Effect of dietary protein supplementation on the development of immunity to Ostertagia circumcincta in growing lambs.

Thirty four-and-a-half-month-old worm-free lambs were used to determine whether the rate of development of immunity to Ostertagia circumcincta infection in growing lambs could be influenced by the addition of a by-pass protein supplement. Sixteen lambs (groups 1 and 2) were fitted with an abomasal catheter and infected daily with 2000 O circumcincta L3 for eight weeks. Group 1 lambs received 45 g of crude protein day-1 (sodium caseinate) as a continuous infusion into the abomasum from week-1 to week 8. At week 9, groups 1 and 2, together with eight naive controls (group 3), were treated with anthelmintic and challenged one week later with 50,000 O circumcincta L3 and killed after a further 10 days. An additional six worm-free lambs provided feed intake and growth rate data. All the lambs were offered a complete ruminant ration (167 g crude protein kg-1) ad libitum. The cumulative liveweight gain of both the trickle-infected groups was less than that of the controls. The mean faecal egg counts were lower in group 1 from day 39 after infection and the mean worm burdens were significantly lower than in group 2. Total Ostertagia populations did not differ significantly between group 3 and either group 1 or 2 lambs. Early L4 stages constituted a greater percentage of the total worm population in group 1 (79.5) and group 2 (48.5) than in the challenge controls (group 3) (20.4). The trickle-infected lambs also had higher concentrations of gastric mast cell protease which correlated positively with the proportion of early L4 stages and negatively with the total worm burden. The provision of by-pass protein supplement accelerated the development of immunity to O circumcincta in these lambs.