Objective—To evaluate efficacy of monthly administration of selamectin, fipronil, and imidacloprid against Ctenocephalides felis in dogs.
Design—Randomized controlled trial.
Animals—44 healthy dogs.
Procedure—Dogs known to be free of fleas were infested with 100 unfed adult fleas on days –28 and –21. On days 0, 30, 60, 90, and 120, dogs (12/group) were treated by topical administration of selamectin (6 mg/kg [2.7 mg/lb] of body weight), fipronil (7.5 mg/kg [3.4 mg/lb]), or imidacloprid (10 mg/kg [4.5 mg/lb]); 8 untreated dogs were used as controls. On day –6 and every 2 weeks after initial treatment, comb counts of viable adult fleas were made, and fleas (≤ 50/dog) were replaced onto the dog from which they were removed. On day 89, fleas were not replaced. On day 91 and every 7 days until the end of the study, dogs were challenged with 20 adult fleas.
Results—14 days after initial treatment, geometric mean flea counts were reduced by 97.5 to 99.1% for all treatments, compared with pretreatment counts on day –6. Selamectin, fipronil, and imidacloprid reduced geometric mean flea counts by 99.7 to 100% from day 29 to the end of the study.
Conclusions and Clinical Relevance—Selamectin is as effective as fipronil and imidacloprid in reducing C felis infestation in dogs housed for 3 months in a flea-infested environment under conditions known to support the flea life cycle, and in protecting against subsequent weekly challenges with C felis for an additional 2 months. (J Am Vet Med Assoc 2000;217: 1669–1671)