Abstract
Background:
Macrocyclic lactones (MLs) are pharmaceutical compounds extensively utilised in the management of Rhipicephalus microplus tick infestations in bovine populations. It is of paramount importance to prevent or delay the development of drug resistance to ML. In vitro techniques are validated by FAO and can serve as an orientative diagnosis of the resistance developed in field conditions. Diluent selection must be considered when sensitivity on field strains is beeing studied. The Syringes Immersion Test (SIT) is a modification of the larval immersion test where syringes are used seeking to reduce the workload.

Aim:
Study the interchangeability of two diluents in the diagnosis of sensitivity to MLs on Rhipicephalus microplus larvae using the SIT technique.

Methods:
Dose-response curves were adjusted using SIT with MLs, on different diluentes (Acetone [ACT] and Dimethyl sulfoxide [DMSO]) on Mozo strain (standard susceptible strain). Slope, potency and discriminating concentration were estimated for each drug on both
diluents. A 4-parameter log-logistic model was applied for model fitting. Ratio between estimated parameters was used to compare results. Field strains were tested on both diluents for each drug, using the discriminating concentration estimated for Mozo strain.

Results:
For the Mozo strain, dose-response models were adjusted for each drug on both diluents using SIT. Ivermectin (IVM) and doramectin (DRM) showed no significant difference for slope when comparing diluents (p > 0.05); Moxidectin (MOX) presents a higher sensitivity for DMSO vs IVM (p<0.05). Significant differences occur when comparing DRM with MOX in both diluents. Potency does not differ for avermectins using ACT 1%, MOX has a higher potency than avermectins (p<0.05). On field populations, we found an increase in larval mortality when using DMSO as opposed to acetone (p<0.05) for IVM, DRM and MOX, a differential sensitivity to detect larvae with survival capacity at equal levels of lethal concentration in both diluents for the same drug on Mozo strain.

Conclusion:
We conclude that the SIT technique is a tool capable of detecting susceptibility/resistance in Rhipicephalus microplus populations regardless of the diluent used.

Key words: Diagnostic, Ectoparasiticides, Pharmacodynamics, Resistance, Ticks