


var.


neoformans has historically been associated with bird (particularly pigeon) excreta (Riley et al. 1992). Cryptococcus terrestris is an unusual organism and it is interesting that the mare in this case report had a history of standing in pond water (Daniel et al. 2020). Cryptococcosis in horses is reported to cause rhinitis,


pneumonia, meningitis and abortion. Sinonasal cryptococcosis was successfully treated in two horses after extensive surgical d


ebridement and long-term treatment with oral fluconazole (Cruz et al. 2009; Stewart et al. 2009).


b)


Fig 3: a) and b) Photomicrographs of an impression smear from a biopsy of a mass in the nasal passage of a horse. The smear is extremely cellular with abundant neutrophils and macrophages. Round yeast cells with large nonstaining capsules and occasional narrow-based budding consistent with cryptococcal organisms are shown (Modified Wright's stain 340 and 100). Images courtesy of Elizabeth Welles, Auburn University, Alabama, USA.


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