Intracellular success: cytologic findings in an ulcerated submandibular mass from a cat

A 1-year-old neutered male domestic shorthair cat had an ulcerated, proliferative lesion in the submandibular area that did not respond to antibiotic therapy. Impression smears from the mass revealed septic pyogranulomatous inflammation, with large numbers of pleomorphic bacteria observed intracellularly within macrophages as well as neutrophils. Bacterial culture was consistent with a diagnosis of Rhodococcus equi, a facultative intracellular coccobacillus capable of replicating within macrophages. The cat's lesion resolved after treatment with rifampin and clarithromycin. R equi should be considered as a differential diagnosis when coccobacilli are recognized within macrophages in cytologic samples.