Flea control is a constant challenge for most pet owners, one that requires diligence and consistency. Recently, a new orally administered month-long flea preventive agent was approved for use in dogs in the U.S. This study examined the effects of spinosad on the cat flea. This collaborative effort among several groups of researchers sought answers to 3 questions: 1) What is the speed of kill on infested dogs? 2) Does spinosad prevent environmental contamination with flea eggs? and 3) Does skin debris from treated dogs affect flea eggs or larvae? To address the first question, dogs were administered spinosad at a minimum dose of 30 mg/kg, and flea comb counts were performed 0.5 to 48 hours after treatment. Flea counts were reduced by 53% at 0.5 hour after treatment as compared with placebo controls and by 100% at time points of 4 hours and later. To answer question 2, dogs were treated with spinosad or placebo, then infested with 600 fleas 3 hours later. Flea eggs were collected first after 3 days and then weekly throughout the next month. Spinosad-treated dogs showed 99.8% reduction in flea egg production over the month-long study period. For the final question, skin debris collected from placebo- and spinosad-treated dogs was added to culture dishes with growth media and flea eggs or larvae. This experiment showed no significant difference in flea development between treated and control groups, suggesting that spinosad is not present in the skin of treated dogs. Study supported by Elanco Animal Health

Commentary: Taken together, the results of this study show that spinosad kills adult fleas fast and that the effects of treatment last throughout the recommended monthly dosing period. By killing fleas before they can lay eggs, spinosad effectively ceases the life cycle and prevents new flea eggs in the environment. Because it is orally administered, spinosad may be a preferable option to pet owners whose dogs require frequent bathing or enjoy water sports, which can wash away topical products. One note of caution: Dogs who are receiving extra-label high doses of ivermectin should not be given spinosad concurrently because of possible exacerbation of ivermectin toxicity.

Effects of orally administered spinosad (Comfortis) in dogs on adult and immature stages of the cat flea. Blagburn BL, Young DR, Moran C, et al. VET PARASITOL 168:312-317, 2010.