The comparative efficacy of afoxolaner, spinosad, milbemycin, spinosad plus milbemycin, and nitenpyram for the treatment of canine cutaneous myiasis.

BACKGROUND: Canine screw-worm myiasis due to Cochliomyia hominivorax or Chrysomya bezziana has traditionally been treated with extra-label use of ivermectin. The larvicidal activities of nitenpyram and spinosad/milbemycin also have been described, but there have been no reports to describe the efficacy of isoxazolines such as afoxolaner. OBJECTIVES: To evaluate and compare the efficacy of spinosad, spinosad/milbemycin, milbemycin, nitenpyram and afoxolaner for the treatment of canine screw-worm myiasis. ANIMALS: Forty client-owned and naturally infested dogs. METHODS: Cases were randomized into five groups of eight dogs, with each receiving an insecticide at the dose recommended by the manufacturer. Each case was evaluated hourly for 7 h and then again 24 h after treatment. Scores of 0 (no visualized effect), 0.5 (partial eradication) or 1 (complete kill and eradication) were recorded at each observation and compared between groups. Time to complete killing of all larvae was recorded for each dog and compared between treatment groups. RESULTS: Nitenpyram killed all larvae at 6 h post-consumption and spinosad/milbemycin at 7 h. In groups receiving afoxolaner or spinosad, all larvae were killed within 24 h. For those receiving milbemycin, two cases were still infested with live larvae at 24 h. Shih tzus and their crosses were most commonly affected. CONCLUSION: Spinosad/milbemycin or nitenpyram seem to be effective drugs for the treatment of canine screw-worm myiasis. Afoxolaner and milbemycin are effective but exhibited slower larvicidal activity. Synergism between spinosad and milbemycin was observed.