Two groups of 7 month old pastured Romney lambs selectively bred for either low (resistant - R) or high (susceptible - S) faecal worm egg count (FEC) were immunised while on pasture by two Oxfendazole abbreviated (15 days) artificial infections of firstly 40,000 Trichostrongylus colubriformis and 20,000 Ostertagia circumcincta and secondly 60,000 T. colubriformis and 30,000 O. circumcincta larvae. Two similar groups were not immunised. All were subsequently grazed together under natural challenge. A significant reduction in FEC was achieved as a result of immunisation of both low and high FEC groups. However, although the two groups of R genotype lambs had a significantly lower worm burden than the S lambs, for each genotype there was no difference in worm burden between the immunised and non-immunised groups. None of the immunological parameters measured [antibody level against excretory/ secretory antigens from infective larvae and adults, response of mesenteric lymph nodes cells to concanavalin A (Con A), lipopolysaccharide (LPS) or T. colubriformis antigen] were found to be associated with the reduction in FEC. Immunised resistant (low FEC) lambs had a significantly higher liveweight gain over the post-immunisation period than the non-immunised resistant lambs.
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