Protection of dogs against canine heartworm infection 28 days after four monthly treatments with Advantage Multi® for Dogs

Background: Monthly heartworm preventives are designed to protect dogs by killing heartworms acquired the month prior to their administration, and after treatment with most products, the drug levels rapidly dissipate to very low levels. Work with Advantage Multi (R) for Dogs (imidacloprid + moxidectin) topical solution showed protection against hookworm infection throughout the month after administration of several monthly doses suggesting that similar protection might occur with heartworms. This study assessed the amount of protection afforded to dogs by the administration of four monthly doses of Advantage Multi for Dogs prior to infection with third-stage heartworm larvae (Dirofilaria immitis) 28 days after the last (fourth) treatment. Methods: There were 16 purpose-bred mongrel dogs in the study that were divided into two groups, 8 control and 8 treated dogs. Dogs were housed in a manner preventing contact between animals and groups, and personal protective gear worn by staff minimised the chance spread of the topically applied product between runs. The dogs in the treated group received monthly applications of Advantage Multi for Dogs as per label instructions on Study Days 0, 28, 56, and 84. On Study Day 112, all 16 dogs received 50 third-stage larvae of D. immitis (Missouri isolate) via subcutaneous inoculation in the inguinal region. The study was terminated on Day 264, and the number of heartworms per dog was determined at necropsy. Results: Moxidectin levels after 4 treatments 28 days apart were near steady state on Study Day 112 when the dogs were inoculated with D. immitis third-stage larvae. At necropsy, 152 days after infection, all the control dogs had adult worms in their pulmonary arteries (geometric mean = 33.9; range 25-41), and none of the dogs treated four times prior to infection, with the last treatment 30 days prior to infection, harbored worms at necropsy. Conclusions: The efficacy of prevention was 100 % when the dogs were infected 28 days after the last monthly treatment. When dogs receive consecutive doses of Advantage Multi for Dogs as prescribed, heartworm infections will be prevented throughout the monthly dosing interval after administration of several monthly doses.