Fatal hepatic sarcocystosis in two polar bears (Ursus maritimus).

Fatal hepatic sarcocystosis was diagnosed in 2 polar bears from a zoo in Anchorage, Alaska. Gross lesions were icterus and systemic petechiae. Marked microscopic lesions were detected only in the liver and included severe random necrotizing hepatitis with hemorrhage. Only asexual stages of an apicomplexan parasite were detected within hepatocytes, and rare extracellular zoites were seen in foci of necrosis. The parasite divided by endopolygeny, and occasionally merozoites formed rosettes around a central residual body. Ultrastructural features of the merozoites included a conoid and low numbers of micronemes at the apical pole, centrally located nuclei, and absence of rhoptries. The parasites failed to react with anti-Neospora sp., anti-Toxoplasma gondii, or anti-Sarcocystis neurona sera. The microscopic and ultrastructural morphology of the parasite are most compatible with an apicomplexan protozoan of the genus Sarcocystis. The life cycle of this parasite in bears is not known.