Organisms in an aspirate from an ulcerated mass: etiologic agent or mass-querade?

Smears prepared from a fine-needle aspirate of an ulcerated, dermal mass between the digits of a 9-month-old Bull Terrier puppy revealed a moderately cellular, mixed-cell inflammation with numerous extracellular organisms. The organisms were 3-8 mu m in length, curved to oval in shape, with prominent nuclei, and (retrospectively) a rare single, narrow-based bud. The cytologic diagnosis was mycotic dermatitis due to infection with an unusual yeast. Histologic sections of the lesion showed pyogranulomatous inflammation without evidence of in infectious organism. Contamination of the cytologic smears was suspected to have taken place in the clinical pathology laboratory and was confirmed by the isolation of a yeast with identical microscopic features from a cactus located on a windowsill near the work bench. The yeast was cultured and identified as Rhodotorula minuta, an airborne contaminant and ubiquitous saprophyte. It can be difficult to differentiate external contamination from infection when interpreting a cytologic specimen, however, it is important to do so, especially for unusual or opportunistic pathogens.