Previous studies of HEV in rats have been fraught with controversy. The earliest report linked serologic evidence of HEV in rats near a village in the former Soviet Union with an epidemic of hepatitis E in the village (20
). Later studies reported transmission of HEV in human feces from Nepal (presumably genotype 1) to laboratory rats (21
) and isolation of genotype 1 HEV sequences from rats trapped in Nepal (22
). However, the second study was retracted (23
To determine whether rats were susceptible to recognized mammalian strains of HEV, we intravenously injected laboratory rats with human genotype 1 strains of HEV from Sargodha, Pakistan (15
); Akluj, India (17
); and Kashi, People’s Republic of China (16
); a human genotype 2 strain from Mexico (19
); and a swine genotype 3 strain from Illinois, USA (18
). Infectious titer of virus administered ranged from ≈102
. None of the animals had evidence of infection, which suggested that rats are not readily susceptible to infection with other mammalian HEVs.
Nevertheless, as reported recently, rats can be infected by HEV strains (11
). Using published primers, we amplified HEV genomic sequence from 1 of 2 HEV strains isolated in urban Los Angeles. This sequence was similar to sequences isolated from 2 rats in Hamburg, Germany; the virus sequence from Los Angeles was as similar to the 2 sequences from Germany as they were to each other. All 3 strains had only ≈60% identity with other mammalian strains, which suggested that rat HEV comprises a new HEV genotype.
On the basis of our extensive attempts to identify the virus in naturally infected wild caught and experimentally infected laboratory rats, we concluded that rat HEV infections were not robust and that the magnitude and duration of viremia and fecal shedding were less than that usually observed in infections with the other mammalian HEV genotypes. A low titer of rat HEV in rat feces in Germany was also reported (11
). Rat HEV caused minimal hepatitis in experimentally infected animals; liver enzyme levels seldom increased above baseline levels, and histopathologic lesions during acute infections, although present, were minimal and not associated with clinical disease. Nevertheless, age-specific antibody prevalence in rats suggests that they are easily infected in their natural environment, and most are infected as juveniles and young adults in a pattern similar to that seen for acquisition of antibody against HEV in swine and humans in hepatitis-endemic areas (24,25
Antibody against HEV in rats was usually directed against epitopes other than the major neutralization epitope in the carboxy portion of a genotype 1 capsid protein (S.U. Emerson, unpub. data). Seroconversion was relatively sensitive in identifying HEV infection in rats; it was in some cases more sensitive than detecting viremia by PCR. However, PCR was ≈10–100-fold more sensitive than infectivity titrations for quantifying HEV, a difference that is common for many virus infections. Overall, PCR confirmed that the magnitude and duration of viremia and viral shedding are not robust in rats. Whether capsid antigen expressed by rat virus will result in a more specific and sensitive assay for rat HEV antibody and whether it will help to better define the specificity of existing tests for antibodies against HEV in humans should be determined.
The high prevalence of antibodies against HEV in humans living in countries to which HEV is not endemic suggests that HEV infection in such areas might be zoonotic. Nevertheless, a direct association between HEV infection in animals and hepatitis E in humans has been limited, for the most part, to exposure to swine through eating undercooked pork and especially undercooked offal or through environmental exposure to swine feces. However, most persons do not eat undercooked pork or come in contact with swine, and their exposure, especially among those living in inner cities or in cultures without pigs, remains an enigma. In such settings, exposure to rats could be the missing link to HEV infection.
To determine whether this link exists, we attempted to transmit rat HEV to rhesus monkeys, a surrogate of humans that are highly susceptible to mammalian genotypes 1–4, including swine HEVs (26–28
). Although we administered >100,000 infectious doses of virus intravenously to monkeys, they were not infected, as shown by lack of viremia and failure to develop antibodies against HEV. We also demonstrated similar lack of transmissibility of avian HEV to rhesus monkeys in previous collaborative studies (29
), and we believe that these studies suggest a lack of zoonotic threat to humans from either avian or rat HEV.