Life Cycle of Isospora canis Nemeséri, 1959 in the Dog*

SYNOPSIS The life cycle of I. canis Nemeséri, 1959 was studied in experimentally infected dogs. Freshly sporulated oocysts were ovoid and 34–40 × 28–32 μm. The endogenous stages were found directly beneath the epithelium of the distal portion of the small intestinal villi. Most of the endogenous stages were in the lower 1/3 of the small intestine, but occasionally they were found in other portions of the small intestine. Three asexual generations were present. First‐generation schizonts were 16–38 × 11–23 μm and contained 4–24 merozoites; mature 1st‐generation merozoites were 8–11 × 3–5 μm. First‐generation schizogony lasted up to 7 days after inoculation. Second‐generation schizonts were 12–18 × 8–13 μm and contained up to 12 merozoites which were 11–13 × 3–5 μm. Second‐generation schizogony was present on postinoculation days 6 and 7. Third‐generation schizonts were formed by nuclear division of 2nd‐generation merozoites. Most 2nd‐generation merozoites underwent nuclear division without leaving the parasitophorous vacuole of the 2nd‐generation schizont. Mature 3rd‐generation schizonts were 13–38 × 8–24 μm and contained 6–72 merozoites. Third‐generation merozoites were 8–13 × 1–3 μm. Third‐generation schizogony was present on days 6–8 after inoculation. Mature macrogametes were 22–29 × 14–23 μm. Mature microgametocytes were 20–38 × 14–26 μm. Gametes were present on postinoculation days 7–10. Oocysts were present in tissue sections on postinoculation days 8–10 and 12. The prepatent period was 9–11 days.