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 bots (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 > or = 20 micrograms/kg/day for 100 days. Twenty-nine days after treatment the sheep were killed and examined for nasal bots. Trial 2--Nasal bot-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 bot 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 bot infestations. No live larvae were collected from any treated animal.

CONCLUSION

Treatment with a single ivermectin controlled release capsule was 100% effective against existing infestations of O ovis and as a prophylactic treatment for this parasite.