title: Some factors influencing the immunisation of sheep with irradiated Haemonchus contortus larvae.
abstract: 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.
mesh_terms: D000818:Animals; D003037:Cobalt Radioisotopes; D005260:Female; D006188:Haemonchiasis / Q000469:parasitology / Q000517:prevention & control / Q000662:veterinary*; D006189:Haemonchus / Q000276:immunology* / Q000528:radiation effects; D007814:Larva; D007978:Levamisole / Q000494:pharmacology; D008297:Male; D011836:Radiation Tolerance; D012756:Sheep; D012757:Sheep Diseases / Q000469:parasitology / Q000517:prevention & control*; D013827:Thiabendazole / Q000494:pharmacology; D014251:Trichostrongyloidea / Q000276:immunology*; D014252:Trichostrongyloidiasis / Q000662:veterinary*; D014613:Vaccines, Attenuated / Q000276:immunology*
chemical_list: D003037:Cobalt Radioisotopes; D014613:Vaccines, Attenuated; D007978:Levamisole; D013827:Thiabendazole