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

Overview

• This study developed a stall-trial method to test how a 1% fipronil pour-on treatment is absorbed in horses and how effectively it controls the tick species Dermacentor nitens.
• The research demonstrated that the treatment was both therapeutically and preventively effective against larvae, nymphs, and adult ticks on treated horses.

Study Objective and Design

• The primary goal was to standardize a stall-trial methodology to evaluate both the pharmacokinetics (how the drug moves through the horse’s body) and efficacy of a 1% fipronil pour-on formulation against Dermacentor nitens ticks.
• Brazilian pony horses were used, housed individually in covered stalls to control environmental variables.
• These horses were experimentally infested with larvae of D. nitens on alternating days from day -31 up to day -1 before treatment, ensuring all animals had tick exposure prior to treatment.
• On day 0, horses received the fipronil pour-on dose at 1 mg per kilogram of body weight.

Measurement of Treatment Efficacy

• Therapeutic efficacy:
  • Involved counting the number of engorged female ticks that naturally detached from both treated and control horses over 29 days post-treatment.
  • This measure indicates how well the treatment kills ticks already on the animals.
• Persistent efficacy:
  • New larval infestations were performed on days 7, 14, and 21 after treatment to test protection over time.
  • Tick counts were recorded up to day 50 to measure how long the treatment remained effective.

Pharmacokinetic Analysis

• Blood plasma samples were collected at various times: before treatment (time 0), 4 hours after, then on days 1, 2, 7, 12, 14, 15, 16, 21, 28, and 48.
• Concentrations of fipronil and its active metabolite fipronil sulfone were measured in plasma.
• Fipronil reached its peak concentration (Cmax) around day 7 with an average of 0.71 ± 0.30 µg/mL.
• Fipronil sulfone peaked later on day 21 with an average concentration of 0.27 ± 0.06 µg/mL.

Results: Efficacy Against Ticks

• The overall acaricidal (tick-killing) efficacy from day 1 to day 29 was approximately 91.83%, indicating a strong immediate effect after treatment.
• Persistent efficacy against tick challenges was:
  • 99.69% on day 7 post-treatment
  • 99.19% on day 14 post-treatment
  • 99.32% on day 21 post-treatment
• These high percentages illustrate sustained protection against new larval infestations for at least three weeks.

Significance of the Stall Trial Methodology

• Housing horses individually in stalls allowed detailed control of tick infestations and minimized outside environmental factors that might affect results.
• This method enabled evaluation of the treatment efficacy against all parasitic stages of the tick—larvae, nymphs, and adults—present on the animals.
• The reduction in detached, engorged ticks demonstrated the treatment’s effectiveness in killing ticks at multiple lifecycle stages.

Conclusions

• The study validated a reliable stall-trial method for testing acaricide efficacy in horses under controlled conditions.
• The 1% fipronil pour-on formulation demonstrated strong therapeutic and preventive efficacy against Dermacentor nitens ticks.
• This formulation can significantly reduce tick infestation burden on horses, protecting animal health over a sustained period after application.