Some Relationships Between Trichostrongylid Infestation and Cobalt Status in Lambs: III. Trichostrongylus Axei and Ostertagia Circumcincta Infestation * *A paper containing data from this series was presented at the First International Congress of Parasitology, Rome, 1964

SUMMARY Two experiments with lambs reared under worm-free conditions and fed on low cobalt diets are described. In the first experiment, a group of cobalt-supplemented lambs harbouring a worm burden consisting of Ostertagia circumcincta superimposed on a long-standing Trichostrongylus axei infestation had significantly lower serum vitamin B 12 levels than cobalt-supplemented lambs some of which were worm-free while others harboured only a recently-acquired O . circumcincta infestation. In the second experiment lambs which were infected with O . circumcincta , and in which cobalt deficiency was kept at a subclinical level, showed higher egg counts, lower haemoglobin concentrations and poorer weight gains than similarly infected lambs receiving adequate cobalt. The results indicated that the effects of low cobalt intake and a mild to moderate O . circumcincta infestation were additive.