Cross resistance between Schistosoma haematobium and S. mansoni in the baboon

Abstract Experiments were carried out to determine whether infection of the baboon with Schistosoma haematobium could stimulate significant acquired immunity to S. mansoni . Baboons immunized with S. haematobium were first shown to have developed a strong immunity to the homologous infection before challenge with the heterologous parasite. The baboons immunized with S. haematobium developed resistance to the S. mansoni challenge infections, as reflected in a reduction of mature worms and of S. mansoni egg excretion, compared with those of the challenge control animals. None of the immunized baboons lost weight or appeared to be sick, their PCV values did not decrease and they produced very little haemorrhagic diarrhoea. The observed pathology of the large intestine was much more severe in the challenge control baboons than in the immunized animals, very little host reaction being provoked by many of the eggs retained in the tissues of these animals. The antibodies being measured by the ELISA test are different to those being estimated by the antibody-mediated cellular cytotoxicity test. It is concluded that a baboon infected with S. haematobium can develop a marked degree of acquired immunity to S. mansoni and that, although the challenge infection may become established, the worm level is markedly reduced and the pathogenicity of the infection ameliorated. It is considered that these findings may have profound immunological and epidemiological implications in endemic areas where man is exposed to the transmission of both parasites.