Schistosoma haematobium in the baboon (Papio anubis): assessment of protection levels against either a single mass challenge or repeated trickle challenges after vaccination with irradiated schistosomula.

Baboons vaccinated intramuscularly with three times 9000 20 krad irradiated Schistosoma haematobium schistosomula at monthly intervals were exposed percutaneously to either a single mass challenge of 3000 (VMC) or ten, weekly trickle challenges of 300 (VTC) normals S. haematobium cercariae. Unvaccinated mass (MCC) or trickle (TCC) challenge controls were exposed simultaneously. Faecal and urine egg production was delayed in the vaccinated groups which also had reduced adult, particularly female, worm recoveries. Total faecal, urine and tissue eggs were lower in the vaccinated groups, as also were the size of granulomata and the gross pathology and severity of inflammatory responses in the bladder and ureters, except for an increased proportion of tissue eggs in the livers of vaccinated animals. Differences in pathology between groups were less marked in the other organs. Most indices were reduced in the trickle versus the mass challenge control groups. Some of these trends were statistically significant, mostly in the trickle vaccination group (VTC), but others were not. Compared with the appropriate unvaccinated controls, the percentage reduction for the trickle challenge (VTC) group (73%) was three times greater than that of the mass challenge (VMC) group (23%). Overall, the protective effect of vaccination was more clearly demonstrated in the trickle than in the mass challenge groups. This conclusion is based on a single experiment. Nevertheless, because trickle infections probably approximate more closely to what humans receive naturally, it is recommended that they should be used for future testing of all potential vaccines in baboons.