Regulation of Teladorsagia circumcincta and Trichostrongylus colubriformis worm populations by grazing sheep with differing resistance status.

In an experiment lasting 4 years, changes in the Teladorsagia circumcincta and Trichostrongylus colubriformis populations were compared in lambs and adult sheep with differing resistance statuses. Two flocks of 30 rams (resistant R and susceptible S) grazed separate pastures and 8 rams were slaughtered in the middle and at the end of each grazing season. Five groups of tracer lambs were added each year to estimate the pasture infectivity and were killed for worm counts. The availability of animals with differing resistance statuses (rams and tracer lambs) and differing levels of infection made it possible to investigate the number, size and fecundity of worms of these two species. The inflammatory response was measured in the rams by counting the globule leukocytes, mast cells and eosinophils in the fundic, pyloric and intestinal mucosa. In the tracer lambs, the daily egg production by the female worms of both species was negatively correlated with the worm burden. Worm length accounted for 60 and 70% of the variation in the number of eggs in utero for T. circumcincta and T. colubriformis respectively. Worm length was closely associated with the resistance status of the host; there were greater differences between lambs, and S and R rams for T. colubriformis. T. circumcincta worm lengths were not affected by the worm number. Globule leukocyte counts were related to the worm burdens, and mast cell counts to worm length in the R and S rams. The number, size and fecundity of the worms may well be regulated by similar mechanisms in both species, but T. colubriformis seemed to be more intensively regulated than T. circumcincta. This finding could be useful in devising more effective methods of parasite control.