The effect of age and dietary protein on immunity and resistance in lambs vaccinated with Trichostrongylus colubriformis.

Development of immunity and resistance to Trichostrongylus colubriformis was measured in sheep, 8-26 or 33-51 weeks of age, that were offered two levels of dietary protein (11 and 20%). Resistance to challenge infection was ascertained by measuring worm burden, faecal egg count and eggs in utero in the nematode. Development of immunity was measured by an in vitro lymphocyte blastogenesis test to T and B cell mitogens and to somatic antigen from infective (L3) larvae. Young lambs offered the low protein diet showed a significantly lower resistance to parasites than older animals. The young animals on the high protein diet developed better resistance. Age and dietary protein influenced in vitro T lymphocyte responses to parasite antigen and mitogens, which were particularly well developed in vaccinated lambs on the high protein diet. Lymphocyte responsiveness to L3 larval antigen was negatively correlated with parasite status in vaccinated older lambs, and may be important in the development of acquired resistance to T. colubriformis.