What is the Paramyxoviridae?

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The Paramyxoviridae is a virus family containing single-stranded, negative-sense RNA (ribonucleic acid),with a helically symmetrical nucleocapsid. The viruses cause a variety of highly contagious and virulent diseases in mammals and birds. However, several previously unknown paramyxoviruses have emerged tocause fatal disease in humans and nonhuman animals.
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Definition

The Paramyxoviridae is a virus family containing single-stranded, negative-sense RNA (ribonucleic acid), with a helically symmetrical nucleocapsid. The viruses cause a variety of highly contagious and virulent diseases in mammals and birds. However, several previously unknown paramyxoviruses have emerged to cause fatal disease in humans and nonhuman animals.

Natural Habitat and Features

Viral particles consist of a single helical strand of RNA, contained within a lipoprotein envelope, a nucleocapsid, and a matrix protein. Typically, paramyxoviral particles are spherical to pleomorphic, ranging from 150 to 200 nanometers (nm) in diameter and 1,000 to 10,000 nm in length. The nucleocapsid is between 600 and 1000 nm, depending on the genus. The genome of paramyxoviridae is made up of an RNA molecule between 15,200 and 15,900 nucleotides long, comprising six genes for six proteins.

Viral particles enter the host cell by binding with the cell using a binding hemagglutinin (H) protein. They fuse the viral envelope with the host cell membrane, facilitated by a fusion (F) protein. Viral particles enter the cytoplasm, where negative-sense RNA genes are turned into messenger RNA, then to nucleocapsid proteins. A positive-sense RNA template is then used to produce more viral RNA.

Paramyxoviruses often produce an excess of nucleocapsids that form large inclusion bodies in the host cells. Individual viruses escape the cell by pushing through the cell’s membranes to form envelopes from the host cell membrane. Paramyxovirus F proteins fuse host cell membranes, which can cause multiple host cells to fuse and form a large, multinucleated syncytium.

Paramyxoviruses do not remain viable for long in the environment, depending instead upon being spread by direct contact between carriers and susceptible persons. Contact is usually through respiratory droplets, and the viruses attack the respiratory system initially and preferentially. As direct contact between susceptible persons is necessary for the survival of paramyxoviruses in populations, diseases caused by these viruses tend to proliferate in areas that are densely crowded, such as in cities.

Pathogenicity and Clinical Significance

Diseases caused by paramyxoviruses include what were once called the childhood diseases: measles and mumps. In addition, common respiratory illnesses such as respiratory syncytial virus and parainfluenzavirus, the cause of childhood croup, are caused by paramyxoviruses. Paramyxoviruses also lead to devastating animal illnesses, such as canine distemper, rinderpest and Newcastle disease; new viruses have emerged to cause fatal infections in humans in Australia, India, Bangladesh, and Malaysia, and to cause massive die-offs of seals and porpoises.

Measles is perhaps the best known of the paramyxoviruses. Measles is considered one of the most highly contagious diseases known, causing disease in more than 90 percent of exposed persons. Measles is usually self-limiting, causing an initial fever, respiratory illness, and a generalized rash, all resolving within fourteen days. Still, as many as 1 in 20 children develops pneumonia, and 1 child in 1,000 contracts encephalitis, which can lead to permanent deafness, retardation, or death.

Vaccination has made measles rare in developed nations, but malnutrition, particularly vitamin A deficiency, keeps measles a serious threat in developing nations. An estimated 10 million cases of measles occur each year, with 197,000 deaths, mostly among children younger than age five years.

Respiratory syncytial virus is the most common cause of lower respiratory tract disease worldwide, and estimates show that all children will have had RSV infection by their second birthday. Parainfluenza viruses are included among viruses that cause the common cold, and they are second only to RSV in number of infections. Like RSV, parainfluenza viruses can also cause more serious infections in the lower respiratory tract (lungs) and can cause croup in children and in the elderly.

Mumps is a virus that causes a localized swelling of the parotid salivary glands. In some cases, the virus will cause testicular swelling that can lead to infertility, particularly in young men. Two new viruses have emerged in the early twenty-first century to form a new class of paramyxovirus. Hendra virus and nipah virus originated in bats and jumped to domestic animal species and then to humans, causing encephalitis and a high rate of fatalities in affected persons.

Drug Susceptibility

No antiviral therapy exists that is effective against the paramyxoviruses. Treatment for any paramyxovirus disease is supportive. Vaccination has been shown to be an effective and long-lasting preventive measure for most paramyxoviral diseases.

Bibliography

Lamb, Robert A., and Griffith D. Parks. “Paramyxoviridae: The Viruses and Their Replication.” In Fields’ Virology, edited by David M. Knipe and Peter M. Howley. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2007.

Schaffer, Kirsten, Alberto M. LaRosa, and Estella Whimbey. “Respiratory Viruses.” In Cohen and Powderly Infectious Diseases, edited by Jonathan Cohen, Steven M. Opal, and William G. Powderly. 3d ed. Philadelphia: Mosby/Elsevier, 2010.

Strauss, James, and Ellen Strauss. Viruses and Human Disease. Burlington, Mass.: Elsevier, 2008.

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