Comparative efficacy of two fipronil spot-on formulations against experimental tick infestations (Ixodes ricinus) in dogs.

A parallel-group-design, randomized, unicentre and blinded controlled study was undertaken to assess the efficacy of a new fipronil-based spot-on formulation applied once to dogs against experimental Ixodes ricinus infestations. Six dogs served as negative controls (group 1), six dogs served as positive controls (group 2) receiving the original fipronil spot-on (Frontline(R) spot-on Dog, Merial) at a dosage of 0.67 mL for a dog weighing from 2 to 10 kg and 1.34 mL for a dog weighing from 10.1 to 20 kg and six dogs were treated with a 10% w/v fipronil-based spot-on solution (Effipro(R) Spot-on, Virbac SA) at an identical dosage (group 3, 0.67 mL for a dog weighing from 2 to 10 kg and 1.34 mL for a dog weighing from 10.1 to 20 kg). Each dog was sedated and subsequently infested with 50 unfed adult I. ricinus on days -7, -2, 7, 14, 21, 28 and 35. Forty-eight hours after the treatment and 48 h after each challenge (days -5, 2, 9, 16, 23, 30 and 37), the population of the remaining ticks was assessed for each animal. Geometric mean tick counts obtained were reduced by 99% and 94% on day 2 in groups 2 and 3, respectively, compared to the negative control group. Dogs were protected from re-infestations with an efficacy of >90% for 3 weeks in group 2 and for 5 weeks in group 3. Both 10% w/v fipronil-based spot-on solutions, despite different vehicles, were equally able to eradicate tick infestation, to prevent new infestations and were equally well tolerated.