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). These viruses are generally called Hepadnaviruses. The natural hosts of Hepadnaviruses include birds, humans and apes. The representative viruses in Hepadnaviridae 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. Hepadnaviruses have no envelope. However, some viruses also possess envelope. They generally replicate in the nucleus and are thus released from their host cell through cell 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 (for example, HIV).
CLICK HERE TO BUY VIROLOGY TEXTBOOK
Table 1. Summary of the causative agents of liver inflammation (hepatitis) in humans
|Hepatitis A||Picornaviridae||ssRNA||Faecal -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 through blood contact|
|Hepatitis E||Hepeviridae||ssRNA||Faecal -oral route|
The replication of HBV 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 which are: hepatitis B surface antigen (HBs Ag) and hepatitis B core antigen (HBc Ag). Both HBs Ag and HBc Ag aid the virulence of HBV 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 through sexual contact, congenitally (i.e., from an infected mother to the unborn child) and parenterally especially through 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. Anyone can be infected by HBV, but the people who are at risk of infection with HBV include 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. HBV infection can be prevented through proper vaccination of susceptible population with hepatitis B vaccine. The treatment of HBV infection in human population is done using antiviral drugs; and supportive treatment is usually included. However, HBV infection is self-limiting in some individuals, and some patients can spontaneously clear the virus from their system.
Nonetheless, proper laboratory diagnosis, treatment and immunization are critical to preventing the spread of HBV infection in a defined human population. 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 virus can be contracted through 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 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 with HBV and other pathogenic viruses.
Acheson N.H (2011). Fundamentals of Molecular Virology. Second edition. John Wiley and Sons Limited, West Sussex, United Kingdom.
Brian W.J Mahy (2001). A Dictionary of Virology. Third edition. Academic Press, California, USA.
Cann A.J (2011). Principles of Molecular Virology. Fifth edition. Academic Press, San Diego, United States.
Carter J and Saunders V (2013). Virology: Principles and Applications. Second edition. Wiley-Blackwell, New Jersey, United States.
Dimmock N (2015). Introduction to Modern Virology. Seventh edition. Wiley-Blackwell, New Jersey, United States.
Kudesia G and Wreghitt T (2009). Clinical and Diagnostic Virology. Cambridge University Press, New York, USA.
Marty A.M, Jahrling P.B and Geisbert T.W (2006). Viral hemorrhagic fevers. Clin Lab Med, 26(2):345–386.
Strauss J.H and Straus E.G (2008). Viruses and Human Diseases. 2nd edition. Elsevier Academic Press Publications, Oxford, UK.
Zuckerman A.J, Banatvala J.E, Schoub B.D, Grifiths P.D and Mortimer P (2009). Principles and Practice of Clinical Virology. Sixth edition. John Wiley and Sons Ltd Publication, UK.