NEMATODE BURDENS AND IMMUNOLOGICAL RESPONSES FOLLOWING NATURAL CHALLENGE IN ROMNEY LAMBS SELECTIVELY BRED FOR LOW OR HIGH FECAL WORM EGG COUNT

Breeding lines of Romney sheep, selected as lambs for consistently low or high faecal nematode egg count (FEC) following periods of natural challenge, have been maintained at Wallaceville for some years. In ord er to determine the extent to which FECs in low and high genotypes ref lected their ability to resist the establishment of gastro-intestinal nematode burdens, we investigated the infection status and immune resp onses in 8- to 9-month-old progeny of selected rams from low and high FEC breeding lines following a period of grazing without anthelmintic treatment in autumn/early winter. In each of the 2 years of the study, outcross male progeny of the two lowest FEC (LFEC) (i.e. most 'resist ant') and two highest FEC (HFEC) (i.e. most 'susceptible') rams from t he divergent lines were slaughtered shortly after autumn/early winter FECs had been analysed. Post-mortem worm counts and examination of int estinal histology were then undertaken. Blood samples collected before slaughter in the second year of the study were assayed to measure ser um levels of Trichostrongylus colubriformis-specific antibody and immu noglobulins (IgG, and IgM), and numbers of circulating eosinophils. Ov erall, correlations between pre-slaughter FEC and total trichostrongyl e burdens in the lambs proved to be very high (0.91 and 0.85, respecti vely, for the 2 years studied). In the first year, LFEC lambs, which w ere shedding only 28.6% as many strongyle eggs as their HFEC counterpa rts at slaughter, were found to harbour 37.6% as many adult trichostro ngyle worms, while in the second year, LFEC lambs, which were shedding 16.1% as many strongyle eggs as their HFEC counterparts at slaughter, were found to harbour 33.5% as many adult trichostrongyle worms. Resu lts, particularly in the second year, confirmed that significantly few er worms of most of the important abomasal and small intestinal nemato de species which infest lambs in New Zealand (i.e. Haemonchus contortu s, Ostertagia circumcincta, Cooperia curticei, Nematodirus spathiger, T. colubriformis, and Trichostrongylus vitrinus) had established in th e LFEC genotypes than in their HFEC counterparts. In addition, in uter o egg counts of female intestinal Trichostrongylus spp. were significa ntly lower in LFEC lambs than in their HFEC counterparts, indicating a reduction in fecundity of those worms which did establish. There was also some evidence of an effect of host response on the developmental composition of burdens in the case of some worm species. In relation t o host responses, numbers of globule leucocytes/mucosal mast cells in the intestinal mucosa were significantly higher (P < 0.01) in LFEC lam bs than in HFEC lambs in both years of the study. Numbers of connectiv e tissue type mast cells and eosinophils in the intestinal mucosa were also significantly higher in LFEC lambs but only in the second year o f the study (P < 0.01 and P < 0.05, respectively). Numbers of circulat ing eosinophils did not differ significantly between the genotypes. T. colubriformis-specific antibodies, IgG, and IgM to both L(3) and adul t worm antigens were all significantly higher (P < 0.01 or P < 0.05) i n LFEC lambs than in HFEC lambs.