Analysis of doramectin in the serum of repeatedly treated pastured cattle used to predict the probability of cattle fever ticks (Acari: Ixodidae) feeding to repletion.

Analysis of doramectin concentration in blood serum of pastured cattle injected repeatedly (12 treatments) at two different dosage rates and 28-day intervals throughout the year was used to predict the probability that cattle fever ticks could successfully feed to repletion during the interval between any two consecutive treatments. Treatment at ~270 μg/kg indicated that serum doramectin concentration dropped below the baseline concentration estimated for tick survival (8 ppb) in 7 of the 12 treatments. However, the longest period between any two treatments during which the doramectin concentration remained below the 8 ppb baseline level for successful tick feeding was 15 days, making it virtually impossible for any ticks to reach ovipositional status prior to a subsequent treatment. At a dosage rate of ~540 μg/kg, the concentration dropped below the baseline tick survival level (8 ppb) only once, following the initial treatment, and the duration during which the concentration remained below the baseline level prior to the subsequent treatment was only 6 days. Thus, at the high dosage rate results indicated, with absolute certainty, that no ticks could successfully feed to repletion between any two consecutive treatments. Based on the data obtained in the study it was concluded that analysis of doramectin concentration in serum of treated animals would be a reliable predictor for assessing the probability that ticks could successfully develop to repletion. More importantly, results demonstrated that the trial policy, instituted by the Cattle Fever Tick Eradication Program, of repeatedly treating cattle with doramectin injections at 25-28 day intervals for eliminating cattle fever ticks would produce little or no risk of any viable ticks developing to repletion and re-infesting the field between treatment applications.