Speed of kill of a new spot-on formulation of selamectin plus sarolaner for cats against induced infestations with Ixodes ricinus.

The speed of kill of a new spot-on formulation containing selamectin plus sarolaner (Stronghold®Plus, Zoetis) for cats was evaluated against Ixodes ricinus ticks in a placebo-controlled, blinded study. Sixteen (16) cats were blocked by pre-treatment tick counts and randomly allocated to the placebo-treated group or the selamectin/sarolaner-treated group. Cats either received a single topical treatment at the minimum dose of 6.0mg selamectin and 1.0mg sarolaner per kg bodyweight or a placebo formulation on Day 0. On Days -2, 7, 14, 21, 28 and 35, cats were infested with approximately 50 unfed, viable and adult I. ricinus ticks. Tick counts were performed in situ 8 and 12h after treatment or re-infestation. Ticks were removed from the cats and counted at the 24h tick count. Acaricidal efficacy at each time point was calculated based on the reduction of mean live tick counts in the selamectin/sarolaner-treated group versus the placebo-treated group. There were no treatment-related adverse reactions during the study. Placebo-treated cats maintained infestations with mean tick counts ranging from 10.3 to 21.9 throughout the study. The new spot-on formulation of selamectin plus sarolaner demonstrated 99.3% efficacy (P<0.0001) within 24h after treatment against pre-existing infestations. For subsequent re-infestations, efficacy was >97.9% for at least 3 weeks and was 89.0% after the re-infestation on Day 28. Mean live tick counts were significantly reduced by 12h after re-infestation for at least 28 days (P<0.0338). Thus, a single application of the new spot-on formulation of selamectin plus sarolaner at the minimum label dose started killing ticks within 24h after treatment and within 12h after re-infestations for 4 weeks. High acaricidal efficacy was achieved within 24h after treatment and this persisted following subsequent re-infestations for a month.