Protective immunity induced by immunization with a live, culturedAnaplasma marginalestrain

Despite significant economic losses resulting from infection withAnaplasma marginale, a tick-transmitted rickettsial pathogen of cattle, available vaccines provide, at best, only partial protection against clinical disease. The green-fluorescent protein expressing mutant of theA. marginaleSt. Maries strain is a live, marked vaccine candidate (AmStM-GFP). To test whether AmStM-GFP is safe and provides clinical protection, a group of calves was vaccinated, and clinical parameters, including percent parasitized erythrocytes (PPE), packed cell volume (PCV) and days required to reach peak bacteremia, were measured following inoculation and following tick challenge with wild type St. Maries strain (AmStM). These clinical parameters were compared to those obtained during infection with theA. marginalesubsp.centralevaccine strain (A. centrale) or wild type AmStM. AmStM-GFP resulted in similar clinical parameters toA. centrale, but had a lower maximum PPE, smaller drop in PCV and took longer to reach peak bacteremia than wild type AmStM. AmStM-GFP provided clinical protection, yielding a stable PCV and low bacteremia following challenge, whereasA. centraleonly afforded partial clinical protection.