Efficacy of oral afoxolaner plus milbemycin oxime chewables against induced infestations with Dermacentor reticulatus in dogs

The efficacy of afoxolaner plus milbemycin oxime (AFX + MO) combination chewables (NexGard SpectraA (R), Merial) and AFX single-entity chewables (NexGardA (R), Merial) against induced infestations with Dermacentor reticulatus ticks was evaluated in dogs. Thirty dogs were assigned to blocks of three animals each based on pre-allocation tick counts and were randomly allocated to one of three groups: untreated (control), treated with a combination of AFX + MO chewables to be as close as possible to the minimum effective dose of AFX + MO (2.5 + 0.5 mg per kg body weight), and treated with a combination of NexGardA (R) chewables to be as close as possible to the minimum effective dose of AFX (2.5 mg per kg body weight). Treatments were administered orally once on day 0. Starting 2 days before treatment administration, each dog was infested with approximately 50 ticks weekly for six consecutive weeks. Live ticks were counted at similar to 48 h post-treatment (removal count) and at similar to 48 h (in situ counts) and similar to 72 h (removal counts) following each post-treatment infestation. Treatment with both AFX + MO and NexGardA (R) chewables rapidly eliminated the existing tick infestations (100 % efficacy) within 2 days following treatment administration. Weekly re-infestations were controlled for a minimum of 5 weeks with the efficacy ranging from 92.2 to 99.7 % based on similar to 48 h post-treatment in situ counts and between 99.0 and 100 % based on similar to 72 h post-treatment removal counts (p < 0.0001 at each occasion). This study demonstrated a high efficacy of both AFX + MO chewable and NexGardA (R) chewable treatments against infestations of dogs with D. reticulatus ticks for at least 5 weeks. In addition, this study indicated no interference between the two compounds with respect to the acaricidal activity provided by AFX.