Evaluation of the adulticidal efficacy of imidacloprid 10%/ moxidectin 2.5% spot-on (Advocate®, Advantage® Multi, Bayer) against Dirofilaria repens in experimentally infected dogs

This study aimed to evaluate the efficacy of imidacloprid 10%/moxidectin 2.5% spot-on (Advocate®/Advantage® Multi, Bayer) against adult Dirofilaria (D.) repens in a blinded, placebo controlled randomised laboratory study. 24 Beagle dogs were experimentally infected with approximately 75 infective D. repens larvae on study day (SD) 0. A modified Knott test was used in monthly intervals to monitor the onset of patency and to follow the course of microfilariaemia after treatment. Treatment was initiated on SD 228 after patency had been confirmed in 21 dogs. 11 dogs received monthly treatments with imidacloprid/moxidectin at the minimum therapeutic dose for six consecutive months and 12 control dogs were treated with a placebo formulation. Approximately one month after the last treatment, all dogs were euthanized and necropsied for the detection of D. repens worms. 11 control dogs harbored live adult D. repens (range 2-11, geometric mean 5.44). 8 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 was 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 in all study animals. It is concluded that six consecutive monthly treatments with imidacloprid/moxidectin topical solution are effective and safe against adult D. repens and provide an option for the prevention of further spreading of this zoonotic parasite.