Recognition of cyclic transmission of Sarcocystis muris by cats.

Sarcocystis muris, obtained from laboratory and house mice, was transmitted experimentally to cats. When cysts in the skeletal muscle of mice were ingested by cats, bradyzoites developed directly into gametes in the intestinal mucosa of the cats. After five to 11 days, infectious sporulated sporocysts (8.5 x 10.3 mum) were shed by cats for up to three months. After ingestion of the sporocysts, S. muris multiplied first in the livers and then in the skeletal muscle of mice. Cysts in skeletal muscle attained a length of 5-6 mm and were visible to the naked eye. With heavy infections the cysts outlined major muscle groups. Lesions in mice were apparently the result of degeneration or rupture of cysts, with myositis and sometimes muscle necrosis, leading to locomotor impairment in heavy infection. S. muris required approximately 76 days of development in mice before becoming infectious for cats. S. muris is perpetuated by an obligatory two-host cycle (mouse-cat). Both cats and mice could be infected repeatedly. Rats, hamsters, and guinea pigs did not become infected.