CONTROL OF GASTROINTESTINAL PARASITISM IN SHEEP WITH IVERMECTIN DELIVERED VIA AN INTRARUMINAL CONTROLLED-RELEASE CAPSULE

The efficacy of ivermectin delivered by an intraruminal controlled-rel ease capsule against gastro-intestinal nematodes of sheep was evaluate d under controlled conditions. In seven Australian studies involving 1 70 Merino or Merino x Border Leicester sheep, intraruminal capsules de veloped for 20-40 kg or 40-80 kg sheep, and delivering 0.8 or 1.6 mg o f ivermectin/day respectively for 100 days (minimum dose 20 mu g/kg/da y), were evaluated. Studies were designed to test the therapeutic effi cacy against naturally acquired and induced infections treated at the adult and fourth larval stage, and the prophylactic efficacy against n aturally acquired and induced infections with third stage infective la rvae. The predominant pathogenic nematodes of sheep were represented. Two studies included known benzimidazole- and levamisole-resistant nem atode strains. Sheep were necropsied for total nematode counts 21-85 d ays after treatment. The efficacy of the ivermectin controlled-release capsule was generally >99% against all nematode species tested, inclu ding those confirmed to be benzimidazole- and levamisole-resistant. Hi gh therapeutic activity was demonstrated against existing adult and fo urth larval stage nematode infections at the time of treatment, and hi gh prophylactic efficacy was shown against incoming third stage larvae of all species and strains tested.