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Hepatitis B

Originally known as serum hepatitis, hepatitis B has only been recognized as such since World War II, and has caused current epidemics in parts of Asia and Africa. Hepatitis B is recognized as endemic in China and various other parts of Asia. Over one-third of the world's population has been or is actively infected by hepatitis B virus (acronym HBV). more...

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Molecular biology

The hepatitis B virus is a member of the Hepadnavirus family. It consists of a proteinaceous core particle containing the viral genome in the form of double stranded DNA and an outer lipid-based envelope with embedded proteins. The envelope proteins are involved in viral binding and release into susceptible cells. The inner capsid relocates the DNA genome to the cell's nucleus where viral mRNAs are transcribed. Three subgenomic transcripts encoding the envelope proteins are made, along with a poorly understood transcript encoding the X protein, whose function is still under debate. A fourth pre-genomic RNA is transcribed, which is exported to the cytosol and translates the viral polymerase and core proteins. Polymerase and pre-genomic RNA are encapsidated in assembling core particles, where reverse transcription of the pre-genomic RNA to genomic DNA occurs by the polymerase protein. The mature core particle then exits the cell via normal secretory pathways, acquiring an envelope along the way.

Hepatitis B is one of a few known non-retroviral viruses which employ reverse transcription as part of its replication process. Other viruses which use reverse transcription include HTLV or HIV, the virus that causes AIDS, but HIV and hepatitis B are not related. Hepatitis B's genome is DNA, and reverse transcription is one of the latter steps in making new viral particles, whereas HIV has an RNA genome and reverse transcription is one of the first steps in replication.

Transmission

Hepatitis B is largely transmitted through exposure to bodily fluids containing the virus. This includes unprotected sexual contact, blood transfusions, re-use of contaminated needles and syringes, vertical transmission from mother to child during childbirth, and so on. The primary method of transmission depends on the prevalence of the disease in a given area. In low prevalence areas, such as the continental United States, IV drug abuse and unprotected sex are the primary method. In moderate prevalence areas, the disease is predominantly spread among children. In high prevalence countries, such as China, vertical transmission is most common. Without intervention, a mother who is positive for the hepatitis B surface antigen confers a 20% risk of passing the infection to her offspring at the time of birth. This risk is as high as 90% if the mother is also positive for the hepatitis B e antigen.

Roughly 16-40% of unimmunized sexual partners of individuals with hepatitis B will be infected through sexual contact. The risk of transmission is closely related to the rate of viral replication in the infected individual at the time of exposure.

Clinical consequences and complications

Hepatitis B virus infection may either be acute (self-limited) or chronic (long-standing). Persons with self-limited infection clear the infection spontaneously within weeks to months.

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Hepatitis B link to cancer is clarified
From Science News, 3/19/05

It's well established that hepatitis B virus can cause liver cancer. Scientists in Taiwan now report that a certain kind of hepatitis B is much more likely than others to lead to cancer and that large amounts of any type of the virus correlate with high cancer risk.

The researchers obtained blood samples from 4,841 men diagnosed with hepatitis B but not yet treated for the condition. Over the next 14 years, 154 of the patients developed liver cancer. The researchers then compared blood from these men with that from 316 similar men in the study who hadn't developed cancer. The comparison indicated that the men with cancer were five times as likely to have a kind of hepatitis B called genotype C.

There are seven genotypes of hepatitis B, which are forms of the virus whose DNA differ from each other, explains study coauthor Ming-Whei Yu, an epidemiologist at National Taiwan University in Taipei.

The blood samples revealed that high amounts of any hepatitis genotype increase the risk of liver cancer. In particular, men whose blood carried a high load of genotype C of hepatitis B virus faced a 27-fold higher risk of developing cancer, compared with men with a low load of one of the virus' other genotypes, Yu and her colleagues report in the Feb. 16 Journal of the National Cancer Institute.

Screening hepatitis B patients for their virus genotype could reveal people at especially high risk of cancer, Yu says. Some medications can suppress the virus.

COPYRIGHT 2005 Science Service, Inc.
COPYRIGHT 2005 Gale Group

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