Immunization with Msp2 HVRs Is Less Effective than the Live Vaccine against Anaplasmosis.

Bovine anaplasmosis, caused by , is the most prevalent tick-transmitted pathogen of livestock globally. In many parts of the world, , a related organism, is used as a live blood-borne vaccine as it causes either no or only a mild clinical disease. does not prevent infection with but does prevent acute disease. is prohibited from being used in the U.S. due to the risk of transmitting emerging pathogens. Both of these organisms encode proteins known as major surface protein 2 (Msp2), which is the most immunodominant protein for the organism. Both organisms persist in their host by evading clearance, i.e., the adaptive immune response, by recombining the hypervariable region (HVR) of with pseudogene alleles. The study goal was to test whether the Msp2 HVRs encoded by are a sufficient source of immune stimulation to provide the clinical protection exhibited by the blood-borne vaccine. Calves were inoculated with recombinantly expressed HVRs. Control groups were inoculated with saponin or infected with the live vaccine and compared with the test group. A Western blot analysis demonstrated that the HVR immunizations and live vaccine stimulated an immune response. All animals in the study became infected upon challenge with -infected ticks. The saponin-immunized control group had a high PPE (5.4%) and larger drops in PCVs (14.6%). As expected, the -immunized animals were protected from acute disease with lower (0.6%) parasitemia and lower drops in PCV (8.6%). The HVR-immunized group had intermediate results that were not statistically significantly different from either the negative or positive controls. This suggests that the HVR immunogen does not fully recapitulate the protective capacity of the live vaccine.