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Human babesiosis has been reported mainly in U.S.and Europe caused by Babesia microti and B.divergens,respectively.While recently human babesiosis is recognized as emergent disease,because genetically closely related species and newly Babesia spp.have been identified.The species B.microti,commonly found in rodents,demonstrates a high degree of genetic and antigeneticdiversity as well.Three lineages,U.S.,Kobe and Hobetsu are known to have zoonotic potential present in rodents in Japan.By using thenested-PCR system,we identifiedspecific tick vector(s)for U.S.and Hobetsu parasites by field investigation at co-infected areas where up to two of the three lineages occur with similar frequency in reservoirs.Transmission experiments using laboratory animals were successfully performed.Interestingly,we found thatJapanese ticks also carried parasites closely related to B.divergens and B.venatorum,a new species found in Europe,both of which had been believed to be absent in Japan.In the conference,I will present these fmdings,which are of importance in the assessment of the regional risk for babesiosis in humans.