The in vivo excystation of Sarcocystis gigantea and S. tenella sporocysts

In vivo studies on the excystation of Sarcocystis gigantea and S. tenella sporocysts indicated that this process was, as in vitro, a diphasic one involving both pretreatment and treatment phases. The studies also tended to support in vitro observations that the requirements for the excystation of these two species are quite different. The results suggested that for neither species was the pretreatment stimulus likely to be provided by conditions in the rumen alone. However, exposure to abomasal conditions only induced moderate levels of excystation in both when they were subsequently treated with trypsin and bile. For S. gigantea, 0.25 to 4 h abomasal exposure was most effective; for S. tenella, 24 hours. The stimuli necessary to complete the excystation process could, apparently, be provided by 1 h placement in the duodenum for S. gigantea but not for S. tenella.