Hepatitis E Virus Genotype 3 in Wild Rats, United States

Justin B. Lack

, Kylie Volk, and Ronald A. Van Den Bussche

Author affiliations: Oklahoma State University, Stillwater, Oklahoma, USA

Abstract

The role of rodents in the epidemiology of zoonotic hepatitis E virus (HEV) infection has been a subject of considerable debate. Seroprevalence studies suggest widespread HEV infection in commensal Rattus spp. rats, but experimental transmission has been largely unsuccessful and recovery of zoonotic genotype 3 HEV RNA from wild Rattus spp. rats has never been confirmed. We surveyed R. rattus and R. norvegicus rats from across the United States and several international populations by using a hemi-nested reverse transcription PCR approach. We isolated HEV RNA in liver tissues from 35 of 446 rats examined. All but 1 of these isolates was relegated to the zoonotic HEV genotype 3, and the remaining sequence represented the recently discovered rat genotype from the United States and Germany. HEV-positive rats were detected in urban and remote localities. Genetic analyses suggest all HEV genotype 3 isolates obtained from wild Rattus spp. rats were closely related.

Hepatitis E virus (HEV) is a major cause of acute hepatitis in developing countries, in which outbreaks arise most often through fecal contamination of drinking water or after flooding (1). Major outbreaks have been reported in India, Southeast Asia, Africa, and Mexico, and mortality rates are considerable (20%–30%) among pregnant women (1). In industrialized countries, HEV infections are reported sporadically and contamination of drinking water is an unlikely source, but cases are increasing as diagnostic tests are being performed more frequently (2). Moreover, zoonotic transmission of HEV through consumption of undercooked pork and deer meat has been confirmed (3,4), and detection of HEV in many mammalian hosts suggests the potential for multiple zoonotic sources of HEV infection in industrialized countries (5).
There are currently at least 4 genotypes of HEV known to infect humans. Genotypes 1 and 2 have been identified only from humans and are responsible for most outbreaks in developing countries (6). Genotypes 3 and 4 are believed to be involved in zoonotic transmission and have been isolated from swine (domesticated pig and wild boar), deer, mongoose, rabbits, cattle, and humans (5). Additional strains not known to infect humans have also been identified in rats and chickens, and the genetic diversity of HEV is only beginning to be understood.
Within the United States, HEV infections have been identified in travelers who have visited developing countries (7), and for several at-risk groups in the United States (i.e., swine veterinarians and farmers), the high number of reported seropositive persons is caused by swine–human contact (8,9). However, seroepidemiologic examinations of blood banks in the United States and other industrialized countries have shown high proportions of samples positive for antibodies against HEV (excluding persons who had traveled to HEV-endemic countries), but this finding was true in urban areas in which swine–human contact is absent (8,10,11).
HEV RNA has been detected in livers from commercially raised pigs (12) and represents an additional potential reservoir of infection. However, consumption of raw pork and wild game is uncommon in the United States, although it is a common practice in other industrialized nations in which high HEV seroprevalence has been reported (i.e., France) (13). This finding suggests that in addition to travel to HEV-endemic regions and swine–human contact, additional reservoirs of HEV infection exist in the United States, and evidence has accumulated indicating rodents as a potential HEV reservoir (14–18). In a survey of 26 rodent species in the United States, Favorov et al. (14) found 14 species of rodents seropositive for antibodies against HEV. Urban populations had ≈2× the proportion of seropositive rats relative to rural populations, and commensal Rattus spp. (R. rattus and R. norvegicus) rats had the highest proportion of seropositive animals (14).
The role of wild Rattus spp. rats as reservoirs in the epidemiology and transmission of HEV is unclear, but their ubiquity in urban environments and unparalleled propensity for carrying zoonotic pathogens makes them an obvious target of investigation. Multiple studies have reported finding IgG and IgM against HEV in R. norvegicus and R. rattus rat populations across the United States and Asia (14–18). Shukla et al. (19) successfully infected cell lines from Mus musculus mice, murid rodents closely related to Rattus spp. rats, with HEV genotype 3. In addition, Maneerat et al. (20) experimentally infected laboratory R. norvegicus rats with HEV isolated from infected humans, although the genotype of the infecting virus was unclear. After infection, the human virus strain effectively replicated in multiple tissues, and HEV RNA was detected in feces and serum for >30 days postexposure, suggesting that human strains of HEV can replicate in and be transmitted by R. norvegicus rats. However, recent discovery of a rat-specific strain of HEV not known to infect humans (21–23) suggests that high seroprevalence of antibodies against HEV may be caused by cross-reactivity rather than widespread infection with a human-infecting HEV genotype.
We used a reverse transcription PCR (RT-PCR) approach to survey R. rattus and R. norvegicus rats for HEV RNA. Our analysis detected HEV RNA in liver tissues from R. rattus and R. norvegicus rats at many localities across the United States. Sequencing of DNA from RT-PCR–positive samples indicated widespread infection with zoonotic HEV genotype 3: one rat in California was positive for the rat-specific strain. These findings suggest that wild Rattus spp. rats are competent hosts for genotype 3 HEV.