Ixodes persulcatus Ticks as Vectors for the Babesia microti U.S. Lineage in Japan.

The U.S. lineage, one of the major clades in the Babesia microti group, is known as a causal agent of human babesiosis mostly in the northeastern and upper midwestern United States. This lineage, however, also is distributed throughout the temperate zone of Eurasia with several reported human cases, although convincing evidence of the identity of the specific vector(s) in this area is lacking. Here, the goal was to demonstrate the presence of infectious parasites directly in salivary glands of Ixodes persulcatus, from which U.S. lineage genetic sequences have been detected in Asia, and to molecularly characterize the isolates. Five PCR-positive specimens were individually inoculated into hamsters, resulting in infections in four; consequently, four strains were newly established. Molecular characterization, including 18S rRNA, β-tubulin, and CCT7 gene sequences, as well as Western blot analysis and indirect fluorescent antibody assay, revealed that all four strains were identical to each other and to the U.S. lineage strains isolated from rodents captured in Japan. The 18S rRNA gene sequence from the isolates was identical to those from I. persulcatus in Russia and China, but the genetic and antigenic profiles of the Japanese parasites differ from those in the United States and Europe. Together with previous epidemiological and transmission studies, we conclude that I. persulcatus is likely the principal vector for the B. microti U.S. lineage in Japan and presumably in northeastern Eurasia. IMPORTANCE The major cause of human babesiosis, the tick-borne blood parasite Babesia microti, U.S. lineage, is widely distributed in the temperate Northern Hemisphere. However, the specific tick vector(s) remains unidentified in Eurasia, where there are people with antibodies to the B. microti U.S. lineage and cases of human babesiosis. In this study, the first isolation of B. microti U.S. lineage from Ixodes persulcatus ticks, a principal vector for many tick-borne diseases, is described in Japan. Limited antigenic cross-reaction was found between the Japan and United States isolates. Thus, current serological tests based on U.S. isolates may underestimate B. microti occurrence outside the United States. This study and previous studies indicate that I. persulcatus is part of the B. microti U.S. lineage life cycle in Japan and, presumably, northeastern Eurasia. This report will be important for public health, especially since infection may occur through transfusion, and also to researchers in the field of parasitology.