Susceptibility of Rhipicephalus microplus to fluralaner: assessment before and after implementation of a strategic control protocol in taurine cattle in a tropical region

BACKGROUND: Fluralaner has been used in the strategic control of the Rhipicephalus microplus tick, an important ectoparasite for cattle farming. However, the effects of its application on the susceptibility profiles of these tick populations after treatment have not yet been investigated. In this study, we evaluated the effect of a strategic control protocol with fluralaner on the phenotypic susceptibility profile of a population of R. microplus before and after its implementation. METHODS: Thirty naturally infested cattle were divided into two experimental groups: T01 (control group): cattle treated with a topical (spray) formulation containing a synthetic pyrethroid and two organophosphates; T02 (strategic control): cattle treated with a formulation (pour-on) containing only fluralaner. For group T01, animals were treated when the average tick count was ≥30 to allow determination of the infestation challenge on the farm, while animals in group T02 were treated when 30% of the animals had adult ticks < 4 mm. Tick counts on animals were performed every 7 days for 266 days. Larvae monitoring in the pasture was performed every 30 days using a dragging flannel for 240 days. Larval immersion tests (LIT) and adult immersion tests (AIT) were used to assess the phenotypic susceptibility profile of ticks before and after the implementation of the strategic control protocol over a tick season in a tropical region. RESULTS: The animals in the group treated with fluralaner (T02) had lower mean tick counts (p ≤ 0.05) than those observed in the control group (T01) in 35 of the 38 evaluations during the 266 days of the study. In the pasture, the number of larvae in T02 was lower (p < 0.05) in seven of the eight evaluations. There was no increase in LC50 values for larvae and engorged females after using the strategic control protocol with fluralaner for 9 months. CONCLUSIONS: The use of the strategic control protocol with fluralaner resulted in a significant reduction in tick numbers on cattle and in pastures, and after 1 year of use, the population remained susceptible to this compound. No upward trend in LC50 values was observed after 9 months of fluralaner use.