Early developmental stages of Sarcocystis cruzi in calf fed sporocysts from coyote feces.

The development of Sarcocystis cruzi was studied in an 11-day-old calf killed seven days postinoculation with 5 x 10(8) sporocysts from feces of coyotes. Uninucleate zoites were found in arteries of mesenteric lymph nodes but not in other organs. Zoites measured 4.9 x 3.0 (3.5-7.0 x 2.1-3.5) micrometers. Of the 36 zoites studied, 31 were in endothelial cells, four were in macrophages in the lumen of arteries, and one was free in the lumen of an artery. Infected endothelial cells were two to three times larger than uninfected cells. Zoites appeared structurally similar to sporozoites. The occurrence of zoites in macrophages suggests that sporozoites of Sarcocystis might use such cells to reach the site of their first merogony.