Observations on the ability of repeated, light exposures to Schistosoma mansoni cercariae to induce resistance to reinfection in Kenyan baboons (Papio anubis).

Using groups of 4 baboons (Papio anubis), 21.0% of a trickle exposure of 480 Schistosoma mansoni cercariae/baboon (10/week for 48 weeks) were recovered as adult worms (Group A) compared with 72.6% from a single massive exposure of 2500 cercariae/baboon (Group C). Group B, exposed first to the trickle and then to the massive exposure simultaneously with Groups A and C, yielded a worm recovery of 54% which was slightly but significantly less than Group C. The trickle infection apparently induced a substantial resistance to light re-exposure but only a partial, although significant, resistance to a massive re-exposure. In Group B, however, there was a marked reduction in the degree of gross pathology, despite high tissue egg production by the mature challenge worms. The parasitological results, in terms of resistance to reinfection by and development of pathology to a trickle infection, are not materially different from those obtained in studies using conventional heavy primary and challenge exposures with S. mansoni. The significance of these findings is discussed in relation to the epidemiology of human schistosomiasis mansoni in which heavy exposures are believed to be the exception rather than the rule.