POXVIRIDAE FAMILY OF VIRUSES (POXVIRUSES)

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Poxviridae family is a distinct family of viral genera that contain viruses that replicate in the cytoplasm of their infected host cells (inclusive of vertebrate and invertebrate cells). Examples of viruses in the Poxviridae family of viruses include: cowpox, smallpox or variola virus, vaccinia virus and monkey pox viruses. The Poxviridae family contains viruses that infect humans, other vertebrates, invertebrates, birds and insects. Viruses in this family especially the poxviruses are the largest in size of all known viruses; and they generally have a double-stranded (dsDNA) genome. Entomopoxvirinae (which infects invertebrates) and Chordopoxvirinae (which infect vertebrates) are the two subfamilies of the Poxviridae family of viruses. The Chordopoxvirinae subfamily contain eight genera of viruses (inclusive of poxviruses which causes infection in humans) while the Entomopoxvirinae subfamily contain only three (3) genera of viruses that parasitize invertebrates.

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The eight genera that make up the Chordopoxvirinae subfamily of Poxviridae family include: Orthopoxvirus (that comprises the human poxviruses, vaccinia, variola, and monkey pox viruses), Parapoxvirus (that parasitize humans and animals), Avipoxvirus (that infect birds), Yatapoxvirus (that infects primates), Capripoxvirus (that infect sheep and cattle), Molluscipoxvirus (that parasitize humans), Suipoxvirus (that infect pigs) and Leporipoxvirus (that infect rabbits). Human poxviruses are unique and different from other DNA-containing viruses because they replicate in the cytoplasm of their infected host cell instead of in the nucleus (as is applicable with most DNA-containing viruses). Poxviruses have a brick-shaped structure; and their size is about 20-450 nm long, and 140-260 nm wide, and 120-240 nm thick. As aforesaid, viruses in the Poxviridae family are the largest in size of all known viruses because of their distinct sizes which is about 20-450 nm long, and 140-260 nm wide, and 120-240 nm thick.

Poxviruses have an enveloped genome and they are released from their infected host cell by budding process, an asexual type of reproduction. The budding of poxviruses from their infected host cell allows them to acquire envelope during the budding process. Infections caused by poxviruses are highly contagious and can be spread by body contact with infected individuals in a human population. However, treatment and vaccination exist for the diseases caused by poxviruses. Though some of the diseases caused by the human poxviruses (for example, measles and smallpox) have been eradicated through massive immunization/vaccination around the world, the re-emergence of these diseases is still possible if proper immunization practices are overlooked and relegated or abandoned.

For example, smallpox (caused by variola virus) was eradicated in 1977; and there has not been any reported case of the disease since then. But the 2018 reported cases of measles uncovered in some parts of Europe is a wake-up call to humanity – that even previously eradicated diseases of man can still reemerge if immunization/vaccination is skipped or under-performed even in developed countries or elsewhere. In terms of measles infection, infected patients become infectious once the maculopapular rash starts appearing, and the incubation period of the disease is usually between 10-12 days after exposure. Sudden onset of fever, back pain and headache are some of the early signs of the disease. Human to human infection or transmission of smallpox virus occurs through the respiratory route after contact with an infected person. Proper immunization/vaccination of the general population (in order to achieve herd immunity) is usually the major way of controlling and preventing infections caused by viruses such as the measles virus and smallpox virus in the Poxviridae family.    

Further reading

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.

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