Studies in Scottish hill sheep: II. Changes in the susceptibility of the lamb to Ostertagia spp.

Abstract

Groups of lambs were removed from a hill farm in late summer and autumn in each of two consecutive years (1972 and 1973) and subjected to experimental challenge with 50 000 infective larvae of Ostertagia circumcincta. In the first year the lambs were as susceptible as lambs of similar age reared parasite-free since birth, but in the second year they were significantly more resistant. During the second year climatic conditions were more favourable for larval development, resulting in higher field infections and a greater antigenic stimulus, which in turn produced greater resistance. Where lambs had acquired a resistance to challenge this was not reduced by anthelmintic treatment.