Schistosoma haematobium in the baboon (Papio anubis): effect of vaccination with irradiated larvae on the subsequent infection with percutaneously applied cercariae.

Groups of five baboons were vaccinated three times at approximately six-weekly intervals at a rate of 1,000 organisms per kg of gamma-irradiated Schistosoma haematobium larvae. Five vaccines were tested: 3 and 20 Krad cercariae applied percutaneously; fresh 3 and 20 Krad mechanically transformed schistosomula injected intramuscularly; and cryopreserved 20 Krad schistosomula injected intramuscularly. These five groups and an unvaccinated control group were challenged percutaneously with 7,500 S. haematobium cercariae three months after the last vaccination. The efficacy of the vaccines was judged by faecal egg excretion, and by adult worm and tissue egg recoveries at necropsy 4.5 months after challenge. Significant protection, with 64 to 89% reductions in worm burden and parallel reductions in egg production, was achieved by all but the cryopreserved vaccine, although egg production was not significantly reduced in those female worms which did mature. Cercariae tended to give more protection than schistosomula and 20 Krad more protection than 3 Krad. No significant pathology could be detected in an additional baboon vaccinated with 20 Krad schistosomula but not challenged with cercariae. This is an encouraging result for the development of a live vaccine against S. haematobium.