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SUMMARY An experiment is described in which 28 lambs reared under worm-free conditions were fed on a low cobalt diet. When the lambs were approximately nine months of age they were randomly divided into four groups, two of which were supplemented with cobalt. Four weeks later the lambs of one supplemented and of one non-supplemented group were given 70,000 H. contortus infective larvae. Though there was no growth response to cobalt administration, liver cobalt levels of non-supplemented lambs were in the range associated with cobalt deficiency. In the early stages of infestation, cobalt-supplemented lambs showed higher egg counts, more severe anaemia and a 43 per cent mortality compared with lambs whose low cobalt diet was not supplemented and in which there was no mortality. It is concluded that a low level of cobalt in the diet of the host is detrimental to H. contortus.

SUMMARY Forty-four lambs were reared under worm-free conditions and fed on a diet low in cobalt. When their average age was 11 weeks they were randomly divided into four groups, two of which were fed a cobalt-supplemented diet. Five weeks later the lambs of one supplemented and of one non-supplemented group were given 3,000 Trichostrongylus axei infective larvae until a total of 51,000 had been given. Serum vitamin B12 levels were indicative of pronounced cobalt deficiency in non-supplemented lambs. Parasitic infestation did not bring about any fall in serum vitamin BI2 in either cobalt-supplemented or cobalt-deficient lambs. Cobalt deficiency caused a highly significant reduction in growth. Infestation in cobalt-deficient lambs, however, caused no significant depression of live-weight at a time when a similar infestation was causing a highly significant decrease in the weight gain of cobalt-supplemented lambs. This result may mean that either a low cobalt status in the host or general malnutrition as a result of cobalt deficiency had the effect of reducing the pathogenicity of T.axei. Infestation, by causing dehydration and concomitant haemoconcentration tended to mask the anaemia due to cobalt deficiency.

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.