Efficacy of ivermectin controlled-release capsules for the control and prevention of nasal bot infestations in sheep

Objective To investigate the therapeutic and prophylactic efficacy of an ivermectin controlled-release capsule against nasal bets (Oestrus ovis) in sheep. Design Trial 1 - A pen study with controls. Trial 2 - A field study with controls. Animals Trial 1 - Forty Merino wethers with natural infestations of nasal bot were used. Trial 2 - One hundred nasal bot -free wethers were used. Procedure Trial 1 - Ten randomly selected animals were slaughtered and the heads split and examined to confirm bot infestation. Fifteen animals were allocated to untreated controls and 15 to treatment with a controlled-release capsule delivering ivermectin at greater than or equal to 20 mu g/kg/day for 100 days. Twenty-nine days after treatment the sheep were killed and examined for nasal bets. Trial 2 - Nasal bet-free sheep were allocated to two groups of 45 animals. One group was untreated the other sheep were treated with capsules as above. The sheep were grazed as a single group exposed to natural challenge from O ovis. Ninety days after treatment the animals were slaughtered and examined for nasal hot infestation. Results Trial 1 - Live O ovis larvae were recovered from 60% of control sheep. No live larvae were collected from treated sheep. Trial 2 - Forty-one percent of untreated sheep harbored nasal bet infestations. No live larvae were collected from any treated animal. Conclusion Treatment with a single ivermectin controlledrelease capsule was 100% effective against existing infestations of O ovis and as a prophylactic treatment for this parasite.