Fine structure of Sarcocystis arieticanis Heydorn, 1985 in its intermediate and final hosts (sheep and dog).

The developmental stages of S. arieticanis (sheep-dog-cycle) were studied by means of electron microscopy. After oral infection of the sheep with sporocysts two generations of schizonts occur around days 14 and 27 p.i. Both are morphologically more or less similar and are situated immediately inside the cytoplasm of their host cells, which are endothelial cells. They produce somewhat more than 100 merozoites reaching a size of about 6-8 micron X 2.5 micron. The tissue cysts are characterized by hair-like protrusions of the primary cyst wall. These protrusions (reaching a length of 11 micron at the maximum) do not contain fibrils, are folded over and thus running along the surface. These cysts may then appear in light microscopical sections with a smooth cyst wall. The fine structure of the cyst wall was significantly different from the other sheep sarcosporidia. No differences were found in morphology of the cyst merozoites reaching a size of 15 X 2.5 micron. Inside the dog gamogony occurred in the cells of the lamina propria of the lower part of the small intestine. Gamogony was mostly completed after 20 h p.i. and sporogony after 12-14 days p.i. The fine structure of the stages in the final host was similar to that of other Sarcocystis species.