MULTIFOCAL MYOSITIS ASSOCIATED WITH SARCOCYSTIS SP IN A HORSE

Multifocal myositis was diagnosed in a 7-year-old Quarter Horse geldin g on the basis of history and findings on physical examination, serum biochemical analysis, electromyography, and microscopic examination of frozen sections of muscle biopsy specimens. Histologic examination of the muscle specimen revealed multifocal accumulations of histiocytes, lymphocytes, and plasma cells, with attendant myofiber degeneration a nd necrosis. Parasitic cysts with morphologic characteristics of Sarco cystis sp were found in regions of myocyte degeneration and necrosis, and in regions of normal muscle. Based on a tentative diagnosis of Sar cocystis sp-induced myositis, the horse was treated with trimethoprim/ sulfamethoxazole and pyrimethamine for 28 days, phenylbutazone for 5 d ays, and paddock rest for 30 days. At the end of treatment, the horse had gained 35 kg, its appetite had returned to normal, and muscle mass was returning to normal. Sarcocystis fayeri is the only Sarcocystis s p reported in equine muscle in the United States and is rarely associa ted with acute myositis or muscle atrophy. The development of clinical signs in this horse could have been the result of an underlying immun osuppression or infection with a particularly pathogenic strain or lar ge infective dose of S fayeri.