Demonstration of vaccine-induced immunity to anaplasmosis without induction of persistent postvaccinal complement-fixing and agglutinating antibodies in yearling steers.
The protective effect and anti-Anaplasma complement-fixing and agglutinating antibody responses induced in yearling steers by vaccination with an inactivated Anaplasma marginale vaccine were evaluated by challenge exposure. Eleven 12- to 14-month-old Hereford X Angus steers were randomly allotted into 6 principals and 5 controls. The principals were injected IM with 5 ml of vaccine on days 0 and 21. On day 70, the 11 steers were challenge exposed by IV inoculation of A marginale-infected blood. After a prolonged prepatent period, the vaccinated steers developed a significantly (P less than 0.01) lower A marginale parasitemia (8.0%) than did the nonvaccinated controls (26.3%). The persistence of a greater than or equal to 0.5% parasitemia was significantly (P less than 0.025) reduced from 34 days in the control to 21 days in the vaccinated group. The percentage reduction in PCV was significantly (P less than 0.025) less in the vaccinated group (34%) as compared with the controls (57%). Complement-fixing and agglutinating antibody responses induced by vaccination did not persist for more than 21 days after the 2nd vaccine injection and did not interfere with positive seroconversion resulting from challenge exposure.