Immune response of twin-bearing Merino ewes when infected with 'Haemonchus contortus': Effects of fat score and prepartum supplementation

The benefit of providing prepartum protein supplementation to twin-bearing Merino ewes with low and high fat score (FS; measure of soft tissue depth related to fatness) was investigated to quantify effects on host resilience and immunity to 'Haemonchus contortus'. The experiment was conducted on low quality pastures from day (d) -91 to weaning at d 52, relative to the mid-point of lambing (d 0). The experimental design consisted of two feeding regimes designed to achieve FS targets of 2 (low) and 4 (high) by d -47 followed by allocation to protein supplementation groups (0 or 200 g/d cottonseed meal; CSM) until d 10. Throughout the experiment there were two infection groups (0 or 750 'H. contortus' third stage larvae (L₃)/week). Peri and postparturient ewes were susceptible to 'H. contortus' shown as a rise in worm egg count (WEC). High FS group ewes had a significant benefit in terms of protection against 'H. contortus', but this was restricted to the prepartum period. There were no benefits of supplemental protein which did not enhance protective immunity. Lower prepartum WEC from high FS ewes was associated with higher levels of circulating eosinophils but a general decline in counts occurred prior to parturition. Infected ewes had elevated antibody titres and a greater number of mast cells in abomasal tissue at slaughter on d 52. The results suggest that sufficient fat and protein reserves are important for twin-bearing Merino ewes during pregnancy to elevate the immune response against 'H. contortus' whereas strategic prepartum protein supplementation had no benefit.