Assessing the susceptibility of 200 Rhipicephalus microplus populations to fluralaner using larval immersion test: A nationwide analysis from Brazil

BACKGROUND: In 2022, a tick control product, against Rhipicephalus microplus, containing fluralaner as the active ingredient was initially launched in the Brazilian market, followed by other markets across Latin America. Once a new molecule is introduced for the control of a parasitic organism, it becomes essential to develop methods for assessing the susceptibility of target parasitic populations. In this context, discriminating dose (DD) tests represent a valuable tool. METHODS: This study evaluated the susceptibility profile of 200 R. microplus populations originating from 21 federal units (20 states and a federal district) across all five regions of Brazil, using a larval immersion test (LIT) with two discriminating doses (DD) of 1.55 and 3.16 µg/mL. Populations showing mortality rates above 95% were classified as susceptible. A field trial was also conducted with one population (São José do Rio Pardo [SJRP]), which had a known history of exposure to fluralaner, to compare laboratory and field test outcomes. RESULTS: The DD at a concentration of 1.55 µg/mL resulted in 100% mortality in 160 populations (80%), whereas the DD at a concentration of 3.16 µg/mL showed more consistent results, with 100% mortality in 182 populations (91%). In the field trial population, SJRP, fluralaner achieved 100% therapeutic efficacy (days +7 to +21) and persistent efficacy (days +28 to +42). CONCLUSIONS: LIT using DDs can provide indicative data on the susceptibility of R. microplus to fluralaner. The discriminating dose at a concentration of 3.16 µg/mL was shown to be the most appropriate for monitoring the susceptibility of R. microplus populations to fluralaner. Laboratory and field data support the classification of the SJRP population as susceptible, demonstrating consistency between the two sets of results. These results can serve as a basis for continuous spatial and temporal monitoring of the susceptibility of R. microplus populations to fluralaner. Continued research, integrating laboratory and field results, is essential to increase the reliability of laboratory-based testing.