ABSTRACT
Hepatitis B virus carries a partially double-stranded DNA genome with four open reading frames encoding core, surface, polymerase, and X proteins. It belongs to the hepadnaviridae family. The replication cycle includes a reverse transcriptase step whereby the viral RNA pregenome is copied into DNA. The virus is present in the bloodstream and every bodily fluid, including within the genital tract. Transmission is via sexual or blood-to-blood contact such as in childbirth or through use of contaminated needles/syringes. Potent initial innate immune responses are important in eliminating acute viral infection, which is also associated with production of neutralizing anti-HBs and virus-specific cytotoxic CD8+ T cells. Acute infection may be asymptomatic, cause acute jaundice, or overwhelming liver failure. Most infected babies, around 10% of infected children and 5% of infected adults fail to eliminate virus and become chronically infected. Chronic infection is associated with significant long-term risks of cirrhosis and hepatocellular carcinoma. Diagnosis of HBV infection is by detection of circulating hepatitis B surface antigen (HBsAg). Acute infection is associated with IgM anti-HBc antibodies. Chronic infection is defined as persistence of HBsAg for more than 6 months. Chronically infected patients are split into those with detectable hepatitis B e antigen (highly infectious) and those with antibodies to e antigen (anti-HBe), the latter usually being much less infectious. Treatment of chronic infection is with either immunomodulatory drugs such as pegylated interferon-alpha, or with nucleos(t)ide analog inhibitors of the viral polymerase. Prevention of infection is by active immunization with HBsAg-based vaccines. These are recommended as universal vaccinations in childhood.
