Efficacy of ivermectin delivered from a sustained-release bolus against inhibited early fourth-stage larvae of Ostertagia ostertagi and other nematodes in cattle

Summary The anthelmintic efficacy of ivermectin ( ivm ) delivered from a sustained-release ( sr ) bolus was evaluated against natural infections with gastrointestinal tract nematodes in 12 crossbred beef heifers in spring. The 12 calves were randomly allotted to 2 groups of 6 calves each. Group-1 calves were treated with an sr bolus designed to deliver 8 mg of ivermectin/d. Group-2 calves were nontreated controls. Cattle groups were kept in separate concrete-floored pens (grass hay nutrition) and slaughter was performed at 35 days after treatment. Fecal egg counts for group-1 calves remained zero after treatment, except for detection of < 1 egg/g of feces in 1 calf at the time of slaughter; counts in nontreated calves increased. Mean and range of Ostertagia ostertagi inhibited larvae in nontreated calves were 27,093 and 10,622 to 56,368, respectively. Efficacy of the ivm sr bolus was 100% against O ostertagi developing fourth-stage larvae (L 4 ) and inhibited early L 4 , Haemonchus placei adults, Cooperia punctata and C spatulata adult males, Cooperia spp adult females, Cooperia spp L 4 , Trichostrongylus colubriformis adults, Bunostomum phlebotomum adults, and Oesophagostomum radiatum adults. Efficacy for O ostertagi and T axei adults was 99.9%. Numbers of nontreated calves infected with C pectinata adult males and Oes radiatum L 4 were too low to evaluate efficacy. Calves treated with the ivm bolus gained 10.2 kg, whereas nontreated calves lost 1.8 kg. Abomasal lesions were clearly greater in nontreated calves on the basis of index comparisons of abomasal weight and total live weight and gross pathologic features.