Jl. Traubdargatz et al., MULTIFOCAL MYOSITIS ASSOCIATED WITH SARCOCYSTIS SP IN A HORSE, Journal of the American Veterinary Medical Association, 205(11), 1994, pp. 1574-1576
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.