Isospora (Toxoplasma) gondii: induction of sexuality

The role of the stomach and small intestine of cats and of the parasite's rate of multiplication on induction of sexuality in Isospora (Toxoplasma) gondii was studied by injecting cats either with free cystozoites or proliferative parasites directly into the intestinal lumen, after laparotomy. Oocyst production was similar in cats infected orally with cysts or cystozoites and in cats infected by inoculation into the duodenal lumen with free cystozoites, obtained by either mechanical rupture of by in vitro pepsin/HCl digestion of the cyst wall. When free cystozoites were injected into the lumen of the posterior part of the ileum, cats became seropositive but oocyst excretion during the first 4 weeks after infection was very low or absent; nevertheless a solid immunity to oral challenge was acquired in the former case. If no oocysts at all were produced after primary infection, the intestine remained susceptible to challenge, in spite of previous seroconversion. This lack of protective immunity in the presence of serum antibodies was observed in all cats primarily infected by direct injection of proliferative parasites into the duodenal lumen. It is concluded that previous gametogony rather than previous infection and seroconversion provokes local intestinal immunity against the development of sexual stages after oral challenge; the switch to gametogony of cystozoites is not triggered off by the low pH of the stomach but is probably related to their reduced rate of multiplication.