Hepatitis is a general term meaning inflammation of the liver. Hepatitis is a disease that can be caused by a variety of different viruses such as hepatitis A, B, C, D and E. Since the development of jaundice is a characteristic feature of liver disease, a correct diagnosis can only be made by testing patients' sera for the presence of specific viral antigens and/or anti-viral antibodies.
Hepatitis E (HEV) was not recognized as a distinct human disease until 1980. Hepatitis E is caused by infection with the hepatitis E virus, a non-enveloped, positive-sense, single-stranded RNA virus.
Although man is considered the natural host for HEV, antibodies to HEV or closely related viruses have been detected in primates and several other animal species.
Travel to Asia presents certain risks with respect to Hepatitis E infection. The following abstract, though very detailed, presents this subject in a complete fashion.
Hepatitis E virus (HEV), the causative agent of hepatitis E, belongs to the family Hepeviridae. At least four major genotypes of HEV have been recognized: genotypes 1 and 2 are restricted to humans and associated with epidemics in developing countries, whereas genotypes 3 and 4 are zoonotic and infect humans and several other animals in both developing and industrialized countries. Besides humans, strains of HEV have been genetically identified from swine, chickens, sika deer, mongeese, and rabbits. The genome of HEV consists of three open reading frames (ORFs): ORF1 codes for nonstructural proteins, ORF2 codes for capsid protein, and ORF3 codes for a small multifunctional protein. The ORF2 and ORF3 proteins are translated from a single bicistronic mRNA and overlap each other but neither overlaps ORF1. The recent determination of the 3D crystal structure of the HEV capsid protein should facilitate the development of vaccines and antivirals. The identification and characterization of animal strains of HEV from pigs and chickens and the demonstrated ability of cross-species infection by swine HEV raise public health concerns for zoonosis. Accumulating evidence indicated that hepatitis E is a zoonotic disease and pigs and more likely other animal species are reservoirs for HEV. This article provides an overview of the recent advances in hepatitis E and its causative agent, including nomenclature and genomic organization, gene expression and functions, 3D structure of the virions, changing perspectives on higher mortality during pregnancy and chronic hepatitis E, animal reservoirs, zoonotic risk, food safety, and novel animal models.
Hepatitis E is an important public health disease in many developing countries of Asia and Africa. Sporadic cases of hepatitis E have also been reported in many industrialized countries. A unique feature of hepatitis E is the relatively high mortality rate reported during pregnancy. The causative agent, hepatitis E virus (HEV), is a small nonenveloped RNA virus that is transmitted primarily via the faecal–oral route.In the past decade or so, progress has been made in understanding the epidemiology, natural history, animal reservoirs, pathogenesis, structure, and molecular biology of HEV. The recent discovery of animal strains of HEV from pigs, chickens, rabbits, deer, and mongeese, and the existence of other animal species seropositive for HEV antibodies has broadened the host range and diversity of HEV. Hepatitis E is now a recognized zoonotic disease, and pigs and more likely other animal species are reservoirs for HEV. The successful construction of HEV infectious clones and the identification of cell lines supporting a limited level of HEV replication provided useful tools to dissect the structural and functional relationship of HEV genes. The development of small animal models for HEV, pigs and chickens, affords the opportunity to define the molecular basis of HEV replication and pathogenesis. The determination of the 3D crystal structure of HEV capsid protein will help rationally design effective vaccines and antivirals against HEV in the future.
This article provides an overview of the recent advances in hepatitis E and its causative agent, including new nomenclature and genomic organization, gene expression and functions, 3D structure of the virions, changing perspectives on higher mortality during pregnancy and chronic hepatitis E, animal reservoirs, cross-species infection and zoonotic risk, food safety, and novel animal models. Despite recent advances, HEV remains an extremely understudied, but important, human pathogen.
Classification and Taxonomy Update
The nomenclature of HEV has been somewhat confusing. HEV was originally classified in the family Caliciviridae. However, because the HEV genome does not share significant sequence homology with caliciviruses, the virus was subsequently declassified from the family Caliciviridae. Currently, HEV is placed in a sole genus Hepevirus within a new family Hepeviridae. However, with the recent identifications of novel HEV strains from various animal species, the HEV taxonomy will probably have to be revised again.
Currently, the species in the genus Hepevirus includes the four recognized major genotypes of HEV in mammalian species: genotype 1 (Burmese-like Asian strains), genotype 2 (a single Mexican strain and some African strains), genotype 3 (strains from sporadic human cases in industrialized countries, and animal strains from pigs, deer, and mongeese), and genotype 4 (strains from sporadic human cases in Asia, and swine strains from pigs). Recently, a novel strain of HEV was isolated from farm rabbits in China. The rabbit HEV appears to be genetically distinct from the four recognized mammalian genotypes, and thus probably represents an additional and fifth genotype within the genus Hepevirus.
The avian HEV is currently classified as a tentative new species in the genus Hepevirus. However, genetically distinct strains of avian HEV belonging to three different genotypes have recently been identified from chickens in Europe. Therefore, the current classification of avian HEV will probably have to be revised as well. Considering that there exist at least three genotypes of avian HEV and that avian HEV shares only approximately 50% nucleotide sequence identity with the mammalian HEV strains, it will be more appropriate to classify avian HEV as a new and separate genus, instead of a species, within the family Hepeviridae.
Emergence of Sporadic Cases of Hepatitis E of Animal Origin
Hepatitis E is no longer just a disease of developing countries. Sporadic cases of human hepatitis E have been reported in both industrialized and developing countries, although epidemics only occur in developing countries of Asia, Africa, and in Mexico. Recently, there has been an increased incidence of sporadic hepatitis E cases in industrialized countries, and most are caused by zoonotic genotypes 3 and 4 strains of HEV. The sources of infection appear to be different for epidemics and sporadic cases: HEV-contaminated drinking water is the main source for HEV epidemics, whereas the risk factors for sporadic cases include shellfish, contaminated animal meats, and direct contacts with infected animals. Recent studies show that the prevalence of anti-HEV antibodies is very high in some developing countries: for example, more than 70% of the general populations in Egypt are positive for anti-HEV. Also, anti-HEV prevalence in some industrialized countries is much higher than expected: for example, in the United States, up to 30% of the normal blood donors in some regions were positive for anti-HEV. The source of seropositivity in individuals from industrialized countries may be of animal origin.
New Perspectives on High Mortality Associated with HEV Infection during Pregnancy
It has long been believed that HEV infection is associated with high mortality in pregnant women, particularly from certain geographical regions in India, where HEV infection causes more severe hepatitis, often leading to acute liver failure (ALF) and death in a significant proportion of pregnant women. However, the results from recent studies in India and elsewhere indicate that the severity of viral hepatitis during pregnancy is similar to that in nonpregnant women. For example, a recent study of a cohort of 2428 pregnant women in Egypt showed that the patients had prior HEV exposures but with no history of liver disease. In India, a large retrospective study was conducted in a single medical centre over a 20-year span to evaluate the risk of HEV-related ALF during pregnancy. Of the 1015 ALF patients in the reproductive age group during the 20-year period, 249 (38.5%) were pregnant women. However, the results showed that HEV-related ALF was independent of the sex or the pregnancy status of the patients; the mortality rate in HEV-related ALF in pregnant patients is 51%, whereas the mortality in non-HEV-ALF in pregnant patients was 54.7% (P > 0.1). The outcome of pregnant ALF patients was also unrelated to the trimester of pregnancy. Under laboratory conditions, the severe and fulminant hepatitis E in infected pregnant women reported in some geographical regions could not be experimentally reproduced in pregnant pigs or monkeys; pregnant sows experimentally infected with a genotype 3 HEV at various stages of gestation had no clinical signs of hepatitis or elevation of liver enzymes. Similarly, pregnant rhesus monkeys experimentally infected with a genotype 1 HEV did not manifest more severe hepatitis than the nonpregnant monkeys. The reasons for the geographical difference regarding the mortality rate associated with HEV infection during pregnancy remain unknown. The socio-economic status of the patients, hormonal changes during pregnancy and the immunological status of the patients, the health care standard in the region, and the existence of other coinfecting agents in HEV-infected patients could all play a potential role in the observed geographical difference of mortality associated with HEV infection during pregnancy.
Chronic Hepatitis E and Organ Transplantation
Unlike hepatitis B and C viruses, HEV is generally believed to cause only a self-limiting acute viral hepatitis that does not result in chronicity. However, recent studies revealed that persistent HEV infections do occur, especially in immune-suppressed organ transplant recipients. Kamar et al. studied 14 cases of acute HEV infection in patients receiving organ transplants. Eight developed chronic hepatitis E as evidenced by persistently elevated aminotransferase levels, detection of HEV RNA in sera, and microscopic evidence of chronic hepatitis. In a retrospective study, two cases of chronic hepatitis E were identified in liver transplant patients. One patient developed unexplained chronic hepatitis 2 months after liver transplantation, which further progressed into cirrhosis. Retransplantation was required in this patient 7 years later, and chronic hepatitis reoccurred. Genotype 3 HEV RNA was detected in sera 3 weeks after the initial transplantation and remained positive in retransplant liver tissues. Another patient also developed unexplained chronic hepatitis after liver transplantation, and retransplantation was needed 5 years later. Genotype 3 HEV RNA was also detected during the period of chronic hepatitis up to the retransplantation. Two cases of rapidly progressive HEV-associated liver cirrhosis were reported in two organ transplant patients. A kidney–pancreas transplant patient developed acute viral hepatitis E 60 months after transplantation, which progressed to chronicity with persistently elevated liver enzymes and detection of HEV RNA in sera. A kidney transplant patient developed acute hepatitis 36 months after transplantation, which persisted for 38 months. HEV RNA was positive in the serum and stool samples of the patient. In a separate retrospective study, persistent HEV infection was also identified in a patient infected with HEV via blood transfusion during chemotherapy against T-cell lymphoma. The HEV sequence from the blood donor was identical to that from the patient obtained on day 170 after the transfusion. Reactivation of HEV infection was also reported in a patient with acute lymphoblastic leukaemia 14 weeks after allogeneic stem cell transplantation. Under laboratory conditions, prolonged viraemia and faecal virus shedding were also observed in a small number of chickens experimentally infected with avian HEV and a few pigs experimentally infected with swine and human HEV.
Taken together, these recent studies indicate that chronic HEV infection may develop in immunosuppressed patients. Acute HEV infection in immunocompromised patients such as organ transplant recipients and cancer patients not only can evolve into chronic hepatitis E, but also can rapidly progress into cirrhosis as well. In some of these patients, anti-HEV antibodies were negative probably because of immune suppression. The contribution of the deficiency of anti-HEV antibodies to the establishing of the chronic infection needs to be established.
Swine Hepatitis E Virus (Swine HEV)
The identification of the first animal strain of HEV, swine HEV from pigs, in the United States opened new avenues for HEV research. Since the initial report in 1997, swine HEV has now been identified essentially from all swine-producing countries of the world. The viruses identified from pigs belong to either genotype 3 or 4, which both cause sporadic cases of hepatitis E in humans. Swine HEV identified from pigs is genetically closely related, or identical in some cases, to the genotypes 3 and 4 strains of HEV recovered from human hepatitis E patients. It is believed that the genotypes 3 and 4 HEV are viruses of swine that also infect humans. Genotypes 1 or 2 HEV strains have not been definitively identified from pigs, although a genotype 1-like sequence was reportedly amplified from a pig in Cambodia, but independent confirmation of this finding is lacking. Swine HEV infection is widespread worldwide and generally occurs in pigs of 2–4 months of age. Approximately 80–100% of the pigs in commercial farms were infected by swine HEV. The infected pigs remain clinically normal, although microscopic evidence of hepatitis was observed in infected pigs.
Avian Hepatitis E Virus (Avian HEV)
In 1999, a HEV-related virus, designated as the big liver and spleen disease (BLS) virus, was identified in chickens from Australia, and the BLS virus was found to share approximately 62% nucleotide sequence identity with HEV. In 2001, avian HEV was first isolated from bile samples of chickens with hepatitis–splenomegaly (HS) syndrome in the United States. HS syndrome is a disease of layer and broiler breeder chickens characterized by increased mortality, decreased egg production, and enlarged livers and spleens. Avian HEV shared approximately 80% nucleotide sequence identity with the Australian BLS virus, suggesting that BLS in Australia and HS syndrome in North America are caused by variant strains of the same virus, avian HEV. The complete sequence of avian HEV is about 600 bp shorter than that of mammalian HEVs. Although avian HEV shares only approximately 50% nucleotide sequence identity with mammalian HEVs, the genomic organization and functional motifs are conserved. Thus far, at least three genotypes of avian HEV have been identified from chickens worldwide. Like swine HEV in pigs, avian HEV infection in chickens is also widespread; about 71% of chicken flocks and 30% of chickens in the United States were positive for IgG antibodies to avian HEV.
HEV Strains from Wild Boars, Sika Deer, Mongeese, and Rabbits
In addition to domestic pigs and chickens, novel strains of HEV have also been genetically identified from wild boars, sika deer, mongeese, and rabbits. Wild boars in several countries including Japan, Germany, Italy, Spain, and Australia are infected by HEV. A full-length genomic sequence of HEV recovered from a wild boar was found to be 99.7% identical to the virus from a deer hunted in the same forest and to four patients who consumed deer meat and contracted hepatitis E, suggesting a potential interspecies HEV transmission between boar and deer in wildlife. The HEV strain identified from wild boars belongs to genotype 3. A genotype 3 HEV was also identified from sika deer, and cases of hepatitis E have been linked to the consumption of HEV-contaminated deer meat. Approximately 8–21% of the mongooses tested in Japan were found seropositive for HEV antibodies. The full-length genomic sequence of HEV recovered from a mongoose was determined and shown to be a genotype 3 strain that was closely related to a genotype 3 swine HEV from Japan.
More recently, a novel strain of HEV was identified from farm rabbits in China, and the rabbit HEV is genetically distinct from the other four known mammalian HEV genotypes with only 73–79% nucleotide sequence identity. It is possible that the rabbit HEV represents a distinct fifth genotype within the Hepevirus genus. Approximately 57% of the farm rabbits in China were seropositive for anti-HEV with approximately 8% of them also positive for HEV RNA. It will be interesting to see whether the rabbit populations in other countries are also infected by HEV and whether the rabbit HEV can cross species barrier and infect other animal species including humans.
Existence of Other Potential Animal Reservoirs for HEV
The host range of HEV is ever-expanding. In addition to the animal species from which HEV strains have been genetically identified including domestic and wild pigs, chickens, deer, rabbits, and mongeese, antibodies to HEV have also been detected in many other animal species such as dogs, cats, sheep, goats, rodents, cattle, and non-human primates, suggesting that these animals have been exposed to HEV or a related agent. However, the source of seropositivity in these animal species could not be definitively identified at the present time because virus was either not recovered from these animal species or the recovered virus could not be sequenced to confirm its identity. It is possible that novel animal strains of HEV will continue to be genetically identified in the near future from these seropositive animal species.
Cross-species Infection and Zoonosis
Genotypes 3 and 4 strains of swine HEV can cross species barriers and infect both rhesus monkeys and chimpanzees. Conversely, the genotypes 3 and 4 strains of human HEV infected pigs. It appears that the genotypes 1 and 2 human HEV have more limited host range and are restricted to humans because attempts to experimentally infect pigs with genotypes 1 and 2 human HEV were unsuccessful. Cross-species infection of HEV has also been reported in other animal species: lambs and Wistar rats were reportedly infected by human HEV isolates, although independent confirmation of these reports is still lacking. Like the genotypes 3 and 4 swine HEV, the avian HEV can also infect across species barriers: avian HEV from a chicken successfully infected turkeys. However, an attempt to experimentally infect two rhesus monkeys with avian HEV was unsuccessfully, suggesting that, unlike swine HEV, avian HEV may have a more limited host range and may not readily infect humans.
Hepatitis E is now recognized as a zoonotic disease and pigs and more likely other animal species are reservoirs. Pig handlers such as pig farmers and swine veterinarians in both developing and industrialized countries have been shown to be at increased risk of HEV infection. For example, swine veterinarians in the United States were 1.51 times more likely to be positive for anti-HEV than age-matched and geography-matched normal blood donors. Individuals from traditionally major swine States are more likely seropositive for anti-HEV than those from traditionally non-swine States; for example, subjects from Minnesota, a major swine State, are approximately five to six times more likely to be seropositive than those from Alabama, which is not a major swine State. Drobeniuc et al. reported that approximately 51% of swine farmers in Moldova were positive for anti-HEV, whereas only 25% of control subjects with no occupational exposure to swine were seropositive. Withers et al. reported that swine workers in North Carolina had a 4.5-fold higher anti-HEV antibody prevalence rate (10.9%) than the control subjects (2.4%). Besides swine, potential transmissions of hepatitis E from a pet cat and a pet pig to human owners were also reported. As more and more animal strains of HEV are genetically identified, their zoonotic potentials must be thoroughly evaluated. Understanding the ecology and natural history of HEV will be a key for effective prevention and control of HEV infections in humans.
Food and Environmental Safety
As a faecal-orally transmitted disease, contaminated water or water supplies are major sources of HEV infections. Historically, waterborne epidemics are the characteristic of hepatitis E outbreaks in humans. It is known that infected pigs and other animals excrete large amounts of HEV in faeces, thus posing a concern for environmental and food safety. HEV-containing animal manure and faeces could contaminate irrigation or coastal water with concomitant contamination of produce or shellfish. Strains of HEV of both human and swine origins have been detected in sewage water. Sporadic cases of hepatitis E have been linked to the consumption of contaminated raw and undercooked pig livers. Approximately 2% of the pig livers sold in local grocery stores in Japan and 11% in the United States tested positive for swine HEV RNA. Most importantly, the contaminating virus present in pig livers sold from the grocery stores in the United States remains fully infectious. The virus sequences recovered from pig livers in grocery stores are closely related, or identical in a few cases, to the viruses recovered from human hepatitis E patients. In Japan, a 53-year-old man developed severe hepatitis E after consumption of contaminated wild boar meat. Another patient, a 70-year-old man, who also ate the same wild boar meat died of fulminant hepatic failure. Neither patient had travelled to an HEV endemic area, but both patients ate uncooked wild boar livers on five occasions. An HEV sequence of genotype 4, which is known to infect domestic and wild pigs, was amplified from the acute phase sera of one patient. Also, a cluster of four cases of hepatitis E were definitively linked to the consumption of raw deer meats in two families in Japan. The viral sequence recovered from the leftover frozen deer meat was 99.7–100% nucleotide sequence identical to the viruses recovered from the four human patients. Taken together, these data provide compelling evidence for zoonotic HEV transmission via direct contact with infected animals or via consumption of infected animal meats.
Journal of Viral Hepatitis