hepadnavirus, any virus belonging to the family Hepadnaviridae. Hepadnaviruses have small, enveloped, spherical virions (virus particles) that are about 40–48 nm (1 nm = 10−9 metre) in diameter. The capsid (the protein shell surrounding the viral nucleic acids) contains a circular double-stranded DNA molecule with a single-stranded DNA region and a DNA-dependent DNA polymerase. The polymerase enzyme functions to repair the gap in the double-stranded DNA molecule that is created by the presence of the segment of single-stranded DNA. The activity of the polymerase is essential for the virus’s replication. Hepadnaviruses are further distinguished by the use of reverse transcriptase for replication and by an abundance of the soluble protein HBsAg (hepatitis B surface antigen).
There are two recognized genera of hepadnavirus: Orthohepadnavirus and Avihepadnavirus. The former includes hepatitis B viruses that have been isolated from mammals, including humans, woodchucks, ground squirrels, Arctic squirrels, and woolly monkeys. The second genus, Avihepadnavirus, consists of hepatitis B viruses that infect birds, including ducks, herons, cranes, and storks. There are also several other hepadnaviruses that infect Ross geese and snow geese, though these are less well characterized.
Humans and other animals that become infected with hepatitis B virus may develop a severe and long-lasting form of liver disease known as hepatitis. In humans the condition may occur as an acute disease, or in about 5 to 10 percent of cases it may become chronic and lead to permanent liver damage. Symptoms usually appear from 40 days to 6 months after exposure to the virus. Those persons at greatest risk for contracting hepatitis B include intravenous drug users, sexual partners of individuals with the disease, health care workers who are not adequately immunized, and recipients of organ transplants or blood transfusions. A safe and effective vaccine against the virus is available and provides protection for at least five years. Passive immunization with hepatitis B immune globulin (antibody) can also provide protection. Approximately 1 in 10 patients infected with hepatitis B virus becomes a virus carrier and may transmit it to others. Those who carry hepatitis B virus are also 100 times more likely to develop liver cancer than persons who do not carry the virus in their blood.