Immune development and performance characteristics of Romney sheep selected for either resistance or resilience to...

Immunological and performance characteristics were explored in Romney sheep from lines selected for either resistance or resilience to parasite infection. At a mean 78 days-of-age, twin lambs from a line selected for resistance (RT) and lambs from a line selected for resilience (RL) were infected with the intestinal nematode Trichostrongylus colubriformis for 100 days (I) while their twin remained as an uninfected control (C). Compared with RL, RT animals had lower levels of circulating CD4⁺ T-cells (P = 0.003) but a greater proportion of these were activated (CD4⁺CD25⁺) in response to infection (P = 0.007). Differences between the lines in humoral immune responses to nematode infection varied with higher levels of T. colubriformis specific immunoglobulin (Ig) E in RT-I than RL-I (P = 0.002) but similar levels of both IgG (P = 0.926) and IgA (P = 0.321) responses. Temporal differences in the immune response also existed between the lines with RT-I animals displaying an earlier peak and more rapid reduction in FEC and an earlier peak in T. colubriformis specific IgA. In addition, compared with their RT-C and RL-C counterparts, infection caused a 22% reduction in feed intake from day 56 (P = 0.001) with total feed intake reduced by 15% and 9% for RT-I and RL-I, respectively. Cumulative liveweight gain was greatest for RL animals (P = 0.026) and relative to RT-C and RL-C was reduced by 5.8 kg and 4.9 kg for RT-I and RL-I, respectively. Overall, the selection lines appear to have differences in immunological characteristics that are both dependent on, and independent of parasite infection. Further, the difference in growth in the uninfected animals coupled with the similar cost of infection suggests the lower liveweight gain of RT-I compared with RL-I may be due to inherent differences between the lines in their growth potential, rather than a greater cost of infection in animals selected for resistance.