Comparative speed of kill of sarolaner (Simparica™) and afoxolaner (NexGard®) against induced infestations of Ixodes scapularis on dogs

BACKGROUND: The black-legged (or deer) tick, Ixodes scapularis, commonly infests dogs and cats in North America and is the main vector for the pathogen that causes Lyme disease in dogs and humans. The speed of kill of a parasiticide is critical to minimize the direct and deleterious effects of tick infestation and especially to reduce the risk of tick-borne pathogen transmission. In this study, speed of kill of a novel orally administered isoxazoline parasiticide, sarolaner chewable tablets (Simparica™), against I. scapularis on dogs was evaluated and compared with afoxolaner (NexGard®) for five weeks after a single oral dose. METHODS: Twenty four dogs were randomly allocated to treatment with either placebo, sarolaner (2 to 4 mg/kg), or afoxolaner (2.5 to 6.8 mg/kg) based on pretreatment tick counts. Dogs were examined and live ticks counted at 8, 12, and 24 h after treatment and subsequent re-infestations on Days 7, 14, 21, 28 and 35. Efficacy was determined at each time point relative to counts for placebo dogs. RESULTS: A single oral dose of sarolaner provided >99 % efficacy within 24 h of treatment and >95 % against subsequent weekly re-infestations of ticks consistently to Day 35. For the earlier time points, sarolaner significantly reduced tick counts versus placebo from Day 0 to Day 21 at 8 and 12 h, and on Day 35 at 12 h (P ≤ 0.0174), while afoxolaner was only significantly lower at 8 h on Days 0 and 14 (P ≤ 0.0309), and at 12 h on Day 0 only (P < 0.0001). Significantly more live ticks were recovered from afoxolaner-treated dogs than from sarolaner-treated dogs at 24 h after infestation from Day 14 to Day 35 (P ≤ 0.0278). At 24 h, efficacy (based on geometric mean counts) of afoxolaner declined to less than 80 % from Day 21 through the end of the study, while efficacy for sarolaner was >95 % for 35 days. There were no adverse reactions to treatments. CONCLUSIONS: In this controlled laboratory evaluation, sarolaner had a faster speed of kill against I. scapularis than afoxolaner. This was noticeably more pronounced towards the end of the monthly treatment period. The rapid and consistent kill of ticks provided by sarolaner within 24 h after a single oral dose and re-infestation over 35 days suggests this treatment will provide highly effective and reliable control of ticks over the entire treatment interval, and should reduce the risk of tick-borne diseases, including Lyme disease whose agent is vectored by I. scapularis.