Efficacy of ivermectin in a controlled-release capsule for the control of breech strike in sheep.

Objective: To investigate the efficacy of ivermectin in an intraruminal controlled-release capsule (CRC) against blowfly strike.

Design: Pen and field trials with controls.

Animals: Pen studies: Two breech strike trials involving 60 Romney and 60 Merino sheep. One body strike trial using 100 Merino sheep. Field trials: Eight trials in New Zealand used 1000 Romney and Romney-cross sheep. Fifty Merino lambs in one trial in Australia.

Procedure: Pen studies: Sheep were allocated to two equal groups. One was not treated, the other sheep received a CRC that delivered ivermectin at > or = 20 micrograms/kg/day for 100 days. In the breech strike trials, each animal was given an oral laxative 2 days before exposure to adult Lucilia cuprina. In the body-strike trial, the sheep sheep were kept wet to increase susceptibility prior to the release of blowflies. Field trials: Fifty or 200 sheep allocated to equal groups of nontreated or treated with the CRC and grazed at pasture exposed to natural blowfly challenge.

Results: Pen studies: Breech strikes developed in 24 of 60 controls but in none of 60 CRC-treated sheep. There was a 35% reduction in the number of CRC-treated sheep struck on the body. Field trials: The average number of breech strikes in CRC-treated sheep was reduced by 86% (P < 0.001). The number of body strikes in the treated groups was a reduced by 27% (P < 0.05).

Conclusion: The ivermectin CRC is a useful aid in controlling breech strike, but provides only moderate reduction in the incidence of body strike.