The effectiveness of a fixed-dose combination pour-on formulation of 1.25% fipronil and 2.5% fluazuron against economically important ectoparasites and associated pharmacokinetics in cattle

The present work consisted of eight studies to evaluate the ectoparasiticidal spectrum and determine the pharmacokinetic parameters of a pour-on combination of fipronil 1.25 mg/kg + fluazuron 2.5 mg/kg for cattle against Rhipicephalus microplus, Haematobia irritans and the larvae of Dermatobia hominis and Cochliomyia hominivorax. The analysis fipronil and fluazuron were performed by liquid chromatography using a mass detector for the detection and quantification of analytes (LC-MS/MS). Additionally, in two of these studies, the animals were artificially infested with R. microplus ticks (stall tests), and the efficacy of this formulation was compared with that of two other standalone pour-on formulations of fipronil 1.0 mg/kg and fluazuron 2.5 mg/kg. In the two stall studies, 28 calves were artificially infested with 5000 R. microplus (different strains), and daily collections of all of the engorged female ticks that detached from each calf were performed until 60 and 100 days post-treatment (dpt). For the R. microplus field trials, 20 bovines were selected by counting the semi-engorged females, and the therapeutic and residual efficacy was evaluated by taking tick counts at 3, 7, 14, 21, 28, 35, 42, 49 and 56 dpt. Forty bovines that were naturally infested with Dermatobia hominis larvae were selected, and the numbers of larvae were counted by visual and tactile inspection on 3, 7, 14, 28, 35, 42 and 49 dpt. To address the efficacy on C. hominivorax larvae, two circular skin incisions (one on each side of the body) measuring approximately 4 cm in diameter each were made in 12 crossbred calves, and the natural exposure of the lesions to C. hominivorax infestations was then allowed. The incisions from the 12 animals were carefully examined daily from 1 to 10 dpt. Based on the PK results obtained for this pour-on combination containing fipronil 1.25 mg/kg + fluazuron 2.5 mg/kg, the maximum concentrations (Cmax) and the half-lives (T1/2) of these two active ingredients were detected on days 2, 5/6 and 19 (± 2)/24, 4 (± 3.5) days for fipronil and fluazuron, respectively. Furthermore, the combination showed higher therapeutic and residual efficacy against R. microplus (P ≤ 0.05) when compared with commercial standalone formulations that were administered separately. A high efficacy for this new combination was also found against C. hominivorax and D. hominis larvae (efficacy ≥ 99%). This study's results show that the combination of these two active ingredients, as opposed to their separate use, could represent a tool for extending the life cycle of these two molecules against cattle ectoparasites, especially R. microplus. Further studies would be desirable to further confirm this.