Threshold levels of artificial infection with Haemonchus contortus impacting lamb physiology and production.

This study examined the effects of levels of artificial Haemonchus contortus infection on lamb behaviour and physiology. Forty 6-month-old Romney-cross wethers were assigned to one of four treatments (n = 10/treatment): 8000 L3 larvae (High; H), 4000 L3 larvae (Medium; M), 1000 L3 larvae (Low; L), or water (Control; C). Lambs were monitored 1 week pre- and 6 weeks post-treatment. Weekly measurements included faecal egg counts, liveweight, blood counts, and cytokine concentrations (e.g., IL-6). Behaviour was continuously monitored using cameras and accelerometers. Faecal egg counts increased in H, M, and L lambs from week 3 post-treatment, exceeding 830 epg in H and M lambs by week 6 (P ≤ 0.05). No behavioural differences were observed (P > 0.19). At week 3, H lambs had lower red blood cells, haemoglobin, and haematocrit, and higher mean corpuscular volume (P < 0.02). IL-6 concentrations tended to be higher in H lambs from weeks 4-6 (P = 0.06). Liveweight gains were lower in challenged lambs by trial end, differing significantly from controls 2 weeks post-trial (P = 0.04). These results suggest that while H. contortus infection may not affect lamb behaviour, higher doses (4000-8000 larvae) cause dose-dependent physiological changes indicative of anaemia, immune activation, and reduced weight-gain, negatively impacting lamb welfare and productivity.