Threshold levels of artificial infection with <i>Haemonchus contortus</i> impacting lamb physiology and production

<p dir="ltr">This study examined the effects of levels of artificial <i>Haemonchus contortus</i> 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 (<i>P</i> ≤ 0.05). No behavioural differences were observed (<i>P</i> > 0.19). At week 3, H lambs had lower red blood cells, haemoglobin, and haematocrit, and higher mean corpuscular volume (<i>P</i> < 0.02). IL-6 concentrations tended to be higher in H lambs from weeks 4–6 (<i>P</i> = 0.06). Liveweight gains were lower in challenged lambs by trial end, differing significantly from controls 2 weeks post-trial (<i>P</i> = 0.04). These results suggest that while <i>H. contortus</i> 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.</p>