Groups of 20 mice were dosed once orally with either 20, 200, 2,000 or 20,000 Sarcocystis muris sporocysts and killed at 8, 16 or 24 weeks post inoculation. Muscle samples were examined for sarcocysts in histologic sections. At 8 weeks, all or most mice in each group had sarcocysts. For mice given 20 sporocysts, there was a significant reduction in the number with sarcocysts and the median sarcocyst count at 16 and 24 weeks. For mice given 200 sporocysts, there was no difference in number with sarcocysts or median sarcocyst count at 8, 16 or 24 weeks. For mice given 2,000 or 20,000 sporocysts, there was no difference in the number of mice with sarcocysts at 8 and 24 weeks, but sarcocyst numbers at 16 weeks were significantly less than at 8 weeks. The median sarcocyst count was less at 16 weeks for mice that received 2,000 sporocysts, but there was no significant difference in the counts at 8, 16 or 24 weeks for the high sporocyst dose groups. In a second experiment, mice were dosed with 10 sporocysts, kept under specific-pathogen-free conditions and groups examined every 2 weeks until week 38. The median number of sarcocysts showed a significant decrease between 8 and 12 weeks followed by a significant increase between 12 and 16 weeks and then a significant decrease by 18 weeks (all p<0.05). These experiments show that from a single oral dose of sporocysts, the number of mice with sarcocysts and the median number of cysts are determined by the sporocyst dose level and the duration of the infection. It is suggested that these observations could be due to cyclical development of sarcocysts.
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