Studies of stage-specific immunity against Trichostrongylus colubriformis in sheep: immunization with adult parasites.

Merino sheep immunized by the adoptive transfer of adult T. colubriformis for 8 weeks were significantly protected against a challenge infection of 20,000 larvae. Two additional groups of sheep received a primary infection of 9000 adult worms which were allowed to persist for 14 weeks before one group was drenched. When both groups were challenged 10 days later with 30,000 larvae, serial necropsies of these and naive sheep revealed that worm rejection did not occur until 7-10 days after challenge. By comparison with the rapid rejection of larval challenges from sheep immunized with normal primary infections, the results suggest that the antigens which elicited rejection in these experiments are stage-specific and were only present or synthesized in sufficient quantities when parasites had developed for 1 week.