Field Challenge of Cattle Vaccinated with a Combined Babesia bovis and Babesia bigemina Frozen Immunogen

To determine the optimal dose of a combined, frozen immunogen containing in vitro culture-derived strains of Babesia bovis and Babesia bigemina, twenty-four 14-month-old Bos taurus steers from a Boophilus microplus-free area in Northern Mexico were used in this experiment. Cattle were randomly allocated into six groups with four animals each, and were intramuscularly inoculated as follows: group 1 (control animals) were administered with normal bovine erythrocytes; group 2 received 1 x 10(7) B. bovis- and B. bigemina-infected erythrocytes as a combined fresh immunogen. Groups 3-6 were inoculated with a combined frozen immunogen containing (previous to cryopreservation at -196 degrees C) 1 x 10(7), 5 x 10(7), 1 x 10(8), and 5 x 10(8) infected erythrocytes of each parasite species, respectively. Four months after immunization, principal and control animals were translocated to a bovine babesiosis endemic zone for field challenge. This was carried out by introducing the experimental cattle to tick-infested pastures for 30 days without ixodicide treatment. Cattle were monitored from day 8 postintroduction to the field (PIF) by recording the manifestation of clinical disease, rectal temperature values (RT), packed cell volume index (PCV), and percent of parasitized erythrocytes (PPE). At challenge, all experimental cattle became infected with both Babesia bovis and B. bigemina. However, except for two animals from group 6, none of the vaccinated animals showed signs of acute clinical babesiosis; therefore, no treatment was instituted. Out of six animals showing acute clinical babesiosis (four group 1 controls and two group 6 vaccinates), two animals (one from each group) died, despite babesiacide treatment, as they manifested classical cerebral babesiosis caused by B. bovis. Regardless of the dose or type of immunogen used (combined fresh or frozen), 90% of vaccinated cattle were determined to be protected against the virulent Babesia sp. field isolates. Nevertheless, by evaluating clinical parameters, such as average of maximum drop in PVC index (28.5%), average duration of parasitemia (3 days for B. bovis; 8.5 days for B. bigemina), and average duration of RT values > or = 39.5 degrees C (2 days), animals receiving 1 x 10(8) infected erythrocytes, as combined frozen immunogen, were more efficaciously protected against challenge with virulent B. bovis and B. bigemina field isolates.