The radiation-attenuated schistosome vaccine induces a high level of protective immunity in rodents. In order to assess its potential relevance to man, we have tested its efficacy in the non-human primate Papio anubis. A vaccination regime consisting of 3 exposures of approximately 9000 cercariae irradiated with 30 or 60krad. of gamma radiation induced > 50% protection to a challenge with normal larvae. A lower attenuating dose of 20 krad., optimal for vaccination of mice, was less effective. All vaccination regimes elicited a population of PBMC which proliferated in vitro in response to antigen. These responses peaked after the third exposure but were significantly lower after challenge. They revealed relatively little cross-reactivity with adult Schistosoma haematobium antigens and provided some evidence for stage-specific antigens. Circulating IgM reactive with adult S. mansoni antigen was detected after the second vaccination but levels remained low throughout. In contrast, IgG levels were boosted by successive vaccinations, although they showed a tendency to decline from 14 days after each exposure. There also appeared to be a lag of about 14 days after challenge before levels began to rise. Thus, both proliferation and antibody data suggest a lower responsiveness after challenge which may reflect either the reduced antigenic load or immunogenicity of normal, compared to vaccinating larvae. The data indicate that the attenuated schistosome vaccine is capable of inducing protection in a highly permissive primate host, with the implication that the mechanisms involved may also be relevant to man.

Groups of South African, farm-reared merino lambs about 4 months old were vaccinated with extracts of adult Haemonchus contortus enriched with H11, an integral membrane protein from the parasite's intestinal microvilli, or with proteins soluble in Tween 20. The lambs were challenged with 5000 infective 3rd-stage larvae. Compared to the adjuvant-injected controls, lambs vaccinated with 350 μg/kg liveweight of the H11-enriched extract showed an 89% reduction in parasite egg production and an 88% reduction in total worm burden at post-mortem 35 days post-challenge. The H11-enriched extract contained some protein also present in the Tween 20 extract. Lambs vaccinated with 600 μg/kg of protein soluble in Tween 20 showed a 40% reduction in faecal egg counts and 51% reduction in total worm numbers 35 days post-challenge. Animals injected with 20 μg/kg of a sub-fraction containing H11 obtained from the H11 extract, showed a 71% reduction in egg output and total worm numbers.