Acaricidal efficacy of fluralaner against Rhipicephalus microplus ticks under laboratory and field conditions in Brazil

Abstract

Background The first isoxazoline-based acaricide (fluralaner) for the control of Rhipicephalus microplus was introduced onto the market in 2022, initially in Brazil, followed by other Latin American countries. Therefore, it is important to establish laboratory methods to monitor the susceptibility of populations of R. microplus to this molecule and to determine the relationship between the results of laboratory tests and those from field trials. Methods A larval immersion test (LIT) was performed on 18 populations of R. microplus. The lethal concentration 50 (concentration causing 50% mortality [LC50]) values were calculated to determine the resistance ratios (RRs) of the populations. The lethal concentration 99 (concentration causing 99% mortality [LC99]) values were calculated to determine the discriminating doses (DDs = 2 × LC99). The DDs were applied in tests with the POA (susceptible) and GYN (resistant) strains, as well as in tests with the population that presented the lowest LC50 value (population 14) and with the two populations that presented the highest LC50 value (populations 10 and 16). Finally, we performed field trials with the population that presented the lowest and two highest LC50 values. Results In the LIT with fluralaner, the LC50 values ranged from 0.144 to 0.481 μg/mL for the 18 R. microplus populations. The mortality rate was 100% in the tests of the DDs in the five populations tested. In the field trials, the efficacy of fluralaner was similar for the three populations of R. microplus tested (populations 14, 10 and 16), with therapeutic efficacy (until day 21) of 100% and persistent efficacy (between days 28 and 42) > 95%. Conclusion We observed natural variability in the susceptibility of larvae from the different populations of R. microplus that had never been treated with this compound. Despite the observed variability in the in vitro results (LC50), a comparable efficacy of > 90% lasting until day 42 was observed in the field trials. Also, based on the results of the laboratory testing (LC50 and DD) and field trials, we can conclude that there was no resistance to fluralaner in the 18 studied tick populations.