This study demonstrated that the oral spinosad formulations provided rapid initial speed of kill against the KS1 flea strain, with 100% efficacy within 24 hours of treatment. However, the initial speed of kill of selamectin was not as rapid; selamectin provided 60.4% and 91.4% reductions in flea populations within 24 and 48 hours of treatment, respectively.
The results for the spinosad formulations in this study were similar to several previous studies where the efficacy was also 100% within 24 hours of oral administration to dogs [12–14]. The results for the speed of kill of selamectin in this study are also comparable to previous studies where efficacy ranged from 83.7% at 24 hours to 100% at 48 hours [15–17]. It is likely that the slower initial speed of kill of selamectin as compared to spinosad is related to the need to be absorbed across the skin to reach effective blood levels.
While the initial speed of kill of selamectin was slower than the spinosad formulations, the residual speed of kill of selamectin against the KS1 flea strain was similar to or better than the spinosad-based oral formulations. The spinosad/milbemycin oxime- and spinosad-based formulations were >90% effective against the KS1 strain from Day 1 to Day 23. Whereas, selamectin was >90% effective against the KS1 strain of from Day 2 to Day 30. The residual speed of kill of selamectin in this study was similar to that seen in previous studies. In two studies the efficacy of selamectin on dogs 48 hours following infestations on Day 28 ranged from 99.2% to 100% [16, 18].
The residual speed of kill of spinosad in this study contrasts with some previous reports. In a study where dogs were fed and then given spinosad at the minimal dose of 30 mg/kg, efficacy 48 hours after the Day 28 flea infestation ranged from 99.1-99.5% . In a study evaluating and comparing the spinosad and spinosad/milbemycin oxime tablet formulations administered to dogs, both formulations provided 100% efficacy 48 hours after the Day 28 flea infestation . However, in a different study, the efficacy of spinosad administered to dogs 24 and 48 hours after the Day 28 infestation was 85.0% and 89.0%, respectively . These data are further contrasted with two previous studies evaluating the efficacy of spinosad using the KS1 flea strain where the efficacy 24 hours after the Day 28 infestation was only 22.1% and 32.5% . It is interesting to note that in the current study using the same flea strain, the efficacy 24 hours following the Day 28 flea infestation was 72.9% and 84.7% for the spinosad and spinosad/milbemycin oxime formulations, respectively. It is unknown why this difference in efficacy was seen between these two studies.
Several studies have demonstrated that the KS1 flea strain has reduced susceptibility or outright resistance to carbaryl, chlorpyriphos, fenthion, fipronil, imidacloprid, permethrin, pyrethrins, and spinosad [4, 7–12]. While these insecticides have performed poorly as residual insecticides against the KS1 strain, dinotefuran, metaflumizone and selamectin topical spot-on formulations have demonstrated excellent residual efficacy against this strain [3, 11, 12, 20]. The good residual efficacy of selamectin against the KS1 flea strain was again demonstrated in this current study.
While laboratory studies are important and often necessary to evaluate and compare the efficacy of flea products, the performance of the products under natural in-home situations is equally important. A large scale clinical field trial was conducted in the U.S. where spinosad was administered orally to 330 dogs and selamectin was applied topically to 140 dogs . By Days 60 and 90 of the 3 month trial, the percent reduction in flea numbers on the dogs were 99.7% and 99.9% for the spinosad-treated dogs and 97.9% and 98.9% for the selamectin-treated dogs, respectively . In another three month clinical field trial conducted in France, the percent reduction in flea numbers on the dogs on Days 60 and 90 were 99.6% and 99.6% for the spinosad-treated dogs and 97.8% and 98.2% for the selamectin-treated dogs, respectively . Both these clinical field trials demonstrated that selamectin and spinosad were highly effective in eliminating flea infestations on client owned dogs.