Hepatic sarcocystosis in a horse

Hepatic sarcocystosis was diagnosed in a horse in association with refracto ry bacterial osteomyelitis and plasma cell tumor of the maxilla and hepatic salmonellosis. Gross lesions included pleural, pericardial, and peritoneal effusions, hepatomegaly, gastric ulceration, colonic edema, and proliferat ive tissues filling 2 maxillary dental alveoli. Histologically, liver was c haracterized by severe suppurative, necrotizing, periportal hepatitis, and severe periacinar necrosis. Hepatocytes frequently contained protozoal schi zonts in various stages of development. in mature schizonts, merozoites wer e often arranged radially around a central residual body, consistent with a sexual division by endopolygeny. Ultrastructural features of merozoites inc luded an apical conoid and polar ring, anterior micronemes, central nuclei, and absence of rhoptries. These protozoa did not react to antisera raised against Neospora caninum, Sarcocystis neurona, Toxoplasma gondii, or Hammon dia hammondi. The microscopic and ultrastructural characteristics and immun oreactivity of this organism are consistent with a Sarcocystis sp. other th an S. neurona. This is the first report of Sarcocystis-associated hepatitis in a horse. The life cycle of this organism and source of infection are un known.