Peritoneal and scrotal carcinomas of unknown origin in two bovine calves.

Two unrelated bovine beef calves, aged 2 mo and 3 mo, were presented to The Ohio State University Veterinary Medical Center because of scrotal swelling and abdominal distension. On postmortem examination, there was abundant peritoneal fluid and numerous small friable masses covering all peritoneal surfaces and extending into the scrotum via the tunica vaginalis, with no identifiable primary neoplasm. Based on light microscopy, differential diagnoses included malignant mesothelioma and anaplastic carcinoma. Immunohistochemically, the neoplasms labeled positive for cytokeratin, and negative for vimentin and calretinin. Neoplastic cells contained periodic acid-Schiff-positive, diastase-resistant cytoplasmic granules, and lacked Alcian blue-positive, hyaluronidase-negative cytoplasmic vacuoles. Ultrastructurally, the cells had features of carcinoma, including secretory granules, and lacked typical features of mesothelioma, such as long slender microvilli. Our final diagnosis was carcinoma in both calves, despite the equivocal gross and light microscopic findings. We propose that a presumptive diagnosis of peritoneal mesothelioma in bovine calves should be avoided without corroboration by a combination of histology, histochemistry, immunohistochemistry, and, if possible, electron microscopy.