LONG-TERM SUBCLINICAL INFECTION WITH TRICHOSTRONGYLUS-COLUBRIFORMIS -EFFECTS ON FOOD-INTAKE, DIET SELECTION AND PERFORMANCE OF GROWING LAMBS

Growing lambs (mean liveweight 22.4 kg) were used to test for the effe cts of long-term subclinical infection with Trichostrongylus colubrifo rmis (2500 larvae per day) on the voluntary food intake (VFI), diet se lection and performance of sheep given a choice between two foods of d ifferent protein content (low versus high), Sheep were either uninfect ed controls or parasitised for 27 weeks (Group 27P). Additional sheep were infected for either 9 or 18 weeks (before and after the developme nt of complete resistance to parasites, as assessed by faecal egg coun ts) and then treated with anthelmintic to remove the parasites (Groups 9P and 18P), Group 18P remained uninfected after anthelmintic treatme nt whereas Group 9P was infected again for the last 9 weeks after a 9 week worm-free period. The VFI declined or remained static from Week 5 up to Week 11/12 of continuous infection, for the 27P and 18P groups, For 9P sheep, VFI recovered within a few days after the anthelmintic administration and was unaffected during re-infection over the last 9 weeks of the experiment. Diet selection changed in such a pattern that the proportion of the high protein food (prop H) in the selected diet started to increase from Week 6 of the infection. The prop H selected remained higher over the interval 10-18 weeks of the continuous infec tion for 27P and 18P groups than the uninfected controls, There were n o significant differences in diet selection among any of the groups be yond Week 18 of the experiment. The infection caused a growth check fr om Week 7 to approximately Week 12/14 of dosing; there was no evidence of compensatory growth following the development of resistance or rec overy. The results are consistent with the view that sheep given a cho ice between two appropriate foods will modify their diet selection in order to moderate the adverse effects of subclinical nematode infectio n. The performance and diet selection of all sheep beyond Week 18 of i nfection suggest that no obvious nutritional penalties are incurred wi th the continuation of the infection once the animals have become resi stant to parasites.