Effect of dietary protein on the regulation of populations of Nematodirus battus by lambs.

Two groups of seven and two groups of six lambs were offered either a complete basal ruminant diet (13.2 per cent crude protein [CP]) (groups 2 and 4) or the same diet supplemented with fish meal (18.3 per cent CP) (groups 1 and 3). Groups 1 and 2 were infected daily for seven weeks with Nematodirus battus larvae (L3) and groups 3 and 4 served as uninfected challenge controls. All the groups were treated with anthelmintic in the eighth week, challenged with a single dose of 30,000 N battus L3 one week later and killed nine days after the challenge. Although protein supplementation tended to enhance the regulation of the population of N battus in the lambs which had been infected continuously, the effect was not statistically significant. The worm burdens in both the groups of previously infected lambs were significantly reduced both in number and size (P < 0.001) and they had a lower proportion of male worms than the uninfected challenge controls. The lambs could be segregated into high or low responders on the basis of their worm burdens, and there was a significant reduction in worm burdens (P < 0.001) and size (P < 0.01) in parallel with the lambs' responsiveness. The identification of high and low responders shows that when the dietary protein supply is adequate the predominant effect of the host on the pathogenicity of its parasites is the host's genetically-determined susceptibility.