Preventive efficacy of six monthly doses of NexGard® PLUS or Simparica Trio® against a macrocyclic lactone-resistant isolate (JYD-34) of Dirofilaria immitis and of a single dose of NexGard PLUS against a susceptible isolate.

Abstract

BACKGROUND: Two studies were conducted assessing the efficacy of NexGard&#xae; PLUS (NP) in preventing heartworm disease. Study 1 evaluated the efficacy of six monthly doses of NP or Simparica Trio&#xae; (ST) against a macrocyclic lactone-resistant isolate of heartworm, Dirofilaria immitis, and study 2 evaluated the efficacy of a single dose of NP against a susceptible isolate. METHODS: In two studies, dogs that were negative for heartworms by antigen test and modified Knott's test were used. In study 1, dogs were randomly allocated into three treatment groups (n&#x2009;=&#x2009;6/group): negative control, NP per label instructions, and ST per label instructions. Dogs were inoculated with 50 third-stage D. immitis larvae (JYD-34 isolate) on day -30. NP and ST were administered orally on days 0, 30, 60, 90, 120, and 150. A necropsy was performed on day 180 for adult heartworm recovery. In study 2, dogs were randomly allocated into two treatment groups (n&#x2009;=&#x2009;10/group): negative control and NP. Dogs were inoculated with 50 third-stage larvae (SC-20 isolate) on day -30. NP was administered orally once on day 0 to target the minimum moxidectin label dose. A necropsy was performed on day 120 for adult heartworm recovery. RESULTS: For study 1, all control dogs had adult heartworms at necropsy (geometric mean, 39.7; range, 28-48). Two of the NP-treated dogs had one live worm, and one of the ST-treated dogs had one live worm. Both treated groups were significantly different from the control group with an efficacy of 99.5% for NP and 99.8% for ST (P&#x2009;<&#x2009;0.0001). There was no significant difference (P&#x2009;=&#x2009;0.8797) between the groups treated with NP and ST. For study 2, all control dogs had adult heartworms (geometric mean, 34.5; range 26-43). None of the dogs treated with NP had live adult worms (efficacy of 100%, P&#x2009;<&#x2009;0.0001). CONCLUSIONS: The results of study 1 demonstrated that NexGard&#xae; PLUS and Simparica Trio&#xae; administered at the label dose provided comparable efficacy against a macrocyclic lactone-resistant isolate of D. immitis. The results of study 2 demonstrated that NexGard&#xae; PLUS administered once near the minimum label dose was 100% effective against a susceptible isolate.