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Resistance to ivermectin in populations of the cattle tick Rhipicephalus microplus in Argentina was diagnosed in this work. The in vitro larval immersion test (LIT) was used to determine quantitatively the levels of resistance to ivermectin 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 with the in vitro assays. The phenotypic response of the populations analyzed was not uniform. Five of them were classified as susceptible, four populations as resistant, and one in the category "incipient resistance". Regarding the field trials, the therapeutic efficacy in a population classified with LIT as susceptible achieved values higher than 94% two weeks after treatment, and no reproductively viable females were observed after the second day post-treatment. Conversely, the values of efficacy percentage in a population (named as "San Martín") classified with LIT in the category "incipient resistance" never exceeded the 70.8%, and engorged females were collected in practically all counts. The population "San Martín" was classified in the category "incipient resistant" with LIT analysis, but the field trial unambiguously shows that this tick population is resistant. The comparison of the results obtained with LIT in vitro assays and through field trials shows that biased estimations of resistance levels may occur when resistance ratios (RR) values are ≤2, and additional field efficacy trials could be needed to know with precision the status of the tick populations evaluated., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The aim of this work was to analyse the spatiotemporal heterogeneity of Rhipicephalus microplus (Canestrini, 1888) (Acari: Ixodidae) resistance to chemical acaricides at intra‐farm level under different environmental (favourable and unfavourable areas for tick development) and management (different schemes of acaricides applications) conditions using ivermectin as a model. The in vitro larval immersion test (LIT) was used to determine quantitatively the levels of resistance to ivermectin in the different populations and subpopulations of R. microplus analysed. In the first case study, differences in resistance levels among tick samples within the same paddock across time and among tick samples from different paddocks were recorded. These results stress the importance of taking successive samples so that they represent the spatial and temporal variabilities in the levels of resistance that can occur within the same farm. In a second case study, the evolution of resistance in R. microplus subpopulations subjected to strategic and threshold control methods based on the application of three annual treatments with alternation of chemical groups was compared. No changes in resistance/susceptibility status were observed in both tick subpopulations. These results show that the application of a particular drug once a year within a scheme of alternation with other chemical groups could be an appropriate strategy to delay the development of resistance. Data of the third case study showed that environmental constraints is not only a key modulator of R. microplus abundance but could also affect the evolution of resistance in the tick populations. The decreasing trend of LC50 values in the tick subpopulation not exposed to chemical treatments but also in that subpopulation exposed to three annual chemical treatments under unfavourable environmental conditions allow us to hypothesise that environmental constraints can modify the levels of resistance in a tick population because it can constitute a stronger selection factor than the treatments themselves. [ABSTRACT FROM AUTHOR]

This study aimed to evaluate how the method of body weight estimation of a herd (average vs individual) for drug dosing may affect the pharmacokinetics and therapeutic efficacy of ivermectin 3.15% against Rhipicephalus (Boophilus) microplus. The trial was performed in two populations of R. (B.) microplus with different levels of susceptibility to ivermectin: San Martín (SM) (incipient resistance) and Colonia Benítez (CB) (susceptible). Twenty-one (SM) and forty-two (CB) 12-month-old Braford heifers, naturally infested with R. (B.) microplus, were individually weighed and divided into three groups (G). G1 and G2 were treated with a dose of ivermectin 3.15% (IVOMEC GOLD®) at a dose rate of 630 µg/kg of body weight (1 mL/50 kg). G1 received a uniform dose of ivermectin calculated according to the average of the individual body weights. G2 received a dose of ivermectin calculated according to the exact body weight of each animal. G3 were the control group. Ivermectin concentrations were measured from blood samples from day 1 to day 21 post-treatment. The efficacy of the treatment was evaluated by counting female ticks. In both populations the differences in the doses of ivermectin administered did not affect the pharmacokinetics and therapeutic efficacy of the drug. Nevertheless, it is necessary to avoid the sub-dosage of cattle to reduce the selection pressure for resistance on the tick populations. [ABSTRACT FROM AUTHOR]