Immune-mediated responses account for the majority of production loss for grazing meat-breed lambs during Trichostrongylus colubriformis infection

• T. colubriformis reduces growth rates of grazing meat-breed lambs.
• Majority of production loss due to host immunity occurs in early stages of infection.
• Greatest losses due to the direct effects of T. colubriformis occur in latter stages of infection.
• The host’s immune response accounted for 75% of overall cost of infection.

The hypothesis tested in this experiment was that Trichostrongylus colubriformis infection would reduce growth rates of grazing meat-breed lambs; however production loss would be reduced by suppression of the host immune response. The experiment had a 3 × 2 factorial design using 6–7 month old meat-breed lambs which remained uninfected or infected (IFY) with 2000 or 4000 T. colubriformis L3/week for 12 weeks and were immunosuppressed (SUPY) using methylprednisolone acetate once weekly or remained non-immunosuppressed (SUPN). Immunosuppression increased worm egg counts (WEC) of infected lambs (SUPY 2421 eggs per gram (epg), SUPN 1154 epg on day 84, p < 0.05) and T. colubriformis burdens (p < 0.05–0.10) and reduced circulating eosinophils (p < 0.05 on days 11, 42, 56 and 84) and intestinal total antibody titres (p < 0.02). There was a significant (p < 0.05) interaction between the main effects of infection and immunosuppression with infection having a larger negative effect on the liveweight of non-immunosuppressed lambs. The immunological response of the host to T. colubriformis infection accounted for 75% of the overall cost of infection (3.1 kg) with the majority of this cost occurring during the first 35 days of infection. In contrast, most of the cost associated with the direct effect of infection occurred after day 35. These results confirm in grazing meat-breed lambs that the host’s immunological response to T. colubriformis infection is the major component of production loss.