The role of immunologically specific and non-specific components of resistance in cross-protection to intestinal nematodes

Vaccination of 6–8-month-old Merino sheep with irradiated Trichostrongylus colubriformis larvae gave a high level of protection (81 %) against single-species challenge with normal infective larvae of the same species. The level of protection (34%) was substantially reduced against challenge with a closely related species (T. vitrinus) and no significant protection occurred against single-species challenge with a generically unrelated nematode (Nematodirus spathiger). These results suggest that antigen(s) which stimulate protective immunity are shared by the related Trichostrongylus species but not by N. spathiger. By contrast with the results obtained for single-species challenge, vaccination with irradiated T. colubriformis produced 98–100% protection against all 3 species in animals challenged simultaneously with infective larvae of the 3 species. Comparison of the levels of protection recorded following the 2 types of challenge indicate that although a specific antigenic trigger is required to provoke an appropriate response, the results obtained, particularly in the case of N. spathiger, suggest that the terminal effector mechanism is not immunologically specific. The implications of these conclusions are discussed in relation to theories of the mechanism of resistance to gastrointestinal nematodes and the potential efficacy of vaccination in the field.