Murray Valley encephalitis virus

Murray Valley encephalitis virus

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Murray Valley encephalitis virus (MVEV) is a zoonotic flavivirus
Flavivirus
Flavivirus is a genus of the family Flaviviridae. This genus includes the West Nile virus, dengue virus, tick-borne encephalitis virus, yellow fever virus, and several other viruses which may cause encephalitis....

 endemic to northern Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 and Papua New Guinea
Papua New Guinea
Papua New Guinea , officially the Independent State of Papua New Guinea, is a country in Oceania, occupying the eastern half of the island of New Guinea and numerous offshore islands...

. It is the causal agent of Murray Valley encephalitis (previously known as Australian encephalitis) and in humans can cause permanent neurological disease or death. MVEV is related to Kunjin virus
Kunjin virus
Kunjin virus is a zoonotic virus of the family Flaviviridae and the genus Flavivirus. It is a subtype of West Nile Virus endemic to Oceania.-History:...

 which has a similar ecology but has a lower morbidity rate.

Vector


MVEV is a mosquito-borne virus
Arbovirus
Arbovirus is a term used to refer to a group of viruses that are transmitted by arthropod vectors. The word arbovirus is an acronym . Some arboviruses are able to cause emergent disease.-Transmission:...

 that is maintained in a bird-mosquito-bird cycle. Water birds from the Ciconiiformes
Ciconiiformes
Traditionally, the order Ciconiiformes has included a variety of large, long-legged wading birds with large bills: storks, herons, egrets, ibises, spoonbills, and several others. Ciconiiformes are known from the Late Eocene...

 order, including heron
Heron
The herons are long-legged freshwater and coastal birds in the family Ardeidae. There are 64 recognised species in this family. Some are called "egrets" or "bitterns" instead of "heron"....

s and cormorant
Cormorant
The bird family Phalacrocoracidae is represented by some 40 species of cormorants and shags. Several different classifications of the family have been proposed recently, and the number of genera is disputed.- Names :...

s, provide the natural reservoir for MVEV. The major mosquito vector is Culex annulirostris. Human infection occurs only through bites from infected mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...

es; the virus cannot be transmitted from person to person.

History


The first epidemics of MVE occurred in 1917 and 1918 in Southeastern Australia following years of high rainfall. The virus was isolated from human samples in 1951 during an epidemic in the Murray Valley
Murray River
The Murray River is Australia's longest river. At in length, the Murray rises in the Australian Alps, draining the western side of Australia's highest mountains and, for most of its length, meanders across Australia's inland plains, forming the border between New South Wales and Victoria as it...

, Australia.

Epidemics usually occur due to either infected birds or mosquitoes migrating from endemic areas to non-endemic areas.

Presentation


The majority of MVEV infections are sub-clinical, i.e. do not produce disease symptoms, although some people may experience a mild form of the disease with symptoms such as fever, headaches, nausea and vomiting and only a very small number of these cases go on to develop MVE. In fact, serological surveys which measure the level of anti-MVEV antibodies within the population estimate that only 1 in 800-1000 of all infections result in clinical disease. The incubation period following exposure to the virus is around 1 to 4 weeks. Following infection, a person will have life-long immunity to the virus. When a patient appears to show MVE symptoms and has been in an MVE-endemic area during the wet season, when outbreaks usually occur, MVE infection must be confirmed by laboratory diagnosis, usually by detection of a significant rise of MVE-specific antibodies in the patient's serum
Blood serum
In blood, the serum is the component that is neither a blood cell nor a clotting factor; it is the blood plasma with the fibrinogens removed...

.
Of those who contract MVE, one-quarter die from the disease.

Clone


The scientific study of the genetics of MVEV has been facilitated by the construction and manipulation of an infectious cDNA clone of the virus.
Mutations in the envelope gene have been linked to the attenuation of disease in mouse models of infection.