These studies demonstrated that the imidacloprid + moxidectin topical solution provided rapid initial speed of kill against the KS1 flea strain, with 100% efficacy within 12 hours of treatment. However, the initial speed of kill of selamectin was not quite as rapid. The results for the initial speed of kill of selamectin in this investigation were similar to results from a 2005 laboratory study against the KS1 flea strain; in this current investigation the 12 hour post-treatment efficacy was 69.4% and the 12 hour post-treatment efficacy in the 2005 study was 59.7% . It is unknown why selamectin has a slower initial speed of kill, but may be related to the systemic activity of the product and its need to be absorbed across the skin to reach effective blood levels.
In addition the residual efficacy of selamectin in this study against the KS1 flea strain was similar to the residual efficacy observed in the 2005 study . In the current investigation the 12 hour post-infestation residual efficacy assessments efficacy ranged from 99.1% on day 7 to 57.3% on day 28. In the 2005 study, at the same post-treatment time points, the efficacy was 99.4% and 70.9%. Also in the 2005 study the efficacy observed when fleas were removed 48 hours after the 28 day infestation was 99.0%,  while in the current investigation the efficacy at the same time period was 98.3%. Additionally, the 24 hour residual efficacy of selamectin on day 28 in the 2005 study was 90.1%, while in the current investigation it was 87.1% in study #1 and 95.3% in study #2. Even though these studies were conducted over 5 years apart, the efficacy assessments for selamectin against the KS1 fleas strain were remarkably similar.
Also of interest is the difference in residual efficacy between the imidacloprid + moxidectin topical solution in this study and the 9.1% w/w imidacloprid formulation evaluated in the 2005 study. In the previous 2005 speed of kill study against the KS1 flea strain, the imidacloprid topical solution did not provide a more rapid residual speed of kill at 12 hours post-infestation at 7, 14, 21 or 28 days post-treatment than the selamectin formulation . While, in this current investigation imidacloprid + moxidectin topical solution provided a more rapid residual speed of kill at every 12 hour post-infestation assessment. In 2005 the 12 hour post-infestation efficacy at 21 and 28 days for imidacloprid mono topical solution was 65.2% and 61.6%, respectively . In this current investigation the 12 hour post-infestation efficacy at 21 and 28 days for imidacloprid + moxidectin was 98.5% and 90.2%, respectively.
While direct statistical comparison between the current and 2005 study cannot be conducted, it appears that the imidacloprid + moxidectin formulation provides a more rapid residual speed of kill against the KS1 flea strain than imidacloprid mono topical solution. Whether the more rapid residual speed of kill is from an additive or synergistic effect of the moxidectin is currently unknown.
It has previously been demonstrated that several flea products do not perform well against the KS1 flea strain either due to resistance or innate reduced susceptibility [6–13]. It has been demonstrated that the residual speed of kill of older pyrethroid and organophosphate based flea products is very poor against this flea strain due to resistance [6, 8, 11]. In addition more modern insecticides such as fipronil, imidacloprid, and spinosad also have reduced activity against the KS1 strain [7, 9, 10, 12, 13]. These newer insecticides were introduced into the U.S. as flea products 6 years (fipronil and imidacloprid) or 17 years (spinosad) after the KS1 strain was colonized. Various studies using other cat flea strains have reported that the 28-30 day residual efficacy of fipronil, imidacloprid, and spinosad flea products should range from approximately 95% to 100% [7, 14–21]. However, when these formulations were evaluated against the KS1, strain the 28-30 day residual efficacy was markedly reduced [7, 10, 12, 13]. While these insecticides have reduced residual activity against the KS1 strain, dinotefuran, metaflumizone and selamectin topical spot-on formulations have demonstrated excellent residual efficacy against this strain [10, 12, 13, 22].