A novel virus in swine is closely related to the human hepatitis E virus

Abstract

A novel virus, designated swine hepatitis E virus (swine HEV), was identified in pigs. Swine HEV crossreacts with antibody to the human HEV capsid antigen. Swine HEV is a ubiquitous agent and the majority of swine >/=3 months of age in herds from the midwestern United States were seropositive. Young pigs naturally infected by swine HEV were clinically normal but had microscopic evidence of hepatitis, and developed viremia prior to seroconversion. The entire ORFs 2 and 3 were amplified by reverse transcription-PCR from sera of naturally infected pigs. The putative capsid gene (ORF2) of swine HEV shared about 79-80% sequence identity at the nucleotide level and 90-92% identity at the amino acid level with human HEV strains. The small ORF3 of swine HEV had 83-85% nucleotide sequence identity and 77-82% amino acid identity with human HEV strains. Phylogenetic analyses showed that swine HEV is closely related to, but distinct from, human HEV strains. The discovery of swine HEV not only has implications for HEV vaccine development, diagnosis, and biology, but also raises a potential public health concern for zoonosis or xenozoonosis following xenotransplantation with pig organs.

Figure 1

Seroconversion of piglets to anti-HEV. Anti-HEV responses of three representative piglets are presented. (A) Piglet born to a seropositive sow with a high titer of IgG anti-HEV. (B) Piglet born to a seropositive sow with a lower titer of IgG anti-HEV. (C) Piglet born to a seronegative sow. The ELISA OD value of IgG anti-HEV in breeder sows is indicated (S).

Figure 2

Liver sections from a naturally infected piglet (no. 14). (A) Multifocal lymphoplasmacytic and necrotizing hepatitis with randomly distributed foci of hepatocellular swelling and vacuolation (arrow) and foci of necrosis (arrowhead) with lymphoplasmacytic sinusoidal and periportal infiltrates (×10). (B) Foci of hepatocellular necrosis (arrow) and lymphoplasmacytic inflammation (×400). Hematoxylin/eosin staining.

Figure 3

Amplification of swine HEV-specific fragment by RT-PCR. Serum samples from two piglets (nos. 4 and 14) obtained 1 week before (−1) and the week of (0) seroconversion in a prospective study were used for RT-PCR of a 344 bp fragment. Serum samples obtained at the same time (weeks 19 and 20) after birth from a seronegative piglet (no. 15) were also included. L, molecular weight marker.

Figure 4

Alignment of the amino acid sequences of ORFs 2 (A) and 3 (B) of swine HEV with human strains of HEV. The sequence of Sar55 strain is shown on top, and only differences are indicated. Deletions are indicated by a minus. The putative hypervariable region (HVR) in the ORF3 is indicated by asterisks. Sequences used in this alignment were Burma (14), Mexico (15), NE8L (Myanmar, ref. 16), Hyderabad (India, ref. 17), Madras (India, GenBank accession no. X99441), HEV037 (isolate from a case of fulminant hepatitis, GenBank accession no. X98292), Sar55 (Pakistan, ref. 18), KS2-87 (China, ref. 19), Hetian (China, GenBank accession no. L08816), and Uigh179 (China, ref. 20).

Figure 5

Phylogenetic tree based on the complete nucleotide sequences of ORFs 2 and 3. The tree was constructed by maximum parsimony methods with the aid of paup software package version 3.1.1. The tree with the shortest length (most parsimonious) was found by implementing the bootstrap (1,000 replicates) with branch-and-bound search option. Branch lengths (number given above each branch) are indicated.