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West Nile virus


West Nile virus (WNV) is a virus that is spread by mosquitoes.

Most cases of WNV are not serious and many people have no symptoms.

In less than 1 in 100 people who are infected, WNV causes serious symptoms, such as inflammation of the:

  • brain
  • spinal cord
  • tissues surrounding the brain and the spinal cord

See West Nile virus - symptoms for more information.

How is West Nile virus spread?

WNV is usually spread by a bite from an infected mosquito. Mosquitoes become infected when they feed on infected birds. They then spread the virus to humans and other animals when they bite. 

It can also be spread through an organ transplant or blood transfusion, although this is rare.

There is evidence that WNV can be spread from a mother to her baby during pregnancy or through breast milk. This is also rare.

See West Nile virus - causes for more information.

Where does West Nile virus occur?

West Nile virus (WNV) is found in: 

  • Africa, including Egypt
  • Asia 
  • Australia
  • the Middle East, including Israel
  • some parts of Europe
  • the USA, including North, Central and South America 
  • some parts of Canada

How common is West Nile virus?

There have been no cases of WNV occurring in the UK in people who have not travelled abroad. There was one UK case of WNV in 2006 and one in 2007, both in people who had visited Canada.

In 2010, there were 981 cases of WNV in North America. 


In around four out of five people, WNV will cause no symptoms. Other people may have mild flu-like symptoms that last a few days, such as a headache and high temperature.

If WNV develops into a severe condition, it can be fatal. Estimates of fatality rates are around 3-15 people in every 100 with a severe case of WNV. It is more likely to be fatal in elderly people.

Flaviviridae viruses

WNV is part of the flaviviridae family of viruses, which also includes:

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In around four out of five people, West Nile virus (WNV) causes no symptoms.

In others, the virus causes mild flu-like symptoms, which may include: 

  • high temperature of 38C (100.4F) or over 
  • headache
  • backache
  • muscle aches 
  • sore throat 
  • nausea 
  • vomiting 
  • diarrhoea 
  • swollen lymph glands in your neck - lymph glands are small oval tissues that are part of your immune system 
  • a rash on your stomach, back and chest

These symptoms usually appear 3 to 15 days after you have been bitten and can last for a few days or for several weeks.

Severe symptoms

In less than 1 in 100 infected people, WNV can cause a more serious infection of the brain and nervous system, including:

  • encephalitis - inflammation of the brain
  • meningitis - inflammation of the membranes surrounding the brain and spinal cord 
  • paralysis - loss of ability to move one or more muscles of the body

Symptoms include:

  • a high temperature
  • headache
  • stiff neck
  • sore eyes
  • disorientation
  • shaking
  • seizures or fits
  • stupor or coma 
  • muscle weakness

People over 50 years of age are more at risk of developing serious symptoms of WNV if they become infected.

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West Nile virus (WNV) is caught from the bite of a mosquito that is carrying the infection. The mosquitoes become infected when they bite infected birds.

The mosquitoes that carry WNV are most active at dusk and dawn, but may also bite during the day. See West Nile virus - prevention for information about preventing mosquito bites.

WNV can also be spread through:

  • blood transfusions 
  • organ transplants 
  • breastfeeding 
  • pregnancy

However, only a very small number of cases have been caused in these ways.

WNV cannot be spread from person to person through close contact, for example from kissing someone who is infected.

Where West Nile virus occurs

WNV is found in: 

  • Africa, including Egypt
  • Asia 
  • Australia
  • the Middle East, including Israel

Outbreaks of WNV have also occurred across Europe, for example in:

  • Albania
  • Austria
  • Czech Republic
  • France 
  • Greece 
  • Hungary 
  • Italy
  • Poland 
  • Portugal
  • Romania
  • Slovakia
  • Spain

In 1999, the virus appeared in New York. It has since spread rapidly throughout North America and more recently to:

  • Canada
  • Central and South America, including Mexico
  • the West Indies

The increase in WNV cases in mainland Europe and the US has led to fears that it may soon arrive in the UK. So far, there have been no reported cases of WNV affecting someone in the UK who had not travelled abroad.

Seasonal changes

In tropical countries, you can catch WNV all year round. In countries in the temperate zones, with changing summer and winter seasons, such as North America, WNV usually occurs in summer and autumn.

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If you have symptoms of West Nile virus (WNV) while you are travelling, contact a doctor, local hospital or health worker as soon as possible, especially if your symptoms are severe. If you are travelling in a remote area, move to a more civilised area as quickly as possible, where you are more likely to find a doctor and medical care.

Medical history and tests

If a doctor suspects WNV, they will ask you about your medical history. For example, they may ask: 

  • where you have been travelling
  • if you have been bitten by mosquitoes 
  • what symptoms you have experienced 

Blood tests

A sample of your blood may be tested for antibodies. Antibodies are special proteins produced by the immune system (the body’s natural defence system) to fight infections. If antibodies are found in your blood, you may have further tests to confirm the results.

Lumbar puncture

If your symptoms are severe, you may need a lumbar puncture (also known as a spinal tap). This is a procedure to take a sample of cerebrospinal fluid (CSF), which surrounds your brain and spinal cord. The CSF is checked for antibodies and signs of infection, and can be used to diagnose meningitis (inflammation of the membranes surrounding the brain and spinal cord).

Imaging scans

In some cases, imaging scans may be taken of your head to rule out other conditions, such as meningitis or encephalitis (inflammation of the brain) caused by the herpes simplex virus. Possible imaging scans include: 

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There is no specific treatment for West Nile virus (WNV).

If you have mild WNV, your symptoms will usually get better without treatment after several days or weeks.

If your symptoms are more severe, you will need to go to hospital. You may be admitted and given supportive treatments such as:

  • intravenous fluids, given through a drip in your arm
  • help with breathing 
  • nursing care

Success of treatment

If your symptoms are mild, you can expect a complete recovery with no long-term side effects.

If your symptoms develop into encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes surrounding the brain and spinal cord), the outlook is less good. These conditions can lead to brain damage and, in some cases, may be fatal.

See the topics about Encephalitis - treatment and Meningitis - treatment for information about treating these conditions and the possible complications.

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If you are travelling to an area where West Nile virus (WNV) is present, the following advice can help reduce your risk of being infected. 

Reducing your risk before you go

Be aware of the potential risks, especially if you are over 50 years of age, and take precautions to protect yourself against mosquito bites.

There is no vaccine for WNV, although scientists are working on developing one.

Travel insurance

Take out adequate travel insurance for the countries you are visiting. Check that your insurance policy gives information about what to do if you become seriously ill, and that it covers you for repatriation on medical grounds (returning you to the UK if you become unwell).

If you are travelling in Europe, make sure you have a European Health Insurance Card (EHIC) as well as travel insurance. An EHIC is free, and it could save you unnecessary costs and difficulties if you need medical help while you are away in Europe. You can apply online for your EHIC here.

Protecting yourself while you are away

There is no effective treatment for WNV, so you will need to protect yourself from biting mosquitoes at all times. If the virus is reported to be in or near the area you are travelling in, be especially careful and pay attention to local news regarding movement of the virus.

  • The mosquitoes that carry WNV are most active around dusk and dawn, but some also bite during the day. You will need to protect yourself if you are outside at these times. 
  • Wear loose-fitting, long-sleeved tops and long trousers to cover up your skin as much as possible. 
  • Use an insect repellent on exposed skin, taking care to avoid your eyes. DEET-based insect repellents are thought to be the most effective. Always follow the manufacturer’s instructions.
  • Mosquitoes can bite through thin clothing, so spraying clothes with insect repellent can help. 
  • Insect-proof screens on windows and doors and mosquito nets over the bed will reduce mosquito bites inside. Air conditioning and spraying insecticide in the room may also help. 
  • If necessary, mosquito coils can be burned in enclosed areas to repel mosquitoes. 
  • Mosquitoes breed in stagnant water. If you notice pools of standing water in waste bins or flower pots, drain them as soon as possible. 
  • Avoid areas where there are likely to be a large amount of mosquitoes, such as near water.
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