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Structure of Hepatitis E Virus
Image: Schematic illustration of non-enveloped and quasi-enveloped HEV particles as well as enveloped virus. The putative model of quasi-enveloped HEV virion includes ORF3 product in its envelope as the existence of pORF3 has been confirmed by capturing quasi-enveloped HEV virion with anti-pORF3 antibodies and further supported by prediction of a putative transmembrane region in the N-terminal of pORF3.
- HEV is classified in the Calciviridae family because of its structural similarity to other calciviruses; however, it is now the sole member of the Hepeviridae family.
- The virus is non enveloped (naked) with icosahedral symmetry measuring 27-30 nm diameter.
- The genome is single stranded RNA genome with positive polarity and measure about 7.2 kb in length.
Genome of Hepatitis E Virus
- The HEV genome is a single-stranded, positive-sense RNA molecule and 7.2 kb in size.
- Genomic RNA is polyadenylated and contains 3 ORFs.
- ORF1 encodes the nonstructural proteins, ORF2 encodes the capsid protein, and ORF3 encodes a small multifunctional protein.
- The ORF2 and ORF3 proteins are translated from a single, bicistronic mRNA.
- Located near the 5′-end, ORF1 encodes a non-structural polyprotein with multiple functional domains, including those for methyltransferase, protease, helicase, and polymerase.
- The viral capsid protein (CP) is encoded by ORF2 near the 3′-end.
- ORF3, which partially overlaps with the other 2 ORFs, codes for an immunogenic protein of unknown function.
- ORF3 encodes a 113 or 114 aa phosphoprotein, depending upon the genotype.
- The ORF2 capsid protein, HEV-CP, contains a total of 660 amino acid residues.
- At the HEV-CP N terminus is a signal peptide followed by an arginine-rich domain that potentially play a role in viral RNA encapsidation during assembly.
- HEV-CP is a key antigen that stimulates the host immune response, and 6 antigenic domains have been identified.
- One neutralization site has been mapped to the polypeptide region between amino acids 452 and 617.
Epidemiology and Transmission of Hepatitis E Virus
- The HEV infection was first reported from the Indian subcontinent and subsequently from other parts of Asia, the Middle East, Central and South America, Africa, Central Europe and Russia.
- People travelling to countries with high prevalence are therefore at risk of acquiring infection during their travel.
- Adult populations in endemic areas are generally susceptible and there is a high infection rate in epidemics.
- The hepatitis E virus is transmitted mainly through the faecal-oral route due to faecal contamination of drinking water or via Ingestion of undercooked meat or meat products derived from infected animals.
- Other transmission route includes: vertical transmission from a pregnant woman to her fetus and transfusion of infected blood products.
Replication of Hepatitis E Virus
- The mechanisms underlying HEV replication are poorly understood.
Figure: Proposed replication of hepatitis E virus. Source: Doi: 10.1111/jvh.12445.
- The HEV capsid protein is believed to bind to a cellular receptor to initiate viral entry and replication.
- ORF2 peptide-binding experiments suggested that the C-terminal region of ORF2 may mediate virus entry by binding to heat shock cognate protein 70 (HSC70) on the cell surface.
- Additionally, HSPGs have been identified as attachment receptors that are located on the cell surface.
- After virus entry into permissive cells, the HEV genomic RNA is uncoated by unknown mechanisms.
- After uncoating, virion releases the positive-sense genomic RNA into the cytoplasm of the cell.
- The positive-sense genomic viral RNA serves as the template to translate the ORF1 nonstructural polyprotein in the cytoplasm.
- The viral RdRp synthesizes an intermediate, replicative negative-sense RNA from the positive-sense genomic RNA that serves as the template for the production of positive-sense, progeny viral genomes.
- The ORF2 and ORF3 proteins are translated from the subgenomic, positive-stranded RNA, and the ORF2 capsid protein packages the genomic viral RNA and assembles new virions.
- The nascent virions are transported to the cell membrane.
- The ORF3 protein facilitates the trafficking of the virion, and the nascent virions are released from the infected cells by lysis.
Pathogenesis of Hepatitis E Virus
- The pathogenesis of hepatitis E is poorly understood.
- Since HEV is presumably transmitted by the fecal-oral route, it is unclear how the virus reaches the liver.
- There is an extra-hepatic site of virus replication.
- The virus could replicate in the intestinal tract before reaching the liver.
- Negative strands of HEV RNA, indicating virus replication, have been detected in the small intestine, lymph nodes, colon, and liver of pigs, indicating extra-hepatic HEV replication.
- HEV then replicates in the cytoplasm of hepatocytes and is released into both blood and bile.
- The liver damage induced by HEV infection may be immune-mediated by cytotoxic T cells and natural killer (NK) cells since HEV is not cytopathic.
- The virus is shed in the stool.
- A serological anti-HEV response is generally detected in patients at the time of onset of illness.
- Anti-HEV IgMs are detected in the early phase of clinical illness, and can persist for several months.
- Anti-HEV IgG appears shortly after the IgM response and can last several years.
- Cross protection is possible due to the existence of only one serotype.
Pathogenesis of fulminant Hepatitis
- The reasons why a hepatitis E infection becomes fulminant are still obscure.
- Stimulating both Th1 and Th2 type immune responses could play a role in liver failure.
- Host factors rather than virus genotype, variants, or specific aminoacids substitutions are responsible for the development of fulminant hepatitis.
- In fulminant hepatitis, there is higher anti-HEV IgM and IgG titers, along with higher concentrations of IFN-γ, TNF-α, IL-2, and IL-10.
- CD4+ T cells are more frequent in the liver and CD8+ T cells have been shown to infiltrate the liver of patients with fulminant hepatitis E.
- Thus, cytotoxic CD8+ T cells could be particularly important in the pathogenesis of fulminant hepatitis.
- Women with acute liver failure (ALF) presents a reduced expression of toll-like receptor (TLR) 3/TLR7/TLR9.
- Impaired monocyte-macrophage function in pregnant women with ALF could contribute to an inadequate innate immune response, and hence to the development and severity of ALF.
- High concentrations of cytokines (TNF-α, IL-6, IFN-γ and TGF-β1) may also be associated with an adverse pregnancy outcome.
- The concentrations of estrogen, progesterone, and β-human chorionic gonadotrophin in HEV-positive pregnant FHF (Fulminant Hepatic Failure) women are higher than in HEV-negative pregnant FHF women.
- An in vitro study have shown that serum from pregnant women, especially those in the third trimester, enhanced the replication of HEV by inhibiting estrogen receptor and type I IFN expression.
Clinical Manifestations of Hepatitis E Virus
- Most HEV infections have a clinically silent course.
- In symptomatic cases, the incubation period ranges from 2 to 8 weeks, with a mean of 40 days.
- Initial symptoms of acute hepatitis E are typically unspecific and include flu-like myalgia, arthralgia, weakness, and vomiting.
- Some patients have jaundice, itching, uncolored stools, and darkened urine, accompanied by increased levels of liver transaminases, bilirubin, alkaline phosphatase, and γ-glutamyltransferase.
- HEV infection can lead to more severe, acute liver disease in pregnant women or patients with underlying chronic liver diseases and sometimes progress to fulminant hepatic failure.
- Infection in organ transplant recipients: Chronic HEV infection has been described in liver and kidney transplant recipients,that lead to persistent increases in levels of alanine aminotransferase, significant histological activity, and fibrosis.
- Patients with HIV Infection: Human immunodeficiency viruses (HIV)–infected individuals more frequently have positive results from tests for anti-HEV than individuals without HIV infection.
- Extrahepatic Manifestations: Muscular weakness and a pyramidal syndrome in a kidney transplant recipient with persistent HEV infection.
- In addition, neurological disorders, including polyradiculopathy, Guillain–Barré syndrome, bilateral brachial neuritis, encephalitis, or proximal myopathy have been reported in patients with acute and chronic HEV infections.
Laboratory Diagnosis of Hepatitis E Virus
- Specimens: Blood, serum, stool
- Definitive diagnosis of hepatitis E infection is usually based on the detection of specific IgM and IgG antibodies to the virus in a person’s blood.
- Additional tests include reverse transcriptase polymerase chain reaction (RT-PCR) to detect the hepatitis E virus RNA in blood and/or stool.
Treatment of Hepatitis E Virus
- There is no specific treatment capable of altering the course of acute hepatitis E.
- The disease is usually self-limiting.
- Hospitalization is required for people with fulminant hepatitis, and should also be considered for symptomatic pregnant women.
- Immunosuppressed people with chronic hepatitis E benefit from specific treatment using ribavirin, an antiviral drug.
- In some specific situations, interferon has also been used successfully.
Prevention and Control of Hepatitis E Virus
- At the population level, transmission of HEV and hepatitis E disease can be reduced by:
- Maintaining quality standards for public water supplies.
- Establishing proper disposal systems for human feces.
- On an individual level, infection risk can be reduced by:
- Maintaining hygienic practices such as hand-washing with safe water, particularly before handling food.
- Avoiding consumption of water and/or ice of unknown purity, and adhering to WHO safe food practices.
- “HEV is preventable by vaccination. HEV239 (Hecolin) is a recombinant HEV vaccine against genotype 1 and 4 that has shown to have more than 95% protection against the virus and to be safe in pregnancy. This vaccine is now available in China”.