In the present study, we were able to isolate a B. microti-like parasite from the first patient with a clinical case of babesiosis detected in Japan. Whether the parasite should be referred to as a geographical variant of B. microti or a new species (or subspecies) remains unresolved. Analysis of the rDNA sequences indicates that the Japanese isolate is closely related, but not identical, to the reference B. microti strains isolated in the United States. The Japanese and the U.S. parasites had significant differences in their antigenicities. Similar findings have also been reported for a B. microti-like parasite isolated in Taiwan (28). Such local antigenic and genetic variations may have resulted from adaptation of B. microti to the local reservoir and vectors or simply may have resulted from the genetic drift of allopatric populations of this pan-Holarctic species (S. R. Telford, personal communication). Regardless of the clear differences in the antigenicities and rDNA sequences, other criteria, such as morphology, pathogenicity, and ecological niche, clearly place the Asian parasites in the species B. microti. At present, therefore, we should be conservative in drawing conclusions about the specific identity of the organism, pending further descriptions of the interpopulation variability of this parasite.
The hu-RBC-SCID mice were found to serve as a useful experimental tool for isolation of Babesia
parasites from humans. Among the various erythroparasitic protozoan species which infect humans, only two, Plasmodium falciparum
) and B. divergens
), can be isolated with in vitro cultivation systems with human RBCs. Since such a system has not yet been available for B. microti
, inoculation of a patient's sample into susceptible laboratory animals is currently the sole method for the isolation and propagation of parasites. Syrian hamsters have been used as the host of choice owing to their excellent susceptibility. However, several reports have documented unsuccessful parasite isolation with hamsters even in cases in which Babesia
parasites were observed in the patient's blood specimens by microscopy (3
). We have also attempted to use hamsters for isolation of parasites from the first Japanese patient found to have babesiosis. Although the parasites were capable of infecting hamsters, the levels of parasitemia varied greatly and did not increase to a level as high as that observed in hu-RBC-SCID mice (M. Tsuji, unpublished data). Our SCID mouse system appears to have some additional advantages over hamsters. For instance, propagation of parasites in immunodeficient mice may be an ideal method for minimization of the possible antigenic variation that has been detected in some Babesia
species, including B. bovis
), B. rodhaini
), and B. microti
). The hu-RBC-SCID mouse system may be a useful method which can directly test whether Babesia
parasites obtained from wild animals have the ability to infect human RBCs. Furthermore, propagation of human-derived parasites in human RBCs may prevent selection of those variants that are more infectious for rodents. The Gray strain of B. microti
, which was isolated from the American index case patient in 1969 (4
), has subsequently been maintained by serial passage in hamsters. Although putatively highly infectious to humans, our preliminary study demonstrated that this strain propagated very poorly in hu-RBC-SCID mice (M. Tsuji, unpublished data), suggesting that it may no longer represent the prototypic agent of human babesiosis. An analogous finding was obtained when we compared the replication of the human-derived Kobe strain and its mouse-adapted substrain, Kobe-Mo, in hu-RBC-SCID mice.
We were able to demonstrate that the patient was infected by a blood transfusion from an asymptomatic carrier. Of the eight blood donors involved in the transfusion, only a single individual was found to be positive for the specific antibody and the parasite DNA. The donor was in good health at about the time of blood donation and did not recall any tick bites, contact with wild animals, or blood transfusion. Thus, how the donor obtained the parasite infection is not clear at this time. However, since neither the patient nor the donor had a history of travel abroad, it is reasonable to assume that the infection occurred in Japan. Even though the case of human babesiosis described here was the first case of babesiosis detected in Japan, it may simply be that human babesiosis has been undetected in the country for many years. More than 15 years ago, Shiota et al. (29
) reported the presence of a Babesia
sp. (probably B. microti
) in Apodemus speciosus
), which seemed to be endemic to Japan. However, rDNA sequence information for that parasite is lacking, and no conclusion may be made about its relationship to the Kobe strain.
Human babesiosis in the United States has been studied extensively since 1969, and the roles of Peromyscus leucopus
and Ixodes dammini
as the reservoir and the tick vector, respectively, have well been established (30
). The northeastern United States is known as the area of endemicity, and special attention has been paid not only to human babesiosis but also to Lyme borreliosis and human granulocytic ehlichiosis in that region because of cotransmission by the same tick vector (13
). By analogy, it may be that Ixodes persulcatus
, the main vector of Lyme borreliosis in Japan (21
), serves as the vector of human babesiosis as well and that human granulocytic ehrlichiosis will soon be discovered as a public health burden in Japan. In the United States, physicians' awareness of tick-transmitted diseases contributes to a rapid diagnosis and appropriate case management. In this first Japanese case of babesiosis, however, the unavailability of information about the disease resulted in a significant delay in the time to diagnosis, and the patient was left untreated for nearly 4 months. Japanese investigators therefore urgently need to study the reservoir, vector ticks, regions of endemicity, and prevalence of babesiosis and associated tick-borne infections. In addition, a practical assay method for the rapid, sensitive, and specific detection of the Japanese B. microti
-like parasites may be required in order to establish appropriate measures for prevention of transfusion-associated infections.