Identification of Anaplasma marginale long-term carrier cattle by detection of serum antibody to isolated MSP-3.

Rapid and accurate detection of Anaplasma marginale-infected cattle would enhance anaplasmosis control procedures and evaluation of vaccines. Current tests based on detection of antibodies in serum are not widely used for several reasons, including the occurrence of either false-positive or false-negative results. We evaluated binding of antibodies in serum to a subunit antigen isolated from A. marginale initial bodies--major surface protein 3 (MSP-3). MSP-3 was detected in lysates of eight geographically different isolates of A. marginale and purified by affinity chromatography with monoclonal antibody AmG75C2. Antibodies from cattle infected with any of five geographically different isolates of A. marginale reacted in immunoblots with MSP-3. Sera from uninfected cattle and cattle infected with another rickettsial organism and two hemoprotozoal organisms failed to react with MSP-3. Six carrier cattle infected with the Florida isolate of A. marginale had antibody titers to MSP-3 ranging from 10(3) to 10(6) during a 5-year evaluation period. Since specific antibodies to isolated MSP-3 persist in high titers in long-term carrier cattle sera and MSP-3 is common among A. marginale isolates, it is recommended as a subunit antigen for an anaplasmosis test.