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

Eprinomectin (EPM) is a macrocyclic lactone used against endo-ectoparasites without withdrawal time in milk and meat after its pour-on administration at 0.5mg/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 1.5mg/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 1mg/kg and 1.5mg/kg EPM treated groups. Female ticks (4.5-8mm) 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 90ng/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<0.05). EPM efficacy calculated using the Henderson-Tilton formula was 98.9% and 99.1% (7 days post-administration) and 100% (23 days post-administration) after EPM treatment at 1 and 1.5mg/kg, respectively. EPM administered at 1.5mg/kg also showed a significantly higher deleterious effect on tick fertility as measured by FEI (P<0.01). Therefore, treatment with EPM may be useful for controlling ticks in cattle, particularly in dairy production systems.