Hepatitis B Virus

STRUCTURE:

HBV can be divided into four major antigenic types :

• adw
• ayw
• adr 
• ayr
• HBV present in India is Ayw, Adr

Three types of particles :

Dane particle

• Double walled sphericle structure, 42 nm in diameter.
• It is the complete hepatitis B virus
• It contains.

1. Core antigen (C antigen – HBc Ag)
2. Surface antigen (HBs Ag)
3. Hepatitis B `e’ antigen (HBe Ag)

Sphericle particle (most common)

• 22 nm in diameter
• It is the surface component of HBV (HBs Ag)

Filamentous or tubular particle

• 22 nm
• It is identical to sphericle particle ie HBs Ag.

Genome:

• There are four known genes encoded by the genome 
• C, P, S, and X
• The core protein
• coded for by gene C (HBcAg)
• Start process from which the pre-core protein is produced. 
• HBeAg is produced by proteolytic processing of the pre-core protein. 
• The DNA polymerase
• encoded by gene P. 
• Reverse transcriptase
• coded on p gene
• P gene is the longest DNA of hepatitis B virus 
• Gene S
• codes for the surface antigen (HBsAg)

TRANSMISSION:

• Transfusion of blood or blood products.
• Most importatnt source
• Sexual transmission
• Percutaneous transmission (eg – needle punctures)
• Vertical transmission from mother to child
• Least likely to cross placenta
• Highest transmission occurs if mother is infected during IIIrd trimester
• Likelihood of perinatal transmission correlates with presence of HBeAg and high-level viral replication

SEROLOGICAL & VIROLOGICAL MARKERS OF HBV:

• HBs Ag(Australian antigen)
• First virological marker detectable in the serum
• Presence precedes elevations of serum aminotransferase activity and onset of clinical illness.
• HBsAg become undetectable 1-2 months after the onset of jaundice and rarely persists beyond 6 months.
• Incubation period of HBV is 45 to 180 days
• Hbs Ag  of HBV causes glomerulonephritis­
• In chronic HBV infection, HBsAg remains detectable beyond 6 months.
• HBsAg + HBeAg suggest highly infective stage

HBc Ag

• It is not demonstrable in the circulation because it is enclosed within the HBs Ag coat.
• HBc Ag remain in the hepatocyte, where it can readily be detectable by immunohistochemical staining.

HBe Ag

• HBe Ag appears concurrently with or shortly after HBs Ag.
• HBe Ag is an indicator of active intrahepatic replication and high infectivity.
• It is a qualitative marker of HBV replication
• HBs Ag carrier mothers who are HBe Ag positive almost invariably (> 90%) transmit hepatitis B infection to their offspring
• Where as HBs Ag carrier mothers with anti HBe rarely (10 to 15%) infect their offspring.
• HBe testing is indicated primarily during follow up of chronic infection.

Anti HBcAg

• Appears within the first 1 to 2 weeks after the appearance of HBs Ag.
• In acute or recent infection IgM anti HBc is detected.
• Ig G anti HBc indicates remote infection.
• Recumbent stage of Hepatitis B is characterized by Anti HBc
• Epidemiologic study of Hepatitis B is by Anti HBc

Anti HBs Ag

• It becomes detectable in blood when HBs Ag disappears.
• Anti HBs Ag is protective antibody.
• It is the only serological marker, present after immunization.

Anti HBeAg

• Disappearance of HBe Ag is followed by the appearance of anti HBe Ag
• Its presence indicates low infectivity and virus replication.

HBV DNA

• It is the quantitative marker of virus replication.
• For the diagnosis of HBV infection, detection of HBs Ag in blood is all that ordinarily necessary.
• The simultaneous presence of Ig M anti HBc indicates recent infection, while Ig G anti HBc indicates remote infection.
• In 1-5% of patients there is a “window” or “gap” period between disappearace of HBs Ag and appearance of anti HBs Ag. During this period diagnosis is made by Ig M anti HBc Ag.