Wednesday, 7 November 2012

Hepatitis Viruses



General Concepts


Viral hepatitis has emerged as a major public health problem throughout the world affecting several hundreds of millions of people. Viral hepatitis is a cause of considerable morbidity and mortality in the human population, both from acute infection and chronic sequelae which include, in the case of hepatitis B, C and D, chronic active hepatitis and cirrhosis. Hepatocellular carcinoma which is one of the ten most common cancers worldwide, is closely associated with hepatitis B, and at least in some regions of the world with hepatitis C virus.

The hepatitis viruses include a range of unrelated and often highly unusual human pathogens.

Hepatitis A virus

Hepatitis A virus (HAV), classified as hepatovirus, is a small, unenveloped symmetrical RNA virus which shares many of the characteristics of the picornavirus family, and is the cause of infectious or epidemic hepatitis transmitted by the fecal-oral route.

Hepatitis B virus

Hepatitis B virus (HBV), a member of the hepadnavirus group, double-stranded DNA viruses which replicate, unusually, by reverse transcription. Hepatitis B virus is endemic in the human population and hyperendemic in many parts of the world. A number of variants of this virus have been described. Natural hepadna virus infections also occur in other mammals including woodchucks, beechy ground squirrels and ducks.

Hepatitis C virus

Hepatitis C virus (HCV), is an enveloped single-stranded RNA virus which appears to be distantly related (possibly in its evolution) to flaviviruses, although hepatitis C is not transmitted by arthropod vectors. Several genotypes have been identified. Infection with this more recently identified virus is common in many countries. Hepatitis C virus is associated with chronic liver disease and also with primary liver cancer in some countries.

Hepatitis D virus

Hepatitis D virus (HDV) is an unusual, single-stranded, circular RNA virus with a number of similarities to certain plant viral satellites and viroids. This virus requires hepadna virus helper functions for propagation in hepatocytes, and is an important cause of acute and severe chronic liver damage in many regions of the world.

Hepatitis E virus

Hepatitis E virus (HEV), the cause of enterically-transmitted non-A, non-B hepatitis, is another non-enveloped, single-stranded RNA virus, which shares many biophysical and biochemical features with caliciviruses. The most similar genome to HEV is found in a plant virus, beet necrotic yellow vein virus, and there are similarities in the functional domains to rubella virus. Final taxonomic classification is yet to be agreed upon.
Hepatitis E virus is an important cause of large epidemics of acute hepatitis in the subcontinent of India, Central and Southeast Asia, the Middle East, parts of Africa and elsewhere. This virus is responsible for high mortality (15–20%), during pregnancy particularly during the third trimester.

The GB hepatitis viruses

The GB hepatitis viruses (GBV-A, GBV-B and GBV-C). The GB hepatitis viruses were cloned recently and preliminary genomic characterization shows that they are related to other positive-stranded RNA viruses with local regions of sequence identity with various flaviviruses. Phylogenetic analysis of genomic sequences showed that these viruses are not genotypes of the hepatitis C virus.

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