Swine hepatitis E virus strains in Japan form four phylogenetic clusters comparable with those of Japanese isolates of human hepatitis E virus.

Japanese patients with sporadic acute hepatitis E are infected with polyphyletic strains of hepatitis E virus (HEV). Hepatitis E is considered a zoonotic disease. Thus far in Japan, only three strains of swine HEV have been identified and an antibody study for HEV antibodies has not been done on Japanese pigs. To determine the prevalence of swine HEV infection in Japan and the extent of genetic variation among Japanese swine HEV strains, we tested serum samples obtained from 2500 pigs from 2 to 6 months of age at 25 commercial swine farms in Japan for the presence of IgG antibodies to HEV and swine HEV RNA. Anti-HEV antibodies were detected in 1448 pigs (58 %). One-hundred-and-thirteen (15 %) of the 750 3-month-old pigs and 24 (13 %) of the 180 4-month-old pigs were positive for swine HEV RNA. The nucleotide sequence of a 412 bp region within open reading frame 2 of the 137 swine HEV isolates was determined. Sequence analyses revealed that the 137 isolates shared 76.6-100 % nucleotide sequence identities and were classifiable into genotype III (93 %) or IV (7 %) and that the isolates from the same farm were > or = 97.1 % similar to each other. Phylogenetic analysis showed that the Japanese swine and human HEV isolates segregated into four clusters, with the highest nucleotide identity being 94.4-100 % between swine and human isolates in each cluster. These results indicate that swine HEV is widespread in the Japanese swine population and further support the hypothesis that swine serve as reservoirs for HEV infection.