Eprinomectin accumulation in Rhipicephalus (Boophilus) microplus: Pharmacokinetic and efficacy assessment

Abstract Eprinomectin (EPM) is a macrocyclic lactone used against endo-ectoparasites without withdrawal time in milk and meat after its pour-on administration at 0.5 mg/kg. Previous experiments evaluated the efficacy of EPM against Rhipicephalus ( Boophilus ) microplus in cattle. This study assessed EPM efficacy against R . (B.) microplus after topical administration at two dose rates and investigated the relationship between EPM systemic exposure in the host and drug concentrations accumulated in ticks recovered from treated animals. A standardized pharmaco-parasitological study was performed in two phases. In phase 1 eighteen Braford cattle naturally infected with R. ( B. ) microplus were divided into three experimental groups with a similar level of infestation (Kruskal–Wallis test, P  > 0.05): control group and treated groups with EPM pour-on (1 and 1.5 mg/kg). Samples of heparinized blood and ticks at different life stages were taken between 0 and 21 days (d) post-administration to measure EPM concentrations by HPLC. The efficacy trial (phase 2) included eighteen Braford calves naturally infected with R. ( B. ) microplus divided into control group and 1 mg/kg and 1.5 mg/kg EPM treated groups. Female ticks (4.5–8 mm) on cattle were counted between 1 and 23 days post-treatment to evaluate the efficacy of EPM. The reproductive efficiency index (REI) and the fertility efficiency index (FEI) were evaluated. Plasma concentrations of EPM showed a linear relationship with the level of dose rate administered. Peak plasma concentrations were within a range between 13.8 and 90 ng/ml, which guarantee milk drug concentrations below the maximum residues level. High EPM concentrations were detected in ticks. EPM concentrations in R. ( B. ) microplus were correlated to plasma concentrations between 1.25 days and 21 days post-administration (r 0.84; P  P