Controlled release of anthelmintic drugs: a new concept for prevention of helminthosis in sheep.

Anthelmintic effects of oxfendazole continuously released at 0:17, 0.28 and 0.47 mg/kg/day from intraruminal capsules were assessed in groups of sheep artificially infected with Ostertagia circumcincta. Removal of worms was directly related to both release rate and plasma concentrations of drug. At the highest level, worm burdens were removed within seven days of administration and anthelmintic efficiency was 99.9 +/- 0.04, 99.3 +/- 0.5 and 98.9 +/- 0.7 per cent against adult worms, developing fourth stage and early fourth stage larvae respectively. In a field experiment, comparisons were made of the parasitological and animal production differences between groups of weaned lambs which were given no treatment, a single oral dose of 5 mg/kg oxfendazole and capsules releasing either 0.24 or 0.48 mg/kg oxfendazole daily. By four days after administration of capsules worm egg counts were reduced and remained below detectable levels for up to 86 days. Worm counts from 'tracer' and flock sheep showed a reduction in worm numbers, especially for Trichostrongylus spp. Compared with untreated controls, live weight gain and fleece weight of sheep given capsules releasing 0.48 mg/kg oxfendazole daily was increased.