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The efficacy as an immunizing agent of the Kenyan strain of Schistosoma mansoni against challenge infection in rhesus monkeys was demonstrated. An initial exposure of 200 cercariae immunized monkeys against a challenge dose of 2,000 cercariae administered 16 weeks later. The penetration rate in rhesus monkeys was 99%, the same as in baboons. The prolongation of the time for immunity to develop in baboons, compared to rhesus monkeys, shown with this strain of S. mansoni is therefore not due to a reduced potential for immunization by this strain in the rhesus monkey.

Parasitological, clinical, and histopathological observations on 54 baboons infected with Schistosoma mansoni are presented. The baboon and S. mansoni constitute a compatible host-parasite system, evidence by the infectivity of cercariae (98% penetration, 42% adult worm recovery), and the long, fertile life of the worms. Baboons tolerated the infection well, with clinical illness a rarity in moderately infected baboons. Pathological findings were generally unremarkable. An acute "toxemic" phase occurred 66 days or less following a large cercarial exposure in three baboons. Worm burdens were not significantly reduced during the course of prolonged infection, but prolonged infections resulted in decreased oviposition by the worms and in an anterior shift in egg deposition from the colon to the small intestine. Concomitant immunity was also a feature of baboon infections. Decreased oviposition and the anterior shift are probably manifestations of a second phase of immunity, distinct from concomitant immunity. The baboon is similar to man and the grivet monkey in that in all three species immunity is slow to develop.