U.S.-Type Babesia microti Isolated from Small Wild Mammals in Eastern Hokkaido, Japan

Abstract

ABSTRACT. Our previous report demonstrated that small wild rodents in Japan harbored two types of novel Babesia microti-like parasites (Kobe and Hobetsu types), but not the type widely distributed throughout the temperate zones of North American and Eurasian Continents (U.S. type). In this study, we surveyed small wild mammals collected at various places in the northern part of Japan, seeking for U.S.-type B. microti. A total of 197 small mammals comprising 10 species, Apodemus speciosus, A. argenteus, Clethrionomys rufocanus, C. rutilus, Eothenomys andersoni, Microtus montebelli, Tamias sibiricus, Sorex unguiculatus, S. caecutiens, and Urotrichus talpoides, were examined. Babesia parasites were detected in A. speciosus, C. rufocanus, C. rutilus, M. montebelli, S. unguiculatus, and S. caecutiens by microscopy of blood smears and by PCR targeting babesial nuclear small-subunit rRNA (rDNA) and β-tubulin genes. Inoculation of their bloods into experimental animals gave rise to 23 parasite isolates, which included 16 from A. speciosus, 4 from C. rufocanus, and 1 each from C. rutilus, M. montebelli and S. unguiculatus. Sequencing analyses of their rDNA and β-tubulin genes revealed that, of the 23 isolates, 20 and 3 were of Hobetsu and U.S. types, respectively. The U.S.-type B. microti strains isolated in Japan, however, were distinguishable from the isolates in the United States when their β-tubulin gene sequences and antigen profiles in Western blots were compared. We conclude that U.S.-type B. microti exists in Japan although it has been genetically and antigenically diversified from that distributed in the United States. The results also suggest that not only rodents, but also some insectivo res may serve as reservoirs for the agent of human babesiosis.

Babesia microti-Group Parasites Compared Phylogenetically by Complete Sequencing of the CCT.ETA. Gene in 36 Isolates

“…This capability makes the gene especially reliable and useful for phylogenetic analysis. While informed by valuable studies relying on 18S rDNA and the β-tubulin gene [8,9,26,30,32,37,38,41,43,44], if we are to identify the true character of the infectious agent B. microti and separate the "related" but noninfectious culprits, then the research arsenal must make use of additional genes such as CCTη in the target parasites.…”

“…The U.S. type included parasites of the Gray, GI, NM69, and AK2273 strains and the Vladivostok38, Korea8, and Xinjiang1637 isolates [30,43,44] in addition to the HK strain [10] and an isolate obtained from Clethrionomys rutilus captured in Russia (Irkutsk16). The HK and Irkutsk16 isolates were provided by Jeremy Gray (University College Dublin, Ireland) and Hiroaki Kariwa (Hokkaido University, Japan), respectively, and assigned tentatively to the U.S.…”

“…In a prestigious study using β-tubulin gene sequences, the B. microti group branched into three or more clades, including parasites common to the Holarctic rodents (clade 1, U.S. type), various carnivores (clade 2), Alaskan voles and their relatives (clade 3), and other hosts [8]. Accordingly, multiple clusters, or clades, also branched from the β-tubulin trees constructed in our laboratory, viz., the U.S., Kobe, and Hobetsu genotypes, and most of the B. microti-like parasites isolated from rodents in Japan, Korea, Far East Russia and northwestern China fell into one of these three genotypes [43,44]. Cluster distinctions described by the two research teams, however, showed inconsistency in certain aspects, probably owing to the sampling deviations; the parasites in the respective studies were obtained mostly from North America [8] or Northeast Asia [37,43,44].…”

“…Accordingly, multiple clusters, or clades, also branched from the β-tubulin trees constructed in our laboratory, viz., the U.S., Kobe, and Hobetsu genotypes, and most of the B. microti-like parasites isolated from rodents in Japan, Korea, Far East Russia and northwestern China fell into one of these three genotypes [43,44]. Cluster distinctions described by the two research teams, however, showed inconsistency in certain aspects, probably owing to the sampling deviations; the parasites in the respective studies were obtained mostly from North America [8] or Northeast Asia [37,43,44]. Today, with escalating recruitment of new genotypes from widespread geographic regions and a multiplicity of wild mammalian hosts, evidence of intra-group genetic diversity is accruing.…”

“…Subsequently, yet a different B. microti-like parasite (Hobetsu type) was found in feral rodents (A. speciosus, Clethrionomys rufocanus and Microtus montebelli) and in shrews (Sorex unguiculatus) in Japan [38,43]. In 2006, 18S rDNA sequence of a European rodent isolate "Munich" (GenBank accession number AB071177) disclosed marked differences from the parasites reported in either the U.S. or Japan [11] but showed high similarity to isolates from rodents (M. arvalis and M. oeconomus) and ticks (Ixodes ricinus) captured in Poland [26,32].…”