Genetic control of the periparturient rise in faecal egg counts in Scottish Blackface ewes facing mixed, natural nematode infections

Abstract

Infection by nematode parasites is an almost-inevitable consequence of sheep grazing improved pasture. Susceptible classes of sheep include growing lambs and ewes during the period of physiological stress caused by gestation and lactation. Nematode infections in lambs are well documented, less well documented is the decrease in resistance in ewes during the periparuturient period. The consequent elevation in faecal egg counts, the periparturient rise (PPR), leads to pasture infestation by worm eggs (Morris et al., 1998) - thus serving as a trigger for the subsequent epidemic in the lambs. Alternative control strategies to complement anthelmintics are continually being sought, with genetic selection of resistant animals a promising option (Woolaston, 1992). Unlike resistance in lambs, relatively little information exists on the genetic control of the PPR in ewes. The aim of this study is to quantify, and estimate the heritability of, the PPR in Blackface ewes grazing improved pasture and facing a natural mixed, nematode challenge.