Immunological unresponsiveness of lambs to infection with Haemonchus contortus. Effect of infection in the ewe on the subsequent responsiveness of lambs.

Specific antigen-induced lymphocyte transformation in vitro and haemagglutinating antibody responses were absent from lambs infected with Haemonchus contortus. However, when lambs were injected with soluble third stage larval antigen of H. contortus, given on five occasions during the first 6 weeks of life, specific antigen-induced lymphocyte transformation and haemagglutinating antibodies were induced to the antigen. In particular, a periparturient increased susceptibility to infection and the resultant presence of heavy infections with H. contortus in ewes appeared to prime their lambs to respond and marked lymphocyte transformation was observed in antigen injected lambs born of heavily infected ewes. The enhancement of this response was associated with the colostral transfer of as yet unidentified factors. The superimposition of an infection with H. contortus in lambs which had been stimulated to respond to antigen, suppressed the in vitro responsiveness of their lymphocytes. Although individual variation was apparent, a direct correlation between the presence of in vitro antigen-induced lymphocyte transformation to third stage larval antigen and susceptibility to infection occurred.