Use of the Bo-RBC-SCID mouse model for isolation of a Babesia parasite from grazing calves in Japan.

SCID mice with circulating bovine red blood cells (Bo-RBC-SCID mice) were used to isolate Babesia parasites from grazing calves in Japan. Although the initial blood samples obtained from the calves contained both Babesia and Theileria parasites, we were able to isolate only the Babesia parasites by repeated blood passages in Bo-RBC-SCID mice, taking advantage of the much more rapid growth of Babesia than of Theileria. The parasites isolated had characteristics of large-type Babesia, showing typical paired pyriform morphology. The Bo-RBC-SCID mice infected with the Babesia parasites developed a high level of parasitemia, showing severe clinical symptoms characterized by hemoglobinuria, jaundice, and hemolytic anemia. In addition, some mice exhibited nervous symptoms, particularly paralysis of both posterior limbs. The results demonstrated that the Bo-RBC-SCID mouse model was useful not only for isolating Babesia from cattle but also for studying the disease caused by the Babesia infection.