Regulation of Trichostrongylus vitrinus and T colubriformis populations in naturally infected sheep in the Netherlands

During a study on the epidemiology of gastrointestinal helminth infections in sheep at the University of Utrecht (1974 to 1977) information was obtained on the population regulation of small intestinal Trichostrongylus species. Comparison of the cumulative burdens in tracer lambs with those in flock lambs showed an accumulation of Trichostrongylus species in the latter until autumn followed by an expulsion of worms. The dominating species, T vitrinus, was expelled earlier than T colubriformis. The proportion of inhibited early third stage Trichostrongylus species larvae in tracer lambs was low, but it tended to increase in autumn. In most flock lambs the levels of inhibition were also low. In non-lactating ewes, populations consisted predominantly of inhibited early third stage larvae but when adults were present they tended to be mainly T colubriformis. Similar populations were seen in some lactating ewes. In other lactating ewes Trichostrongylus species populations with low proportions of inhibited early third stage larvae which were dominated by T vitrinus, indicated a relaxation of resistance around parturition.