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Two topically applied spot-on products–65% permethrin (Defend Exspot Treatment for Dogs, Schering-Plough Animal Health, Union, NJ) and 9.7% fipronil (Frontline Spot On Dog, Merial Limited, Iselin, NJ)–used for canine flea and tick control were evaluated for repellency against Ixodes ricinus, the tick species that is the primary vector of
Lyme disease in Europe. Eighteen dogs were randomly assigned to the following treatment groups (n = 6): (1) 65% permethrin, (2) 9.7% fipronil, or (3) untreated control. Dogs were exposed to ticks in individually assigned cages with a carpet that covered =70% of the cage bottom. Dogs in treatment groups 1 and 2 were treated in accordance with label directions for each re-spective product on study day 0. Fifty unfed, male and female adult ticks were placed in the cages 15 to 30 minutes before the dogs. The dogs were placed in the cages for a 2-hour exposure period at 2, 7, 14, 21, 28, 35, and 41 days after treatment. After a 2-hour exposure period, dogs were removed from the cages and live (attached and unattached) and dead ticks were counted on the dogs, on the carpets, and in the cages. Cages were thoroughly cleaned and new carpet was used for each tick exposure period. Treatment of dogs with 65% permethrin reduced tick numbers on dogs by 99.1% at 2 days, 99.0% at 1 week, 95.9% at 2 weeks, 88.5% at 3 weeks, 87.1% at 4 weeks, and 48.0% at 6 weeks after application. In contrast, treatment of dogs with 9.7% fipronil reduced tick numbers on dogs by 61.4% at 2 days, 51.6% at 1 week, 37.0% at 2 weeks, 33.7% at 3 weeks, 10.8% at 4 weeks, and 0% at 6 weeks after application. The efficacy of 65% permethrin was significant (P < or = .05) when compared to the control at all challenges, whereas the efficacy of 9.7% fipronil was not significant as compared to controls at 21, 28, 35, and 41 days after treatment. The 65% permethrin killed significantly more Ixodes ricinus ticks (P < or = .05) than 9.7% fipronil from 2 to 41 days after treatment. The 65% permethrin repelled 1.9-, 2.0-, 3.0-, 43.3-, 3.9-, 8.9-, and 17.3-fold more Ixodes ricinus than did 9.7% fipronil at 2, 7, 14, 21, 28, 35, and 41 days after treatment, respectively, and all differences in repellency were significant (P < or = .05).