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

Over the last 6 years a number of experiments in which sheep were successfully immunised with 2 doses of 10 000 Haemonchus contortus larvae, irradiated at 60 krad in a gamma-source, have been reported by the authors. In this paper, the failure of such a regimen of immunisation, using the same strain of larvae, is reported together with an investigation of the possible causes. Technical errors were eliminated as a cause, on the grounds that the failure occurred in 2 separate laboratories using different 60Co sources, as was increased radiosensitivity of the continuously passaged strain of H. contortus as measured by the percentage development of irradiated larvae in 6 naive lambs. From experiments utilising 18 lambs and using both irradiated and normal larvae, it was postulated that the strain had lost its original degree of immunogenicity. Subsequently, in 2 experiments involving 27 lambs, it was shown that irradiation at 40 krad was completely successful in restoring immunogenicity and producing a degree of protection against challenge similar to that previously reported with 60 krad. It was concluded that, with the current strain of H. contortus, 60 krad is now too high a level of irradiation of larvae to produce a consistently high degree of protection against challenge.