Some factors influencing the immunization of sheep with irradiated Haemonchus contortus larvae

Over the last six years, a number of experiments in which sheep were successfully immunized with two doses of 10,000 Haemonchus contortus larvae, irradiated at 60 kR in a gamma source, have been performed by the authors. The failure of such a regimen of immunization with the same strain of larvae is reported in the paper, together with an investigation of the possible causes. From experiments using both irradiated and normal larvae, it was postulated that the strain had lost its original degree of immunogenicity. Subsequently, it was shown that irradiation of the larvae at 40 kR was successful in restoring immunogenicity and producing a degree of protection against challenge similar to that previously reported with 60 kR. It was concluded that, for the strain of H. contortus used, 60 kR was too high a level of irradiation to produce a consistently high degree of protection against challenge. (author). 12 refs, 6 tabs