Effect of age on the immune response of cattle experimentally infected with Babesia bigemina

Two different age groups of Holstein Friesian cattle were experimentally infected with Babesiabigemina. Calves of group A (6 months old) did not show noticeable symptoms of babesiosis and had relatively low (0.6%) numbers of parasites in their red blood cells (RBCs). Group B calves (1 year old) had typical signs of the disease; parasites were found in 6.6% of their RBCs. Blood from both groups inoculated into splenectomized calves at 3, 6, 12 and 18 months following initial inoculation demonstrated the presence of B. bigemina, while after 22 months no parasites could be demonstrated.The indirect fluorescent antibody (IFA) test detected babesial antibodies at 4–5 days post inoculation (PI) and reached a maximum titre of 1 : 640 at 2 weeks PI. Following challenge at 2–3 months after initial inoculation, the antibody titre rose sharply to 1 : 2560, then decreased gradually but was still detectable 22 months PI. No correlation was found between antibody titre and the presence of the parasite hin the peripheral blood.