Nasal cryptococcosis in a cat
History: 4 year-old female spayed DSH cat developed a nasal swelling and inspiratory noise. Biopsies were submitted.
Nasal turbinate: The submucosa is expanded by large numbers of fungal yeasts forming sheets. The yeasts are 15-20 um in diameter with a 4-6 um thick clear capsule and a 4-5 um diameter oval basophilic nucleus. There are small numbers of macrophages, neutrophils, and lymphocytes in the submucosa. The nasal mucosa is multifocally eroded.
This is a typical case of nasal Cryptococcosis in a cat. There is generally little inflammation, and the histologic appearance of Crpytococcus neoformans is typical: large sheets of foamy yeast bodies with basophilic nuclei. The mucicarmine stain colors the polysaccharide inner capsule and cell wall of the yeast red, but the outer capsule remains clear. The nucleus is not visible. In capsule deficient forms of Crpytococcus Fontana masson stain is used to detect melanin in the yeast, and is an important diagnostic tool which differentiates the capsule deficient Cryptococcus from Histoplamsa and other yeasts. In my experience the mucicarmine stain is variable, and often does not stain more than the cell wall and parts of the inner capsule. There is typically a large clear capsular zone even in the mucicarmine stain. This is the most helpful diagnostic sign.