Development of the sheep-canid cycle of Sarcocystis tenella

Thirty-seven sheep were inoculated orally with 10 4 to 66 × 10 6 Sarcocystis tenella sporocysts from dogs or coyotes and necropsied between 1 and 276 days postinoculation (DPI). Two single zoites were found in submucosal arteries of small intestines of lambs 3 and 6 DPI. At 9 to 21 DPI, numerous first-generation meronts were found in mesenteric arteries, intestinal blood vessels, and occasionally in other organs. First-generation meronts were 37.0 × 27.3 μm in sections and 54.4 × 46.6 μm in smears. At 16 to 40 DPI, second-generation meronts were found in capillaries of several organs; renal meronts were 20.6 × 12.0 μm in sections. Merozoites were found in peripheral blood at 14 to 16 DPI and at 25 to 32 DPI. Earliest sarcocysts were found at 35 DPI and contained one to three metrocytes. At 40 DPI, sarcocysts were up to 97 × 10 μm and contained 1 to 13 metrocytes. Bradyzoites formed between 52 and 66 DPI. At 75 DPI, sarcocysts had cross-striated walls and were infective for coyotes. Dogs and coyotes fed infected meat shed sporocysts in feces 9 to 10 days later. Gamonts were found in the small intestine between 8 and 21 h postinoculation and sporulation was completed within 8 days.