The effect of dietary protein and some amino acids on immunity in parasitised lambs : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

Two experiments were carried out to investigate the effect of dietary protein and methionine and lysine on the immune system of lambs infected with gastrointestinal parasites. Both experiments were run concurrently. In experiment I, the effect of dietary protein on the immune system of lambs was investigated: Thirty two, 8 week old lambs were individually penned, divided into two groups (n=16) and each group was fed either a low or high level of dietary protein [low protein group (79g MP/kg DM) and high protein group (157g MP/kg DM)]. These two groups of lambs were further divided in two groups (n=8), one acting as an unsensitized control, the other being sensitized with parasite larvae. Infective larvae (L3) of Trichostrongylus colubriformis (95 %) and Ostertagia circumcincta (5 %) were administered as a trickle infection at rates of 600, 1200, 1800 and 3000 L3/week for the first 4, second 4, third 3 and last 4 weeks, respectively. All lambs including unsensitized control animals were challenged with 30,000 L3 of T. colubriformis 17 days after the end of the trickle infection and lambs were slaughtered 11 days after this challenge. In experiment II, the effect of methionine and lysine (ML) in the diet on development of immunity in lambs was investigated. The same low protein (LP) diet used in experiment I was supplemented with ML that had been protected from rumen degradation. Neither diet significantly increased live weight gain (LWG) in the 8-27 week old lambs over the whole period of the trial. However, there was a strong trend (p<0.09) for an enhancement due to HP at a number of different time points during sensitization and ML supplementation tended to improve (p=0.06) growth rate 6-9 week into the trickle infection. The effect of the high dietary protein on established small intestinal and abomasal worm burdens was not statistically significant, but the HP group tended to have decreased survival of T. colubriformis after challenge (p=0.091). The HP diet had no effect on abomasal worm population and faecal egg numbers. However with ML supplementation there was a significant difference in the worm counts of T. colubriformis in the small intestine acquired during trickle infection and the worm count of the ML group was half that of the LP group. Abomasal worm population of O. circumcincta were also similar after sensitization and challenge irrespective of ML intake. Faecal egg output was decreased during trickle infection and following challenge due to ML intake. In general, both protein and ML supplementation improved parasite (L3) specific and nonspecific (mitogen) lymphocyte proliferation in vitro during the early trickle period (p<0.05). All sensitized animals irrespective of diet had elevated blood eosinophils and higher proliferative responses to L3 antigen (p<0.05). Levels of eosinophils correlated negatively with challenge L3 survival but L3 proliferation correlated negatively and positively with HP and ML diet groups respectively.