Resistance of the cattle tick Rhipicephalus (Boophilus) microplus to ivermectin in Argentina

TORRENTS, JORGELINA; SARLI, MACARENA; ROSSNER, MARIA V.; TOFFALETTI, JOSÉ R.; MOREL, NICOLÁS; MARTÍNEZ, NORBERTO C.; WEBSTER, ANELISE; MANGOLD, ATILIO J.; GUGLIELMONE, ALBERTO A.; NAVA, SANTIAGO

RESEARCH IN VETERINARY SCIENCE

ELSEVIER SCI LTD

Lugar: Amsterdam; Año: 2020 vol. 132 p. 332 - 337

0034-5288

Resistance to ivermectin in populations of the cattle tick Rhipicephalus microplus in Argentina was diagnosed inthis work. The in vitro larval immersion test (LIT) was used to determine quantitatively the levels of resistance toivermectin in different populations of R. microplus. Additionally, field trials to control natural infestations of R.microplus on cattle with a commercial formulation of ivermectin 3.15% were carried and jointly analyzed withthe in vitro assays. The phenotypic response of the populations analyzed was not uniform. Five of them wereclassified as susceptible, four populations as resistant, and one in the category ?incipient resistance?. Regardingthe field trials, the therapeutic efficacy in a population classified with LIT as susceptible achieved values higherthan 94% two weeks after treatment, and no reproductively viable females were observed after the second daypost-treatment. Conversely, the values of efficacy percentage in a population (named as ?San Martín?) classifiedwith LIT in the category ?incipient resistance? never exceeded the 70.8%, and engorged females were collectedin practically all counts. The population ?San Martín? was classified in the category ?incipient resistant? with LITanalysis, but the field trial unambiguously shows that this tick population is resistant. The comparison of theresults obtained with LIT in vitro assays and through field trials shows that biased estimations of resistance levelsmay occur when resistance ratios (RR) values are ≤2, and additional field efficacy trials could be needed toknow with precision the status of the tick populations evaluated.