Efficacy of four commercially available heartworm preventive products against the JYD-34 laboratory strain of Dirofilaria immitis

Background: Heartworm disease in dogs can be severe and life threatening. Resistance to available heartworm preventives was considered among potential causes of increased reports of failed heartworm prevention in dogs. The objective of the present study was to compare the efficacy of four commercially available heartworm disease preventives against the JYD-34 strain of D. immitis. Methods: Forty laboratory-reared dogs approximately 6 months old were used. Each dog was infected with fifty, third-stage heartworm larvae on study day (SD) -30. On SD-1, the dogs were randomized to five groups of eight dogs each. On SD-0, dogs in groups 1-4 were treated as follows: Group 1: ivermectin/pyrantel pamoate chewable tablets; Group 2: milbemycin oxime/spinosad tablets; Group 3: selamectin topical solution; and Group 4: imidacloprid/moxidectin topical solution. Dogs in Group 5 were not treated and served as controls. The dogs were treated according to their current body weights and labelled dose banding for each product. Groups 1, 2, and 3 were retreated with their respective products and current body weights on SD 31 and 60. On SDs 124-126 the dogs were euthanized and necropsied for recovery of adult heartworms. Results: Adult heartworms were recovered at necropsy from each of the dogs in the control group (13-32 worms/dog, geometric mean (GM) = 18.4 worms/dog). Adult heartworms and/or worm fragments were also recovered from each of the dogs treated with ivermectin/pyrantel pamoate, milbemycin oxime/spinosad or selamectin. Geometric means of worms recovered from dogs in each of these groups were 13.1, 8.8, and 13.1, resulting in efficacies compared to controls of 29.0, 52.2, and 28.8 %, respectively. All dogs in Group 4 (imidacloprid/moxidectin) were free of adult heartworms (100 % efficacy). Conclusions: The combination of imidacloprid/moxidectin was 100 % effective in this study in preventing development of JYD-34 laboratory strain of D. immitis in dogs following a single treatment, while three monthlytreatments of the three other commercial products provided less than 100 % efficacy. The high efficacy achieved with imidacloprid/moxidectin was likely due to the unique pharmacokinetic properties of the topical formulation delivering greater and sustained drug concentrations necessary to prevent development of D. immitis larvae.