Seasonal fluctuations and inhibited development of gastro-intestinal nematodes of sheep.

Abstract

Summary Groups of parasite-free lambs grazing permanent sheep pasture in south-west Scotland for monthly intervals from July 1967 to March 1968 developed worm burdens consisting predominantly of the abomasal nematodes Ostcrtagia spp. and Trichostrongylus axei and the small intestinal parasites Trichostrongylus spp. and Ncmatodirus filicollis. All genera were present throughout the period of observation, maximum numbers occurring between mid-September and mid-November. The proportion of inhibited larvae of all genera, except Trichostrongylus spp. increased markedly during late autumn to reach a maximum in December and January. The increase in inhibition occurred independently of either the age or sex of the host or size of the worm burden. It is suggested that the primary factor involved in inhibition is an environmental one producing endocrine or metabolic changes in the larva.