TOGAVIRIDAE FAMILY: Chikungunya virus, Rubella virus

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Togaviridae family comprises viruses that are usually transmitted through arthropods such as insects. Viruses of the Togaviridae family have a wide range of natural hosts including vertebrate and non-vertebrate hosts. Arthropods such as mosquitoes serve as the natural host of viruses in the Togaviridae family. Other natural hosts of viruses in the Togaviridae family include birds, humans, nonhuman primates, equids, amphibians, reptiles, rodents, pigs, sea mammals, salmonids and other arthropods. But some viruses in this family lack arthropod transmission. There are basically two genera of viruses in the Togaviridae family; and they are Alphavirus and Rubivirus.

The viruses in the genera Alphavirus are basically transmitted to humans through arthropods especially blood-sucking mosquitoes while those in the genera Rubivirus lack arthropod transmission. The Alphavirus genus includes a large number of species, while the Rubivirus genus includes the single species Rubella virus – which is transmitted through the respiratory routes in human population. Chikungunya virus (found in Asia and some parts of South America and Africa), Mayaro virus (found in South America), Ross River virus (found in Australia) and Sindbis virus (found in Australia, Asia and Africa) are some examples of viruses that make up the genera Alphavirus. Rubella virus is the only viral species in the genera Rubivirus. Pestiviruses and Flaviviruses are other members of the Togaviridae family.

Rubella virus which causes German measles in children is one of the most important members of the Togaviridae family, and this is because of the disease they cause in human population. Rubella virus is found in the genera Rubivirus. Viruses in the Togaviridae family have an icosahedral nucleocapsid and they are generally enveloped viruses. The major transmission routes for viruses in the Togaviridae family are zoonosis, mosquito bite, and respiratory routes. The viruses in this family exit their host cell through the plasma or cytoplasmic membrane by budding process; and they replicate within the cytoplasmic space of their host cell.

They have a 10–12 kb of positive-sense, non-segmented RNA [ss(+)RNA] genome and their replication is in the cytoplasm of the infected host cell. Viruses in the Togaviridae family measure between 50-70 nm in diameter. The viruses in the Togaviridae family have a worldwide distribution. Chikungunya virus (an Alphavirus) and rubella virus (a Rubivirus) are the two viruses in the Togaviridae family that causes infection in humans. It is noteworthy that viruses in the genera Alphavirus are usually geographically limited to some regions of the world. German measles or rubella is a benign form of measles in children and young adults, and the disease is usually characterized by the appearance of maculopapular rash on the skin.

Rubella used to be a worldwide epidemic but since the advent of vaccination, the disease has been significantly contained in countries where it is formerly endemic. Pregnant women especially those in their first trimester of pregnancy who are infected with rubella virus can induce serious congenital defects due to rubella virus infection in their unborn child in utero. Rubella is a highly infectious disease and its transmission route is through the respiratory tract of humans (in respiratory droplets). Arthralgia (joint pains), arthritis, fever, and maculopapular rash are some of the clinical symptoms of rubella (German measles). There is no active treatment for the disease since most cases are self-limiting in affected individuals. Active rubella immunization/vaccination still remains the basis for the effective control of rubella infection in human population. Vaccines are also available for the prevention of infection caused by Alphaviruses.             

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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