Evaluation of Anaplasma marginale from tick cell culture as an immunogen for cattle.

Anaplasma marginale has been propagated and continuously passaged in an Ixodes scapularis cell line. Anaplasma development was characterized and cultures with a high density of rickettsiae were harvested at a predictable rate. Culture-derived A. marginale (CAM) remained infective for cattle and was used effectively as antigen in diagnostic tests with the sensitivity to identify bovine carriers of A. marginale. This study presents results of an initial trial using the CAM as an immunogen for cattle. CAM was mechanically disrupted, frozen at -70 degrees C, and inactivated with beta-propiolactone. Two intact yearling cattle were immunized with CAM and Freund's adjuvant, receiving 4 subcutaneous injections at 3-4 week intervals. Two control yearling cattle received adjuvant and PBS. Serum samples were evaluated by competitive ELISA (C-ELISA) using CAM as antigen and the standard complement fixation test (CFT). All cattle were subsequently challenged with A. marginale-infected blood from a carrier cow. An additional intact calf was inoculated with live CAM from the same passage and screened by C-ELISA and CFT. Sera collected from immunized cattle were negative or suspicious by CFT throughout the immunization study. The same sera were strongly positive by C-ELISA two weeks after the first injection and throughout the study. All cattle became infected following challenge-exposure with blood, but immunized cattle exhibited longer prepatent periods as well as lower parasitemias and percent reduction of packed cell volumes as compared with the controls. The calf receiving live CAM became infected and underwent a mild clinical reaction with positive C-ELISA and CFT results and did not become clinically ill following blood challenge. This preliminary study suggests that the CAM antigen is highly immunogenic in cattle. Furthermore, the CFT did not identify immunized animals whereas the C-ELISA (using CAM) was highly sensitive for detection of both immunized and infected animals.