Pharmacokinetics of a fipronil pour-on formulation and its efficacy against Dermacentor nitens (Acari: Ixodidae) in horses.

The objective of this study was to standardize a stall-trial methodology for horses in order to evaluate the pharmacokinetics and efficacy of a 1 % fipronil pour-on formulation against Dermacentor nitens. Brazilian pony breed horses were housed in individual stalls within a covered barn and artificially infested with D. nitens larvae on alternate days from day -31 to -1 before treatment. The animals were treated on day 0 with a fipronil 1 % pour-on at a dose of 1 mg/Kg body weight. To determine the therapeutic efficacy, the number of engorged females naturally detached from control and treated groups was recorded for 29 days after treatment. To evaluate the persistent efficacy, further larvae infestations were performed on days 7, 14 and 21 after treatment and tick counts were recorded until day 50. The determination of fipronil and its metabolite fipronil sulfone concentration in the plasma was performed at times of 0, 4 h and on days 1, 2, 7, 12, 14, 15, 16, 21, 28, and 48 after treatment. The overall acaricidal efficacy was 91.83 % from day 1-29. The persistent efficacy was 99.69; 99.19 and 99.32 % against tick infestation challenges performed on days 7, 14 and 21, respectively. In plasma, the Cmax of fipronil was 0.71 ± 0.30 µg/mL on day 7 and 0.27 ± 0.06 µg/mL for fipronil sulfone on day 21. The implementation of a stall trial with horses enables the evaluation of the effectiveness of acaricides against all life stages of ticks without interference from environmental factors. The observed reduction in the number of detached, engorged ticks indicates that Fipronil 1 % pour-on is effective against all parasitic stages present on the animal: larvae, nymphs, and adults. Therefore, fipronil 1 % pour-on is both therapeutically and preventively effective against D. nitens in horses.