Is the suckling period and application pattern relevant for fluazuron efficacy against ticks in cows and their suckling calves?

Background: Fluazuron is a chitin synthesis inhibitor administered as a pour-on formulation in cattle for tick control. This study analyzes under endemic tick infestation, the incidence of the pour-on application pattern on the plasma levels of fluazuron in cows and calves in the lactation period of the beef cow. Three hundred and thirteen beef cows around parturition were treated with commercial pour-on formulation of fluazuron at a rate of 2.5 mg/kg of body weight. A total of 4 treatments were carried out with a dosing period set in a range of 32 to 45 days. At each administration time, the cows were grouped according to the pour-on administration pattern: long (~60 cm pour-on application surface) and short (~30 cm pour-on application surface). Fluazuron levels in cattle and calves plasma were determined before the third and fourth application for each subgroup (n=10) by HPLC-MS/MS. During the entire study cow–calf pairs were maintained under field conditions and qualitatively examined for tick infestation on the day of each treatment. Both treatment (long and short) schemes were designed to prevent the annual persistence of ticks. Results: : None of the groups presented ticks during the first 117 days of the study. There were no differences after 40 days post-treatment of the second application (30±5 ppb vs 28.5±12 ppb, P>0.05) and 45 days after the third application (147±55 ppb vs 140±46 ppb, P >0.05) between groups of cows treated with the long or short pour-on application, respectively. Plasma concentration of fluazuron at second and third application was increased (3.3 and 2.9 times, respectively) in calves under free suckling compared to cows. Nevertheless, both groups of cows (4.9 times) and calves (2.8 times) showed a significant increase in plasma concentration of fluazuron between times (P<0.05). In both groups, tick prevalence was 0% throughout the trial, except for day 77, which reached 1%. Conclusions: : The main conclusions of this study were the following: 1) Different administration patterns (long vs short) did not differ in plasma levels of fluazuron.; 2) Given that only the cattle were treated, and lactating calves presented higher plasma levels of fluazuron than cows, passage through milk appears to be relevant and possibly due to a cumulative effect and continuos drug intake.