Eosinophilic myositis due to Sarcocystis hominis in a beef cow.

A case of eosinophilic myositis (EM) in an 8-year-old beef cow was investigated. The animal originated from a herd in which a high incidence of the disease had been observed in slaughtered adult females over a period of 2 years. Histologically, the lesions in the muscles were characterized as granulomas with a central core of degenerate eosinophils and remnants of necrotic muscle fibres, surrounded by a rim of epithelioid cells and fibrous tissue with an infiltrate consisting predominantly of eosinophils radiating outwards. Degenerate sarcocysts with a thick (7-9 microm) wall were present in the suppurative centre of most lesions. Intact sarcocysts with similar morphology were present in adjacent muscle fibres but without an associated inflammatory reaction. By transmission electron microscopy the sarcocysts were identified as Sarcocystis hominis, based on the morphology of villar protrusions of the sarcocyst wall, which were broad-based and cylindrical, with a blunt distal end, and contained numerous long microfilaments. Circumstantial evidence indicated a human source of infection, human faecal material having been spread on the pasture grazed by the cattle. The findings supported a causal relationship between S. hominis infection and EM in cattle.