Bovine leukemia virus (BLV) infection of cattle has been increasing yearly in Japan although several European countries have successfully eradicated the infection. In the present study, phylogenetic analysis on the env gene obtained from 64 tumor samples found in different regions in Japan was carried out in order to define the genetic background of BLV strains prevailing in the country. Most of the Japanese isolates were found to reside in the consensus cluster or genotype 1 of BLV strains (Rodriguez et al., 2009). Out of them, 21 isolates and 10 isolates exhibited the identical sequences, respectively. Only one isolate was classified into the different genotype related to the US isolates. Analysis on the deduced amino acids of gp51 demonstrated the sequence diversity in the neutralizing domain. These data may indicate that two major populations of BLV prevailed throughout Japan, whereas antigenic variants also exist. It was further proved that multiple invasion of the genetically different BLV strains have occurred in Japan.
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