Study of the effect of different diluents for syringe immersion test on larvae against macrocyclic lactones.

Background: Macrocyclic lactones (MLs) are pharmaceutical compounds extensively utilized in the management of tick infestations in bovine populations. It is of paramount importance to prevent or delay the development of drug resistance to ML. 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 being studied. The syringe 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 larvae using the SIT technique.

Methods: Dose-response curves were adjusted using SIT with MLs, on different diluents [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 four-parameter log-logistic model was applied for model fitting. The 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 in slope when comparing diluents ( > 0.05); moxidectin (MOX) presents a higher sensitivity for DMSO versus IVM ( < 0.05). Significant differences occur when comparing DRM with MOX in both diluents. Potency does not differ for avermectins using ACT 1%, and MOX has a higher potency than avermectins ( < 0.05). On field populations, we found an increase in larval mortality when using DMSO as opposed to ACT ( < 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 populations regardless of the diluent used.