The Physiology of the Periparturient Relaxation of Immunity to Sheep Worms

Abstract

The temporary relaxation of immunity to nematode parasites of the gastrointestinal (GI) tract in ewes during late gestation and/or lactation manifests itself as a characteristic rise in faecal worm egg count and is termed the periparturient rise (PPR). Apart from direct effects of the parasites on the ewe which may cause wool breaks and decreased milk production for the lamb, periparturient ewes become a potent source of pasture infectivity which may subsequently increase the exposure of grazing lambs to nematode infection. It can therefore severely affect production and, as a consequence, profitability. Although modulators of the PPR have been identified (such as protein supply and genetic selection), the definitive causal factor/s are still unknown. This lack of knowledge has hampered the development of targeted strategies that act to mitigate the effects of the PPR. The aim of this PhD program was to identify the causal mechanisms responsible for the increased susceptibility to the GI nematode 'Trichostrongylus colubriformis'. A series of experiments were undertaken to characterise the physiology underpinning the periparturient relaxation of immunity by measuring contemporaneous profiles of parasitological, immunological, production and endocrine parameters. Experimental contrasts known to influence parasite resistance, the early-weaning of lambs and manipulation of the nutritional environment, were utilised as a means of identifying the key mechanisms responsible for the PPR. In all experiments, procedures commenced at approximately day -50 relative to the midpoint of lambing and ewes were sampled until approximately day 42 after lambing. Analysis of data was conducted in three discrete periods to avoid confounding time with physiological status, viz. (1) prior to lambing, (2) during the lambing period, and (3) during the postpartum lactation period.