Oesophagostomum columbianum: gamma-irradiated third-stage larvae for immunization in lambs

40-kR gamma-irradiated third-stage larvae of Oesophagostomum columbianum were used for the immunization of Kashmir Merino lambs. Male lambs (aged from 8 to 12 weeks) were immunized in two separate experiments by two doses of irradiated larvae, given 21 days apart, and subsequently challenged with normal larvae. Judging by the establishment of worms resulting from the challenge infections in the immunized and control groups of lambs in the two experiments, a high degree of immunity was shown to develop in young lambs vaccinated with 500, followed 21 days later with 2000, 40-kR irradiated larvae. Lambs from the immunized groups showed more nodules in the intestine, a high percentage of which were positive for histotrophic stages of O. columbianum, than did controls. The importance of this finding in relation to the possible use of a vaccine for the immuno-prophylaxis of oesophagostomiasis in sheep and other animals is discussed.