Persistent challenge with Trichostrongylus colubriformis and Haemonchus contortus larvae does not affect growth of meat-breed lambs suppressively treated with anthelmintics when grazing

- Suppressive anthelmintic treatments prevented establishment of gastrointestinal nematodes (GIN) in lambs challenged with susceptible GIN larvae.
- No production losses for meat-breed lambs despite an evident immunological response to larval challenge.
- Sheep suppressively treated with effective anthelmintics are a valid substitute for GIN-free sheep in field experiments.

This experiment tested the hypothesis that persistent challenge with anthelmintic susceptible Trichostrongylus colubriformis and Haemonchus contortus larvae would not affect growth of grazing, meat-breed lambs when suppressively treated with anthelmintics. The experiment was a 2 × 2 factorial design using 6-7 months old White Suffolk X Border Leicester/Merino (meat-breed) lambs which were either infected with 2000 T. colubriformis and 300 H. contortus L3/week (IF) or remained uninfected (UIF) for 9 weeks and were either treated (TX) with a combination of short and long-acting anthelmintics or remained untreated (UTX). Lambs grazed as one flock and were rotated between paddocks to avoid autoinfection from pasture. Lambs were humanely euthanised on day 63 and the abomasum and small intestine collected to determine total worm burdens and tissue antibody response specific to T. colubriformis. As expected, worm egg count (WEC) and worm burden were significantly higher in IF UTX lambs (p < 0.001). WEC was dominated by H. contortus and peaked at 2325 epg on day 63 but remained at zero for the other treatment groups for the duration of the experiment. Tissue antibody responses were evident in IF lambs (titres; 9982 vs 2767, p = 0.012) but treatment had no effect (titres; 5912 vs 5349, p = 0.829). Lambs grew an average of 2.6 kg during the experiment with no difference between IF TX and UIF TX groups (p = 0.432). Elevated tissue antibody responses were not associated with differences in growth. Results from this experiment support the hypothesis that persistent larval challenge with anthelmintic susceptible H. contortus and T. colubriformis will not affect growth of grazing, meat-breed lambs when suppressively treated with effective anthelmintics. Therefore the use of sheep suppressively treated with effective anthelmintics appears to be a valid substitute for gastrointestinal nematode-free lambs in field experiments.