HEPADNAVIRIDAE FAMILY

Hepadnaviridae family consists of two viral genera which are Orthohepadnavirus (which contain viruses that infect humans and other mammals) and Avihepadnavirus (which contain viruses that infect birds); and these viruses are generally called hepadnaviruses. The viruses in this family include human hepatitis B virus, animal hepatitis viruses and duck hepatitis virus. Hepadnaviruses are spherical in shape, and they measure between 40-48 nm in diameter. They are sensitive to ether, heat, acid, organic solvents and detergents; and they have no envelope. However, some viruses also possess envelope. They generally replicate in the nucleus and are thus released from their host cell via lysis. The human hepatitis B virus is the main viral representative of the Hepadnaviridae family; and the virus has a worldwide distribution. Hepatitis B virus (HBV) is the only causative agent of viral hepatitis that has a dsDNA genome (Table 1).

The other causative agents of hepatitis (liver inflammation) in humans are caused by viral agents whose genome type is ssRNA. HBV is a unique DNA virus because it shares an encoding reverse transcriptase (RT) with the retroviruses (e.g. HIV); and their replication is through an RNA-DNA route because of the RT they encode. HBV has a high affinity for liver cells (i.e. hepatocytes); and they have two major proteins hepatitis B surface antigen (HBs Ag) and hepatitis B core antigen (HBc Ag) that both aid the virulence of the pathogens in humans. While HBc Ag antigens are basically expressed within hepatocytes, the HBs Ag is expressed outside the liver cells when the virus replicates in the liver. The main route of transmission of human HBV is via sexual contact, congenitally (i.e. from an infected mother to unborn child) and parenterally especially via exposure to contaminated blood.

The incubation period of HBV infection in humans is between 6 weeks to 6 months; and the disease can be acute or chronic in nature depending on the persistence of the virus in the infected host. The clinical outcome of the disease is usually characterized by liver dysfunction and jaundice. Prolonged and untreated disease can result to liver cancer and cirrhosis. Intravenous drug users, healthcare workers, people who receive tattoos and acupuncture, blood recipients, homosexuals, people who keep multiple sex partners, those who share sharp objects and sex workers are people who are at risk of infection with HBV. HBV infection can be prevented via proper vaccination of susceptible population with hepatitis B vaccine, and treatment is done using antiviral drugs. However, HBV infection is self-limiting in some individuals, and some patients can spontaneously clear the virus from their system.     

Table 1: Summary of the causative agents of liver inflammation (hepatitis) in humans

VIRUS FAMILY GENOME TRANSMISSION ROUTE
Heptitis A Picornaviridae ssRNA Feacal-oral route
Hepatitis B

 

Hepadnaviridae dsDNA Sexual contact, blood and from mother to child
Hepatitis C

 

Flaviviridae ssRNA Sexual contact, blood and from mother to child
Hepatitis D  Deltavirus ssRNA Parenteral or via blood contact
Hepatitis E  Hepeviridae ssRNA Feacal-oral route
 

HBV infection is a common nosocomial viral infection that can spread within a particular hospital environment or healthcare setting owing to the ease with which the viruses can be contracted via blood and other blood-contaminated body fluids or instruments such as syringes, scissors, and other hospital equipment. It is therefore critical that healthcare workers and laboratory personnel’s wear the correct protective coverings such as gloves and hospital/laboratory gowns when attending to patients or processing patient’s specimens. And all blood samples or blood-containing body fluids should be treated as “HIGHLY INFECTIOUS” in order to avoid contamination.

SELECTED REFERENCES

Acheson N.H (2011). Fundamentals of Molecular Virology. Second edition. John Wiley and Sons Limited, West Sussex, United Kingdom.

Ahmad K (2002). Norwalk-like virus attacks troops in Afghanistan. Lancet Infect Dis, 2:391.

Alan J. Cann (2005). Principles of Molecular Virology. 4th edition. Elsevier Academic Press,   Burlington, MA, USA.

Alba R, Bosch A and Chillon M (2005). Gutless adenovirus: last-generation adenovirus for gene therapy. Gene Ther, Suppl 12:S18-S27.

Alberts B, Bray D, Johnson A, Lewis J, Raff M, Roberts K and Walter P (1998). Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. Third edition. Garland Publishing Inc., New York.

Balows A, Hausler W, Herrmann K.L, Isenberg H.D and Shadomy H.J (1991). Manual of clinical microbiology. 5th ed. American Society of Microbiology Press, USA.

Barrett   J.T (1998).  Microbiology and Immunology Concepts.  Philadelphia,   PA: Lippincott-Raven Publishers. USA.

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