The developmental cycle of a species of Sarcocystis occurring in dogs and sheep, with observations on pathogenicity in the intermediate host.

Twelve dogs were fed mutton containing small sarcocysts, and killed 1, 3, 4, 6, 7, 10, 15, 16, 17 days after infection (DAI). Beginning 13-15 DAI sporocysts 14.7 times 9.0 mum were passed in the faeces of the dogs killed 15-17 DAI. Histological examination showed that developing stages were most numerous in the subepithelial tissue at the tips of villi in the proximal third of the small intestine. Macrogametes containing tiny PAS + granules, and microgametocytes with peripheral developing microgametes were present 1 DAI. By 4 DAI oocysts, with a small nucleus and vacuolate cytoplasm were seen. Sporulation was observed 7-10 DAI. The first nuclear division resulted in 2 polar nuclei which divided laterally, resulting in 2 sporocysts each with 2 polar nuclei. This process was repeated once more to produce 4 nucleated sporozoites in each of 2 sporocysts. PAS + granules were seen at the periphery of sporulating oocysts and sporocysts. There was a large PAS + granule in the mid zone of sporozoites, with a smaller granule at one tip. Numerous sporulated sporocyst pairs were present beneath the epithelium at the tips of villi in dogs killed during patency. Four lambs were inoculated orally with sporocysts passed by dogs following feeding of infected mutton. Fifteen DAI schizonts were seen in the endothelium of arteries and arterioles in many organs, but not brain. Twenty-four DAI, smaller schizonts were seen in capillary endothelium in many organs, including brain. The two other lambs died 42 and 104 DAI, after an illness characterized by anaemia and ill-thrift. Mature schizonts were found in cells in the brain 42 DAI, associated with nonsuppurative meningoencephalitis. Developing sarcocysts were found in muscle, associated with myositis. Sarcocysts in muscle 104 DAI were mature. In the brain there were degenerate cysts and mature sarcocysts, and nonsuppirative meningoencephalitis.