PSX-27 Growth performance and immune response dynamics in weaned lambs with subclinical <i>Haemonchus contortus</i> parasite infection

Abstract Parasite infections, particularly from Haemonchus contortus, hinder sheep production in Canada by reducing weight gain and immunity. Current diagnosis and treatment methods rely on observing symptoms and the blind administration of anthelmintic drugs, resulting in escalating costs and the development of drug-resistant parasites. This study aimed to identify the exact time-point when subclinical H. contortus parasite infection affects growth performance and triggers an immune response. A total of 60 Rideau Arcott ewe lambs (~7-8 mo old), confirmed to be parasite-free, were randomly assigned in a 2 x 2 factorial design assessing inoculation treatment (TRT): oral inoculation (IN) with a solution containing 5,000 L3 strain of H. contortus or sham inoculation (CT) using water solution as control; and drench (DRENCH) administration: oral drench (D), 28-days post-inoculation, with a combination of 0.08% ivermectin solution (200 mcg per kg of BW; Durvet®) and 10% oral suspension of fenbendazole (2.5 mL per 50 kg of BW; Safe-guard®), or water solution (non-drenched, ND), resulting in four treatment groups: IN-D, IN-ND, CT-D, and CT-ND (n=15/treatment). Lambs were equally assigned to four feedlot pens (two replicates/treatment) for 57 days (d) with ad libitum access to a grain-based total mixed ration and water. Animals were individually weighed (BW) and blood sampled on days -1 and 0 (baseline), as well as on days 7, 21, 28, 35, 42, 56, and 57 post-inoculations (DAY); average daily gain (ADG) was calculated for the entire experimental period. White blood cells count (WBC) was used as an indicator of infection and red blood cell count (RBC) as indicator of anemia. Data was analyzed using a mixed-effects model including TRT, DRENCH, DAY (and their interactions) as fixed effects. Overall, CT lambs were heavier (TRT effect; P &amp;lt; 0.05) and had a tendency of greater (TRT effect; P = 0.08) ADG than IN lambs. The IN lambs had a tendency of greater (TRT*DAY; P = 0.07) WBC on d 7 than CT lambs. Additionally, IN-ND had lower (TRT*DRENCH; P &amp;lt; 0.05) WBC than IN-D lambs (1.9 and 2.0 ± 0.042 x 109/L, respectively). A significant (TRT*DRENCH*DAY; P &amp;lt; 0.01) interaction was found on RBC. The CT-D had lower (P &amp;lt; 0.05) RBC on d 7 and greater (P &amp;lt; 0.05) on d 28, 35, 42, and 57 than IN-ND as well as on d 28 than IN-D. Furthermore, CT-ND had greater (P &amp;lt; 0.05) RBC on d 21, 28 and 35 than IN-D and from d 21 to 57 than IN-ND. The IN-D had greater (P &amp;lt; 0.01) RBC from d 35 to 57 than IN-ND. These findings suggest compromised growth in both drenched and non-drenched infected groups. Infection slightly peaked in the infected lambs on d 7 post-inoculation, but drench administration was effective at mitigating overall infection by day 21 and the anemia by day 35.