West Nile virus
Can cause fatal neurological disease

West Nile virus disease is caused by infection after being bitten by an infected mosquito.  West Nile virus causes a wide spectrum of symptoms. This can range from no symptoms at all, in most cases, to flu like illness. Although less common, some people develop severe neurological issues, which can lead to death.

While the disease is widely prevalent and can cause large outbreaks of neuroinvasive disease, little is known about how many people are infected yearly, due to most infected being asymptomatic. The numbers of severe illness and death are equally unknown due to lack of testing and reporting.



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What causes West Nile Virus

West Nile is a viral disease, spread by several species of female mosquitoes of the Culex genus. Its primary host are birds.

West Nile virus is an RNA virus of the family Flaviviridae, genus Flavivirus, which also contains Japanese Encephalitis virus, Dengue virus, Yellow Fever virus, and Zika virus. The virus has two main distinct lineages and various variants that are region specific. Viral strains of both lineages are virulent.

Besides birds and humans, the virus can infect a wide range of other hosts such as horses, non-human primates, cats and dogs.

Photo: Culex mosquito on arm.

Where is West Nile Virus found

The virus is found throughout the majority of the world, including Africa, the Americas, Europe, Asia, Australia, the Middle East, and the Caribbean.

The viruses are not endemic to some of these regions, but they can be introduced through migratory birds. Several of the virus strains are established and maintained in Africa. The lineage 1 virus has been found in Africa, Europe, the Middle East, and certain parts of Asia. The lineage 2 virus has been found primarily in the region south of the Sahara desert in Africa.

The presence of West Nile virus in regions like the Americas and the Caribbean is likely because the virus circulates in flocks of migratory birds who move from one region to another.

Image: Geographic distribution of West Nile Virus.

What are the symptoms of West Nile Virus

80% of people infected with West Nile virus are asymptomatic. About 20% develop West Nile Fever and have mild symptoms of flu-like ilness. In 1% the disease will become neuroinvasive and cause symptoms similar to encephalitis or meningitis.

West Nile Fever is characterized by weakness, fever, headaches, nausea, diarrhea, and sometimes sore throat. Some infected patients may have a skin rash that develops on the neck. Severe illness is occasionally observed in older patients, and associated symptoms include cardiac arrhythmia, respiratory failure, and stroke.

West Nile neuroinvasive disease is more severe than West Nile Fever. There are a total of three syndromes associated with the neuroinvasive disease: encephalitis, meningitis, and acute flaccid paralysis. Symptoms of West Nile encephalitis include disorientation, ataxia, incoordination, and involuntary movements. West Nile meningitis has been described by headaches, fever, and a stiff neck. Finally, West Nile acute flaccid paralysis is characterized by weakened limbs, muscle aches, abnormalities and bowel and bladder function, and eventually, paralysis.

How can West Nile Virus be prevented

Controlling mosquito populations is the main way to prevent and reduce West Nile virus. This includes source reduction, pesticide spraying for larval control and fumigation for adult control, and/or the use of mosquito traps.

Wear long-sleeved clothing and long trousers when outdoors.

Use mosquito netting over the bed if the bedroom is not air conditioned or screened, and for additional protection, treat the mosquito netting with the insecticide permethrin.

There is no vaccine for West Nile virus.

How is West Nile Virus diagnosed

West Nile virus can be diagnosed by a number of different tests:

IgG antibody sero-conversion (or significant increase in antibody titers) in two serial specimen collected at a one week interval by enzyme-linked immunosorbent assay (ELISA). IgM antibody capture enzyme-linked immunosorbent assay (ELISA) and neutralisation assays.

However, these antibodies may be able to persist for more than a year, so they may not be able to show that an individual has been recently infected.

An active infection of West Nile virus can be detected by Polymerase chain reaction (PCR) tests, such as the ones made available by the WoIDM. The PCR tests are more accurate than antigen detection.

How is West Nile Virus treated

There are no specific antiviral drugs for West Nile virus. Treatment depends on symptoms and is supportive for patients with neuroinvasive West Nile virus, involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections.

Immunoglobulin therapy and other means of passive immunization are being tested in clinical trials.

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