Human babesiosis in Japan: Epizootiologic survey of rodent reservoir and isolation of new type of Babesia microti-like parasite

We have carried out epizootiologic surveys at various sites in Japan to inv estigate wild animals that serve as reservoirs for the agents of human babe siosis in the country. Small mammals comprising six species, Apodemus speci osus, Apodemus argenteus, Clethrionomys rufocanus, Eothenomys smithii, Croc idura dsinezumi, and Sorex unguiculatus, were trapped at various places, in cluding Hokkaido, Chiba, Shiga, Hyogo, Shimane, and Tokushima Prefectures. Animals harboring Babesia microti-like parasites were detected in all six p refectures. Inoculation of their blood samples into hamsters gave rise to a total of 20 parasite isolates; 19 were from A. speciosus, and the other 1 was from C. rufocanus. Sequencing of the parasite small-subunit rRNA gene ( rDNA) sequence revealed that 2 of the 20 isolates were classified as Kobe t ype because their rDNAs were identical to that of the Kobe strain (the stra in from the Japanese index case). The other 18 isolates were classified as a new type, designated the Hobetsu type, because they all shared an identic al rDNA sequence which differed significantly from both that of Kobe-type i solates and that of northeastern United States B. microti (U.S. type). The parasites with Kobe-, Hobetsu- and U.S.-type rDNAs were phylogenetically cl osely related to each other but clearly different from each other antigenic ally. The isolates from rodents were demonstrated to be infective for human erythrocytes by inoculation into SCID mice whose erythrocytes had been rep laced with human erythrocytes. The results suggest that a new type of B. mi croti-like parasite, namely, the Hobetsu type, is the major one which is pr evalent among Japanese wild rodents, that A. speciosus serves as a major re servoir for both Kobe- and Hobetsu-type B. microti-like parasites, and that C. rufocanus may also be an additional reservoir on Hokkaido Island.