Comparative efficacy of six monthly doses of Simparica Trio® (sarolaner, moxidectin, and pyrantel chewable tablets) versus NexGard® Plus (afoxolaner, moxidectin, and pyrantel chewable tablets) against a macrocyclic lactone-resistant Dirofilaria immitis isolate in dogs.

BACKGROUND

The emergence of macrocyclic lactone (ML)-resistant Dirofilaria immitis highlights the need for informed and considered use of currently available heartworm preventives. Moxidectin provides robust protection against multiple ML-resistant D. immitis isolates, with efficacy dependent upon both the dosage and the number of monthly treatments administered. In this study the efficacy of six monthly treatments of Simparica Trio® (24 µg/kg moxidectin, 1.2 mg/kg sarolaner, and 5 mg/kg pyrantel pamoate) was compared with NexGard® Plus (12 µg/kg moxidectin, 2.5 mg/kg afoxolaner, and 5 mg/kg pyrantel pamoate), both administered at the recommended label dose.

METHODS

A total of 24 dogs were randomly allocated to negative control, NexGard Plus, or Simparica Trio groups. Each dog was inoculated with 50 ML-resistant D. immitis L3 larvae (ZoeLA isolate) on day -30. All dogs were dosed orally on days 0, 30, 60, 90, 120, and 150. Upon study completion (day 237), all adult worms were recovered and counted.

RESULTS

Mean moxidectin dosages administered to each group were 17.6 ± 1.6 µg/kg for NexGard Plus and 37.9 ± 8.4 µg/kg for Simparica Trio. Geometric mean (GM) adult worm counts in both NexGard Plus (0.7) and Simparica Trio (0.2) groups were significantly lower than negative control (P < 0.0001), with NexGard Plus and Simparica Trio providing 98.1% and 99.5% efficacy, respectively. However, D. immitis adult worms were recovered in 6/8 (75%) NexGard Plus-treated dogs compared with 2/8 (25%) Simparica-Trio-treated dogs, and the GM worm counts were significantly lower (P = 0.0316) in Simparica-Trio-treated dogs. Additionally, four (50%) dogs treated with NexGard Plus were positive for D. immitis antigen, and one of these four was microfilaremic. In contrast, in the Simparica-Trio-treated group, a single dog was positive for D. immitis antigen, and all eight dogs were amicrofilaremic.

CONCLUSIONS

Six consecutive administrations of the higher label dosage of moxidectin delivered by Simparica Trio (24-48 µg/kg) protected more dogs from infection with the ML-resistant D. immitis isolate ZoeLA than the lower label dosage of moxidectin provided by NexGard Plus (12 - 24 µg/kg).