Nematodes and nutrient partitioning

Subclinical infection of sheep with gastrointestinal nematodes results in the diversion of nutrients from growth and development towards the repair of damaged intestinal tissues and to sustain the metabolic shifts (nutritional, hormonal and immune) occuring in tissues affected by the parasites. These metabolic effects include decreased nitrogen retention; increased amino acid utilisation and oxidation in the gastrointestinal tract; increased endogenous protein loss and altered protein synthesis in the gastrointestinal tract; increased amino acid utilisation and protein synthesis in the liver and activation of the immune system. These observations strongly suggest that there is competition between the key tissues involved in parasitism and that metabolic decisions are made resulting in the re-prioritisation of nutrient utilisation between the tissues in this interorgan system. Nutritional status of the host can influence the pathogenesis of parasitic infection and well-nourished animals generally withstand parasitism better than those less adequately fed. Recent studies have focused on the effect of restricting intake on the acquisition of immunity in sheep selected either for resistance or susceptibility to nematodes. The major outcome of these studies was the increase of peripheral eosinophil counts in resistant animals infected with Trichostrongylus colubriformis when fed on an ad libitum diet compared with similar animals fed the same diet at a maintenance level. This was inversely correlated to the numbers of adult parasites recovered. Intake seems to be the primary determinant of the expression of immunity in sheep selected for resistance against nematodes.