Expression of worm resistance in sheep selected for low worm-egg counts fed at maintenance or above-maintenance level

The present study reports on changes in faecal worm-egg counts (WEC), larval composition, and the number of worms at different developmental stages in young sheep sourced from a flock selected for reduced faecal worm-egg counts over 15 years. The sheep were individually penned and fed a maintenance (1.0 M) or a 1.5 times maintenance (1.5 M) diet over two periods, namely, worm-free and infection phases. They were dosed weekly with 10 000 Trichostrongylus colubriformis and 10 000 Teladorsagia circumcincta L3 infective larvae for 11 weeks. Sheep on the 1.5 M diet had lower WEC and higher bodyweights than did sheep on the 1.0 M diet. A significant decline in the percentage T. colubriformis occurred during the experiment, but no concomitant change in T. circumcincta was noticed. Resistant sheep had significantly (P <0.001) fewer worms at necropsy, and also shed significantly (P <0.001) fewer worm eggs during the experiment. Restricted feeding reduced bodyweight significantly (P <0.001) and had a small but significant (P <0.04) effect on the faecal worm-egg output and on the number of T. colubriformis worms (P <0.01) in both the control and resistant sheep. The study showed that sheep selected for low WEC resulted in significantly (P <0.001) lower WEC than in an unselected control line fed at a maintenance (1.0 M) and at an above-maintenance (1.5 M) level. Restricted feeding reduced bodyweight and had a small negative, and inconsistent, effect on the faecal worm-egg output in both the control and resistant lines.