Identification of genotype 3 hepatitis E virus (HEV) in serum and fecal samples from pigs in Thailand and Mexico, where genotype 1 and 2 HEV strains are prevalent in the respective human populations

Hepatitis E virus (HEV), the causative agent of hepatitis E, is an important public health concern in many developing countries. Increasing evidence indicates that hepatitis E is a zoonotic disease. There exist four major genotypes of REV, and HEV isolates identified in samples from pigs belong to either genotype 3 or 4. Genotype 1 and 2 HEVs are found exclusively in humans. To determine whether genotype 1 and 2 HEVs also exist in pigs, a universal reverse transcription-PCR assay that is capable of detecting all four REV genotypes was used to test for the presence of REV RNA in serum and/or fecal samples from pigs in Thailand, where genotype 1 human REV is prevalent, and from pigs in Mexico, where genotype 2 human REV was epidemic. In Thailand, swine REV RNA was detected in sera from 10/26 pigs of 2 to 4 months of age but not in sera from 50 pigs of other ages. In Mexico, swine HEV RNA was detected in 8/125 sera and 28/92 fecal samples from 2-to 4-month-old pigs. Antibodies to swine REV were also detected in about 81% of the Mexican pigs. A total of 44 swine REV isolates were sequenced for the open reading frame 2 gene region. Sequence analyses revealed that all swine REV isolates identified in samples from pigs in Thailand and Mexico belong to genotype 3. Phylogenetic analyses revealed that minor branches associated with geographic origin exist among the swine REV isolates. The results indicated that genotype 1 or 2 swine HEV does not exist in pigs from countries where the respective human REV genotype I or 2 is prevalent. It is likely that only genotype 3 and 4 REV strains have zoonotic potential.