IMMUNIZATION OF NEMATODE RESISTANT OR SUSCEPTIBLE ROMNEY LAMBS WITH OXFENDAZOLE ABBREVIATED INFECTIONS AGAINST PASTURE CHALLENGE

Two groups of 7 month old pastured Romney lambs selectively bred for e ither low (resistant - R) or high (susceptible - S) faecal worm egg co unt (FEC) were immunised while on pasture by two Oxfendazole abbreviat ed (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 natura l challenge. A significant reduction in FEC was achieved as a result o f immunisation of both low and high FEC groups. However, although the two groups of R genotype lambs had a significantly lower worm burden t han the S lambs, for each genotype there was no difference in worm bur den between the immunised and non-immunised groups. None of the immuno logical parameters measured [antibody level against excretory/secretor y antigens from infective larvae and adults, response of mesenteric ly mph nodes cells to concanavalin A (Con A), lipopolysaccharide (LPS) or T. colubriformis antigen] were found to be associated with the reduct ion in FEC. Immunised resistant (low FEC) lambs had a significantly hi gher liveweight gain over the post-immunisation period than the non-im munised resistant lambs.