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Complete concentration analysis of eprinomectin in serum obtained from blood samples collected at the USDA, ARS, Cattle Fever Tick Research Laboratory (CFTRL) in collaboration with the ARS scientists in support of the continuation of research studying the interaction between Bm86 vaccine and a long-acting eprinomectin injectable formulation. Blood samples collected during the study will be processed at the USDA, ARS, CFTRL to obtain serum. ARS will provide the serum samples to the University of Texas at San Antonio (UTSA) to conduct the analysis per the one year field study. The maximum number of days between treatments with an injectable product for cattle that kills fever ticks used by the Cattle Fever Tick Eradication Program (CFTEP) is twenty-eight (28) days. Treatments to kill all fever ticks infesting cattle that last longer are needed by the Program to improve eradication efforts. A field study conducted with Texas A&M AgriLife University (TAMU) to confirm previous findings indicating that treatment combining a long-acting eprinomectin injectable formulation with immunization using the Bm86-based vaccine approved in the U.S. could be used by the program to expand the interval between treatments will generate the samples needed to determine the eprinomectin levels circulating in treated cattle. Two premises raising cattle will be selected in south Texas in collaboration with Program personnel, TAMU, ARS Principal Investigators, and ranchers interested in participating in the study. Cattle procured for the study by TAMU will be kept at the USDA, ARS, CFTRL before they are moved to the test premises. ARS and TAMU will establish separate agreements with the owner of each premises raising cattle to conduct the study as per the protocol approved by the Cooperator and ARS Animal Care and Use Committee. Thirty (30) cattle will be used at each premises. Cattle will be held at each premises for three weeks and inspected for ticks before the initial treatment. Treatment here means the application of eprinomectin+vaccine. The experimental schedule synchronizes the use of eprinomectin with the immunization protocol followed by the Program to vaccinate at 0, 30, 180 days, and every 6 months thereafter. Cattle will be inspected for ticks every time they are treated and at the end of the study to determine treatment effects on infestation status. The field study will last one year.