Estimating cost of immunity to gastrointestinal nematodes in meat sheep differing genetically in resistance and resilience to infection

Abstract

Gastro-intestinal parasites cost southern Australian prime lamb producers an estimated $65.7 million annually (Carmichael, 2009). This cost, and escalating drench resistance has led to significant investment in breeding sheep for genetic resistance to parasites. However, unfavourable correlations between worm resistance and growth rate / body weight have been identified in meat sheep (Morris et al., 2000). In addition, Greer et al. (2005, 2008) have demonstrated that production loss associated with 'Trichostrongylus colubriformis' and 'Teladorsagia circumcincta' infection is largely due to host immune response to infection. Resistance to gastro-intestinal nematode infection may therefore incur a growth penalty (cost of immunity). However, as it is possible to identify sires with high Australian sheep breeding values (ASBV) for growth and low ASBV for worm egg count (WEC), the cost of immunity may vary depending on sheep genotype. ... Identification of phenotypes that can control worm infections with a low metabolic cost will raise the possibility of improved targeting of such animals for selection purposes and functional studies to determine the molecular/physiological basis for this. The novelty in the present project is to apply the cost of immunity model to defined genotypes of animals varying in resistance and resilience, and to undertake these studies for the first time with the major Australian sheep parasite 'Trichostrongylus vitrinus'.