TF and infection: Four calves experimentally infected with a low dose (5 × 103 infected erythrocytes) of B. bovis, two with the Mo7 strain (C1287 and C1291) and two with the attenuated Tf-137-4 strain (C1290 and C1292), had variable parasitemia for longer than 11 months when tested by nested PCR, demonstrating long-term persistence of B. bovis after infection (Fig. 5).