Vaccination of young lambs by means of a protein fraction extracted from adult Haemonchus contortus.

Lambs aged 48-150 days which had been injected with extracts enriched in a functional antigen (contortin) obtained from adult Haemonchus contortus developed specific circulating antibodies and were less susceptible to haemonchosis when challenged 1 month later with a single dose of 20,000-25,000 infective 3rd-stage larvae. Sera from 18 out of a total of 19 lambs injected with the extract contained precipitating antibodies to 2-5 components of the extract. None of these lambs died. The one extract-injected lamb which did not develop antibodies and 9 of the 13 lambs used as controls died of haemonchosis. The average weight of worms recovered was 1.45 g from the immune lambs and 5.72 g from the non-immune lambs.