Efficacy of spinosad on the treatment of myiasis caused by Cochliomyia hominivorax (Diptera: Calliphoridae) in dogs

•Spinosad showed 79.7% efficacy on Cochliomyia hominivorax myiasis in dogs.•Spinosad showed an average 73.3% expulsion rate on C. hominivorax larvae.•A single spinosad dose is useful on clinical management of primary myiasis. The purpose of this study was to evaluate the overall efficacy, larval expulsion and larvicidal effect of spinosad on the treatment of myiasis caused by New World screwworm Cochliomyia hominivorax in naturally infested dogs. Six Beagle dogs presenting with myiasis were treated once with spinosad 560 mg, with dosages ranging from 43 to 577 mg/kg. Dogs were observed with 15 min post treatment, and with intervals of 15 min within the first hour followed by hourly evaluations up to six hours post-treatment. After this period, the remaining larvae were mechanically removed. Spontaneous expulsion of larvae was variable among dogs. All dogs received support treatment and were followed up for healing of the myiasis-associated lesion. On average, the highest larval expulsion occurred four hours after treatment. A single spinosad treatment had an overall efficacy of 79.7% (56.7–100%). The average larval expulsion rate was 73.3% (56.7–100%), reached its maximum within 6 h post-treatment, and the average larvicidal effect was only 23.7% (0–58.3%). No animal needed a second dosage 24 h post-treatment, lesions were completely healed within 8 days post-treatment. Despite not reaching the desirable 100% efficacy, spinosad treatment is an option in managing NSW myiasis, in conjunction with mechanical removal of maggots that remain in the lesion and support treatment.