Comparative curative and preventive efficacies of ivermectin and closantel on Oestrus ovis (Linné 1758) in naturally infected sheep.

A field trial was undertaken to assess the efficacy of each of two formulations of ivermectin and of closantel in prevention and treatment of Oestrus ovis in a naturally infected flock grazing on the foothills of the Pyrenees mountains, in south-western France. Within the flock, 875 sheep were randomly divided into four groups, and treated twice during the fly season, with an interval of 60 days between treatments. Group 1 sheep were treated with albendazole (ABZ) at a dose rate of 3.8 mg/kg to maintain control of trichostrongylid parasites without affecting O. ovis; Group 2 received closantel at a dose rate of 10 mg/kg because of its known persistent activity against O. Ovis; Groups 3 and 4 received ivermectin at a dose rate of 200 mcg/kg bodyweight by subcutaneous injection (Isc) and orally (Io), respectively. All sheep were managed as a single group throughout the study. In order to assess the prophylactic effect of each product, immediately prior to the scheduled second treatment on Day 60 (D60), five sheep from each group were chosen at random and necropsied. Similarly, to assess the therapeutic effect, another five sheep from each group were selected on D70 and necropsied for parasite counts. During the 120 days of the trial, a significant number of animals from each group were regularly individually examined to assess their clinical status with regard to O. Ovis infection. Clinical signs of infection had significantly declined in Groups 2, 3 and 4 by 10 days after treatment reaching their lowest level at D30. In the control group during this period, clinical signs increased. Ten days after the second treatment, (D70), there was also evidence of a significant response to treatment. Finally the between-treatment differences in clinical scores of the closantel and ivermectin groups were small, although scores in Group 1 sheep was suggestive of a higher challenge in the second half of the study. On the basis of the postmortem counts and arithmetic means, prophylactic efficacies for the treatments relative to ABZ treated group, were 97.7, 62.5 and 0%, for the closantel, Isc and Io groups respectively. Therapeutic efficacies for the closantel, Isc and Io were 100, 100 and 98% respectively.