Malakoplakia mimicking neoplasia in the urinary tract of an adult cat

An 8-year-old neutered female shorthaired cat presented with sudden worsening of a 3-week history of polyuria/polydipsia, dysuria, and lethargy. Abdominal ultrasound revealed a trigonal mass with extensive involvement of urethra and ureterovesical junctions resulting in bilateral hydronephrosis; the main differential was transitional cell carcinoma. A suction biopsy indicated marked septic inflammation, with no evidence of neoplasia. Escherichia coli, followed by a multi-resistant Pseudomonas aeruginosa were cultured from the urinary tract. Antibiotic treatment was initiated and bilateral subcutaneous ureteral bypasses were placed. Sequential traumatic catheterizations of the bladder demonstrated a transition from neutrophilic to granulomatous inflammation. Ultimately, fine needle aspirates of the mass revealed numerous, large, round cells with abundant, fine, purple cytoplasmic granules; occasional cells contained leukocytes, hemosiderin, and calcium-like crystals. Malakoplakia, or a granular cell tumor, were considered the most likely differential diagnoses. Histologically, the submucosa was expanded by numerous round cells with abundant, eosinophilic and PAS positive cytoplasmic granules (von Hansemann cells). Occasional intracellular and extracellular, 2 to 10 microns, basophilic structures were noted, which were positive for calcium (von Kossa and Alizarin Red stains), and frequently had a target-like appearance (Michaelis–Gutman bodies). Positive staining for CD18 confirmed histiocytic origin of the cells, supporting a final diagnosis of malakoplakia. Malakoplakia is a rare, granulomatous disease that can mimic neoplasia. Exact pathogenesis remains unclear, but impaired bactericidal function of macrophages is the proposed underlying mechanism. Only four cases have been reported in veterinary literature and, to our knowledge, this is the first cytological description of malakoplakia in domestic animals.