Hepatitis A Virus- An Overview

Structure of Hepatitis A Virus

• HAV (Hepatitis A Virus) is a distinct member of the picornavirus family, assigned to genus hepatovirus.
• HAV is a 27 nm to 32 nm spherical particle with icosahedral symmetry containing a linear single-stranded RNA genome with a size of 7.5 kb and non-enveloped.

• Hepatitis A virions have a primary buoyant density of 1.32 to 1.34 g/cc in CsCl and a sedimentation coefficient of 156S to 160S in neutral sucrose solutions.

Genome of Hepatitis A Virus

• HAV can be divided into three parts
  • a 5′ noncoding region (NCR) that comprises approximately 10% of the genome, is uncapped, and is covalently linked at the 5′ terminus to viral protein VPg
  • a single open reading frame that appears to encode all of the viral proteins, with regions designated as P1 for capsid proteins and P2 and P3 for nonstructural proteins
  • a short 3′ NCR terminating in a polyA tail.

• The regions P1 contains four segments for structural proteins which make up the capsid protein; 1A-VP4, 1B- VP2, 1C-VP3, 1D-VP1.
• P2 comprises of three non structural proteins; 2A, 2B, 2C which play a role in viral replication.
• P3 makes up four non structural proteins
  • 3A- anchors the replication complex to cell membrane
  • 3B- it is VPg protein
  • 3C- it is cysteine protease that cleaves the protein from polypeptides
  • 3D- it is RNA dependent RNA Polymerase.

Epidemiology of Hepatitis A Virus

• Hepatitis A occurs throughout the world.
• It is highly endemic in some areas, particularly Central and South America, Africa, the Middle East, Asia, and the Western Pacific.

Transmission of Hepatitis A Virus

• Transmission via fecal-oral route; ingestion of fecally contaminated food (eating uncooked shellfish harvested from sewage) or contaminated water.

Replication of Hepatitis A Virus

• HAV is spread primarily through the ingestion of fecally contaminated food or water.
• Once HAV reaches the intestine, it is thought to be absorbed into the bloodstream and to reach the liver through the portal system.
• Attachment of the virus to host cell receptors (HAV cr-1) mediates endocytosis of the virus into the host cell possibly by clathrin- dependent endocytosis.
• Upon endosomal acidification, the capsid undergoes a conformational change and release VP4 that opens a pore in the host endosomal membrane and the viral genomic RNA penetrates into the host cell cytoplasm.
• VPg protein is removed from the viral RNA, which is translated into a processed polyproten.
• The IRES allows direct translation of the polyprotein.
• A ds RNA genome is synthesized from the genomic ssRNA(+).
• The dsRNA genome is transcribed thereby providing viral mRNAs/new ssRNA(+) genomes.
• New genomic RNA is believed to be packaged into preassembled procapsids.
• Cell lysis occurs and virus is released.

Pathogenesis of Hepatitis A Virus

• Viral replication occurs primarily within hepatocytes and the secretion of virus into bile results in large quantities of virus being shed in the faeces.
• During the incubation period, viremia is observed at about the same time that fecal shedding of HAV is occurring.
• Viremia terminates shortly after hepatitis develops, whereas feces may remain infectious for another 1 to 2 weeks.
• Acute hepatitis includes features like inflammatory cell infiltration, hepatocellualr necrosis and liver cell regeneration.
• Portal infiltration by lymphocytes, plasma cells and periodic acid Schiff (PAS)-positive macrophages are prominent features in early biopsies.
• Parenchymal cells undergo ballooning degeneration.
• These hepatocytes are swollen and have indistinct plasma membranes, enlarged nuclei, and a featureless cytoplasm, except for some cytoplasmic remnants condensed around the nuclei.
• Disruption of bile canaliculi may lead to bile retention after liver cell enlargement or necrosis.
• In some cases, extension of the inflammatory infiltrate from the periportal region into the hepatic parenchyma with significant erosion of the limiting plate is seen.
• HAV replication in the liver triggers a substantial immune response, both humoral and cell mediated.
• CD8 +, cytotoxic T cells that are capable of lysing autologous HAV infected cells , but not of controlling uninfected cells, are present both in circulation and in the liver at the site of disease.
• These virus specific T cells also produce interferon gamma and other cytokines at the site of infection that may be responsible for much of liver injury.
• In addition to cell mediated immune response, there is vigorous antibody response to the virus during later stages of infection which are directed against conformational epitopes.
• Neutralizing antiviral antibodies play an important role in clearance of the virus.
• Serum antibody responses are first noted at onset of symptoms and include virus specific IgM as well as IgG and IgA.

Clinical Manifestations of Hepatitis A Virus

• HAV causes an acute, self limiting infection that does not progress to chronic phase.
• Following exposure, an incubation period of 15-45 days precedes the development of clinical symptoms.
• Virus is present in blood and shed in stools within a few days of exposure.
• Symptoms typically occurs abruptly, with liver injury heralded by fever, myalgia, nausea, anorexia and vomiting accompanied by right upper quadrant abdominal pain.
• Disruption of hepatobilary metabolism results in passage of dark coca cola like urine, light gray coloured stool and frank icterus.
• Appearance of biochemical abnormalities including abnormal elevation of liver derived enzymes like ALT- alanine aminotransferase, ALP- alkaline phosphatase, GGTP- gamma glutamyl transpeptidase and increased level of serum bilirubin.
• The severity of infection is associated with age of patient indicating individuals over 50 years of age at increased risk.
• They may developed fulminant hepatitis with clinical pictures including ascites, bleeding diathesis or hepatic coma.
• Severe complication includes cholestatic hepatitis characterized by persistent jaundice associated with pruritis, anorexia and weight loss.

Lab Diagnosis of Hepatitis A Virus

Sample: blood, stool, bile, liver biopsy, serum

1. Antigen detection

• Detection using PCR and nucleic acid hybridization assay.

2. Antibody detection

• Demonstration of IgM antibodies to the virus, which are almost always present at the onset of symptoms and which persist for up to 6 months following infection.
• IgG antibody usually persists for many years and is a useful indicator of immunity.
• Antibody detection done by Enzyme linked immunsorbent assay (ELISA).

3. Liver function test- detection of level of liver enzymes like ALT, ALP, GGTP, and serum bilirubin.

Treatment of Hepatitis A Virus

• Supportive treatment to reduce other non specific symptoms
• No antiviral therapy

Vaccination of Hepatitis A Virus

• Inactivate or live attenuated vaccine confers 90% of prevention.
• Formalin inactivated HAV vaccine given in two doses- an initial dose followed by booster dose after 6-12 months.
• Two inactivated whole-virus hepatitis A vaccines are available: HAVRIX (GlaxoSmithKline) and VAQTA (Merck).

Prevention and control of Hepatitis A Virus

• Persons exposed to HAV (e.g., those who have been served food by an HAV-infected food handler) can be offered administration of serum immune globulin.
• If given soon after HAV exposure, the anti-HAV antibodies in serum immune globulin can prevent HAV infection or reduce its extent and severity.
• This form of preventive intervention is known as postexposure prophylaxis.
• Personnel hygiene and sanitation should be followed.