Unilateral luteoma of the ovary in a pregnant Risso’s dolphin (Grampus griseus)

A white, lobular mass was found in the right ovary of a pregnant Risso's dolphin (Grampus griseus) at necropsy. The mass was unilateral and occupied most of the pre-existing ovarian tissue. Histologically, the mass was composed of diffuse sheets of polyhedral cells with abundant eosinophilic cytoplasm and oval nuclei, separated by fibrous connective tissue. Only a few ovarian follicles were observed at the periphery of the mass. Immunohistochemically, the large eosinophilic cells were positive for vimentin and negative for pan-cytokeratins. Based on the histopathological features, the present case was diagnosed as luteoma. In human medicine, luteoma of pregnancy, a tumor-like proliferative lesion occurring in pregnant women, is well described. In veterinary medicine, luteoma associated with pregnancy has never been described. The present study would provide useful information for understanding the characteristics of luteoma in animals.