Evaluation of the Adulticidal Efficacy of Imidacloprid 10 %/Moxidectin 2.5 % (w/v) Spot-on (Advocate®, Advantage® Multi) against Dirofilaria repens in Experimentally Infected Dogs

This study aimed to evaluate the efficacy of imidacloprid 10 %/moxidectin 2.5 % (w/v) spot-on (Advocate®/Advantage® Multi, Bayer) against adult Dirofilaria repens in a blinded, placebocontrolled randomised laboratory study. Twentyfour Beagle dogs were experimentally infected with approximately 75 infective D. repens larvae each on study day (SD) 0. Treatment was initiated on SD 228 after patency had been confirmed in 21 dogs, using a modified Knott Test. Eleven dogs received monthly treatments with imidacloprid/moxidectin at the minimum therapeutic dose (10 mg/kg imidacloprid and 2.5 mg/kg moxidectin) for six consecutive months and 12 control dogs were treated with a placebo formulation. Approximately one month after the last treatment, all dogs were euthanised and necropsied for the detection of D. repens worms. Eleven control dogs harboured live adult D. repens (range 2–11, geometric mean 5.44). Eight of 11 imidacloprid/moxidectin-treated dogs were free of live worms. The live worm count was reduced by 96.2 % (range 0–1, geometric mean 0.21). The majority of dead worms were encapsulated and degenerated. After the first treatment, Knott Tests were negative in all imidacloprid/moxidectin-treated dogs and this status was maintained in 10 dogs until study end. One dog showed a low microfilariae count (1 and 4/mL) on four occasions but was also negative before necropsy. The treatment was well tolerated by all study animals. It is concluded that six consecutive monthly treatments with imidacloprid/moxidectin spot-on are effective and safe against adult D. repens and provide an option for preventing the further spread of this zoonotic parasite.