Failure to eliminate the Anaplasma marginale carrier state in beef cows following multiple treatments with long-acting injectable oxytetracycline

Thirty-four multiparous beef cows naturally infected with Anaplasma marginale and confirmed as both seropositive and polymerase chain reaction (PCR) positive for the organism were randomized to treatment. Five cows served as untreated controls, and 29 were treated three times at three-day intervals with a long-acting oxytetracycline formulation at a dose of 10 mg/lb (22 mg/kg). On day 14, 100% of control cows and 90% of treated cows were found to have A. marginale present by PCR testing. All control cows and 86% of treated cows were found to have A. marginale present by PCR testing on day 74. PCR results suggest that utilization of a three-dose regimen of injectable long-acting oxytetracycline at three-day intervals was not effective for eliminating the carrier state of A. marginale from naturally infected animals.