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To determine the source of infection for the Japanese index case of human babesiosis, we analyzed blood samples from an asymptomatic individual whose blood had been transfused into the patient. In addition, we surveyed rodents collected from near the donor's residence. Examination by microscopy and PCR failed to detect the parasite in the donor's blood obtained 8 months after the donation of the blood that was transfused. However, we were able to isolate Babesia parasites by inoculating the blood sample into SCID mice whose circulating red blood cells (RBCs) had been replaced with human RBCs. A Babesia parasite capable of propagating in human RBCs was also isolated from a field mouse (Apodemus speciosus) captured near the donor's residential area. Follow-up surveys over a 1-year period revealed that the donor continued to be asymptomatic but had consistently high immunoglobulin G (IgG) titers in serum and low levels of parasitemia which were microscopically undetectable yet which were repeatedly demonstrable by inoculation into animals. The index case patient's sera contained high titers of IgM and, subsequently, rising titers of IgG antibodies, both of which gradually diminished with the disappearance of the parasitemia. Analysis of the parasite's rRNA gene (rDNA) sequence and immunodominant antigens revealed the similarity between donor and patient isolates. The rodent isolate also had an rDNA sequence that was identical to that of the human isolates but that differed slightly from that of the human isolates by Western blot analysis. We conclude that the index case patient acquired infection by transfusion from a donor who became infected in Japan, that parasitemia in an asymptomatic carrier can persist for more than a year, and that A. speciosus serves as a reservoir of an agent of human babesiosis in Japan.

We have isolated piroplasms from a patient who developed the first case of human babesiosis in Japan by using NOD/shi-scidmice whose circulating erythrocytes (RBCs) had been replaced with human RBCs (hu-RBC-SCID mice). Following inoculation of the patient's blood specimen into hu-RBC-SCID mice, parasites proliferated within the human RBCs in the mice, resulting in a high level of parasitemia. Parasite DNA was prepared from blood samples of the patient and the mice, and the nuclear small-subunit rRNA gene (rDNA) was amplified and sequenced. Both DNA samples gave rise to identical sequences which showed the highest degree of homology (99.2%) with the Babesia microti rDNA. Because the patient had received a blood transfusion before the onset of babesiosis, we investigated the eight donors who were involved. Their archived blood samples were analyzed for specific antibody and parasite DNA; only a single donor was found to be positive by both tests, and the parasite rDNA sequence from the donor coincided with that derived from the patient. The donor's serum exhibited a high antibody titer against the isolate from the patient, whereas it exhibited only a weak cross-reaction against B. microti strains isolated in the United States. We conclude that the first Japanese babesiosis case occurred due to a blood transfusion and that the etiological agent is an indigenous Japanese parasite which may be a geographical variant of B. microti. Our results also demonstrated the usefulness of hu-RBC-SCID mice for isolation of parasites from humans and for maintenance of the parasite infectivity for human RBCs.

This new method is preferable for screening for granulocyte antibodies. In addition, it has the advantage of requiring only 5 microl of serum for each test.

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