The hepadnaviurses include three viruses of mammals and two viruses of birds. The mammalian viruses are closely related. For instance, Hepatitis B and Woodchuck Hepatitis Virus are 60% genetically identical. All have a very narrow host range.

Members of the Hepadna family include:

Hepadnaviruses are characterized by:

Envelope-- spherical, 42nm in diameter, contains cellular lipids, glycoproteins, and a virus-specific surface antigens such as HBsAg.

Icosahedral Capsid-- 27nm core. The core contains contains an antigen (HBcAg), DNA polymerase, and a small, circular, partially double stranded DNA. Capsid is closely surrounded by the viral envelope. T=3 triangulation number.

Viral genome-- Circular, 3.2kb, partially double-stranded DNA containing four overlapping open reading frames. It has the smallest genome of DNA viruses. One DNA strand, the minus strand has a protein covalently attached to the 5' end. The plus strand is of variable length and is shorter than the negative strand, and has an RNA oligonucleotide at its 5' end.

The rest of discussion will be based on Hepatitis B Virus (HBV) as more is known about this virus than other viruses in the Hepadnavirus family.

What is Hepatitis B?

Hepatitis B virus (HBV) is a virus that is transmitted sexually, in contaminated blood, and from mother to child. HBV can cause serious health problems, although many treatment options do exist. Hepatitis B infection is the 10th leading cause of death in the world, affecting approximately 2 billion people across the globe. The most serious complication of HBV infection is liver cancer (hepatocellular carcinoma), which alone causes more than 300,000 deaths per year. The risk of developing liver cancer after being infected with HBV is low, occurring in 1-4% of patients with HBV infections. Other serious complications that can arise from HBV infection are cirrhosis or scarring of the liver and a rare immunological disorder called immune complex disease. Cirrhosis (scarring of the liver) is also a serious health problem associated with HBV.

Pathogenesis
Hepatitis B virus is dangerous because it attacks the liver, thus inhibiting the functions of this vital organ. The virus causes persistent infection, chronic hepatitis, liver cirrhosis, hepatocellular carcinoma, and immune complex disease.

HBV infection in itself does not lead to the death of infected hepatocytes. HBV in a non-cytolytic infection. Liver damage however, arises from cytolytic effects of the immune system's cytotoxic T lymphocytes (CTL) which attempt to clear infection by killing infected cells. The strength of the CTL response has been noted to determine the course of the infection. For example, a vigorous CTL response results in clearance and recovery, although often with an episode of jaundice. A weak response results in few symptoms and chronic infection (and hence higher susceptibility for hepatocellular carcinoma).

The younger a person is when she becomes infected with HBV, the more likely she is to be asymptomatic and become a chronic carrier of the disease. Babies born to infected mothers are at very high risk of to becoming carriers and developing liver pathology.

Vertical is thus one common way that HBV is transmitted, along with transmission through sexual intercourse and mixing of blood products. Vertical transmission can be prevented by administering vaccine the same day of birth. Different modes of transmission are more prevalent in certain areas of the world and among certain high-risk groups, yet all areas of the world see HBV transmission through all of these avenues. About 90% of adults who acquire HBV recover from it completely and become immune to the virus. The other 10% of cases are the people who become chronic carriers.

Epidemiology
Groups at high risk of contracting Hepatitis B: