What is the Paramyxoviridae?

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.