Identification of a Genotype III Swine Hepatitis E Virus That Was Isolated from a Japanese Pig Born in 1990 and That Is Most Closely Related to Japanese Isolates of Human Hepatitis E Virus

In 1997, Meng et al. (4) first reported the discovery of hepatitis E virus (HEV) in pigs in the United States. Swine HEV is a potential zoonotic agent that is possibly transmitted to humans from infected pigs, as suggested by the close genetic relationship between swine and human viruses and experimental cross-species infection of swine HEV to a chimpanzee and rhesus monkeys (4, 5, 10). Swine HEV isolates exhibit extensive genetic heterogeneity, and HEV variants of genotype III or IV have been isolated from farm pigs in other countries, including Australia, Canada, China, India, Japan, Korea, Mexico, The Netherlands, New Zealand, Spain, and Taiwan (3, 7, 8, 12). However, it remains unknown when such variants emerged in pigs in these countries.

We analyzed serum samples from 24 Japanese pigs of 1 to 6 months of age that had been born in 1990 and raised in a swine farm. The serum samples, which had been stored below −20°C, were tested for the immunoglobulin G (IgG) class of antibodies to HEV (anti-HEV IgG) by in-house enzyme immunoassay by using purified recombinant open reading frame 2 (ORF2) protein that had been expressed in the pupae of silkworm (6) as the antigen probe; the samples were tested for HEV RNA by reverse transcription-PCR by the method described previously with primers targeting the ORF2 region (6). The amplified product was sequenced directly on both strands.

Anti-HEV IgG was not detected in the serum samples from the seven 1-month-old pigs or in those from the four 2-month-old pigs, but it was detected in samples from two (50%) of the four 3-month-old pigs, all three 4-month-old pigs, and all six 6-month-old pigs. Among the samples, one serum sample obtained from a 2-month-old pig was reproducibly positive for HEV RNA.

The swine HEV isolate (swJC1990) from the infected pig was close to known swine and human genotype III isolates, with 83.2 to 93.9% identity in a 412-nt sequence, and it was most closely related to the HE-JA5 isolate of genotype III that had been isolated from a Japanese patient who contracted sporadic acute hepatitis E in 1998 and who had never traveled abroad (6). Among swine strains, it was closest to swJ681 of Japanese origin, with 93.0% identity. The phylogenetic tree constructed by the neighbor-joining method (9) based on the partial ORF2 sequence of 301 nt confirmed that the swJC1990 isolate belonged to genotype III; it segregated into a cluster consisting of eight HEV strains that had been isolated from six Japanese patients with no history of travel to areas of endemicity who developed sporadic acute hepatitis between 1982 and 2001 (1, 6, 11) and from two Japanese farm pigs (swJ570 and swJ681) in 2001 (7) (Fig. 1).

FIG. 1.

Phylogenetic tree constructed by the neighbor-joining method based on the partial nucleotide sequence (301 nt) of the ORF2 region of 74 human and swine HEV isolates. The nucleotide sequences of 73 known human and swine HEV isolates were retrieved from the GenBank/DDBJ/EMBL databases on 27 November 2002. The partial nucleotide sequence of swJC1990 obtained in the present study is deposited under accession no. AB096756 and is boxed for visual clarity. All human and swine HEV strains of Japanese origin are indicated by boldface type. Bootstrap values of >70% are indicated for the major nodes as a percentage of the data obtained from 1,000 resamplings (2).

These results indicate that a presumably Japan-indigenous swine HEV isolate was circulating in Japan even in 1990. The finding that a domestic HEV strain(s) with a high nucleotide sequence identity to human HEV in Japan has been present for more than 10 years in Japanese farm pigs further supports the idea that swine serve as reservoirs for HEV infection in humans.

Nucleotide sequence accession number. The partial nucleotide sequence of swJC1990 obtained in the present study is deposited in GenBank under accession no. AB096756.