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

Authors: Davey, Ronald; Pound, Joe; Klavons, Jerome; Lohmeyer, Kimberly - Kim; FREEMAN, JEANNE - Former ARS Employee; Olafson, Pia

Submitted to: Experimental and Applied Acarology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2012
Publication Date: 4/1/2012
Citation: Davey, R.B., Pound, J.M., Klavons, J.A., Lohmeyer, K.H., Freeman, J.M., Olafson, P.U. 2012. 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. Experimental and Applied Acarology. 56(4):365-374.

Interpretive Summary: The Cattle Fever Tick Eradication Program has instituted the use of doramectin injections in cattle at 25 day intervals as an alternative to dipping cattle in coumaphos every 14 days to reduce the financial burden on producers from having to gather their cattle so frequently. Therefore, a long-term study was conducted to determine the doramectin concentration in the serum of pastured cattle treated repeatedly at 28 d intervals at two dosage rates to predict the probability that cattle fever ticks could successfully feed to repletion during the interval between treatments. At ~270 µg/kg, the doramectin concentration dropped below the baseline level for successful tick feeding (8 ppb) on numerous occasions, but the longest period it remained below the baseline level was 15 d, making it impossible for ticks to attain ovipositional status prior to a subsequent treatment. By contrast, at ~540 µg/kg the concentration dropped below the baseline level of 8 ppb only once for a period of only 6 days. Results clearly indicated that no ticks could successfully feed to repletion between subsequent treatment applications. Doramectin was metabolized more rapidly during the summer and fall months, when temperatures were higher, than during the spring and early summer months, when temperatures were lower. Thus, analysis of doramectin in the serum of treated animals was a reliable predictor for assessing the probability of successful tick development. More importantly, the Cattle Fever Tick Eradication Program trial policy of treating cattle with doramectin injections at 25-28 d intervals produced virtually no risk of allowing viable ticks to reach repletion and sustain the field population.

Technical Abstract: Doramectin concentration in the serum of pastured cattle treated repeatedly at 28 d intervals at two dosage rates was used to predict the probability that cattle fever ticks could successfully feed to repletion during the interval between treatments. At ~270 µg/kg, the doramectin concentration dropped below the baseline level for successful tick feeding (8 ppb) in 7 of the 12 treatments. However, the longest period during which the concentration remained below the baseline level was 15 d, making it impossible for ticks to attain ovipositional status prior to a subsequent treatment. At ~540 µg/kg, the concentration dropped below the baseline level of 8 ppb only once, and the duration during which it remained below the baseline level was only 6 days. Thus, results clearly indicated that no ticks could successfully feed to repletion prior to application of a subsequent treatment. Results also suggested doramectin was metabolized more rapidly during the summer and fall months (July through October), when temperatures were higher, than during the spring and early summer months (late February through mid June), when temperatures were lower. Based on these data, 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 U.S. Cattle Fever Tick Eradication Program trial policy of treating cattle with doramectin injections at 25-28 d intervals would produce virtually no risk of allowing viable ticks to reach repletion and sustain the field population.