West Nile virus

You can get West Nile virus (WNV) if you are bitten by a mosquito carrying the infection.

Information written and reviewed by Certified Doctors.

Contents

Causes

You can get West Nile virus (WNV) if you are bitten by a mosquito carrying the infection. The mosquitoes become infected when they bite infected birds.

Mosquitoes that carry WNV are most active at dusk and dawn, although they may also bite during the day. Read more about preventing mosquito bites.

Rarely, WNV can also be spread through:

  • blood transfusions
  • organ transplants
  • breastfeeding
  • pregnancy

However, only a small number of cases have occurred in these ways.

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

At risk areas

WNV is found in:

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

Outbreaks of WNV have also occurred in the following European countries:

  • 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

Seasonal changes

In tropical countries, it is possible to catch WNV all year round. In countries with changing summer and winter seasons, such as North America, WNV is usually more common during summer and autumn.

Introduction

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

It is part of the flaviviridae family of viruses which also includes:

Most cases of WNV are not serious and many people have no symptoms or mild flu-like symptoms, such as muscle aches and a fever.

Serious symptoms occur in less than 1 in 100 people infected with the virus and include inflammation of the:

  • brain (encephalitis)
  • spinal cord
  • tissues surrounding the brain and spinal cord (meningitis)

Read more about the general and severe symptoms of West Nile virus.

How is West Nile virus spread?

WNV is usually spread through bites of infected mosquitoes. Mosquitoes become infected when they feed on infected birds. They can then spread the virus to humans and other animals they bite.

In rare cases, WNV can be spread through an organ transplant or blood transfusion. The virus can also be passed from a mother to her baby during pregnancy or through breast milk, although this is also rare.

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

Read more about the causes of West Nile virus.

Treating West Nile virus

There is no specific treatment for WNV. If your symptoms are mild, they will usually get better without treatment after a few 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 and nursing care.

WNV can be fatal if it becomes severe. It is estimated that around 3-15 people in every 100 die. It is more likely to be fatal in elderly people.

Read more about treating West Nile virus and how West Nile virus is diagnosed.

Preventing West Nile virus

As there is no specific treatment for WNV, it's best to protect yourself from being bitten by mosquitoes when visiting affected areas. The mosquitoes that carry WNV are most active around dusk and dawn.

To avoid being bitten by mosquitoes you should:

  • wear loose-fitting, long-sleeved tops and long trousers to cover 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 most effective
  • be aware mosquitoes can bite through thin clothing, so spraying clothes with insect repellent can help
  • use insect-proof screens on windows and doors and mosquito nets over the bed; air conditioning, mosquito plugs and spraying insecticide in the room may also help
  • avoid areas where there are likely to be large numbers of mosquitoes, such as near water

Read more about preventing WNV.

Treatment

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

Success of treatment

If you have mild symptoms of WNV, such as a high temperature, a headache and aching muscles, 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 how encephalitis is treated and how meningitis is treated for more information about treating these conditions and possible complications.

Symptoms

In around 4 out of 5 people, West Nile virus (WNV) does not cause any symptoms.

In some people, 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 nodes in your neck - lymph nodes are small oval glands that form part of your immune system
  • a rash on your stomach, back and chest

These symptoms usually appear 3-15 days after you have been bitten by an infected mosquito, and can last for several days or weeks.

Severe symptoms

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

  • encephalitis - inflammation of the brain
  • meningitis - inflammation of the membranes that surround the brain and spinal cord
  • paralysis - loss of the 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 have an increased risk of developing serious symptoms of WNV if they become infected.

Diagnosis

If you have symptoms of West Nile virus (WNV) while travelling, contact a doctor, local hospital or health worker as soon as possible, particularly if your symptoms are severe.

If you are travelling in a remote area, move to a more built up area as soon 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 about your medical history. For example, they may ask:

  • where you have been travelling
  • whether 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) 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 fluid will be checked for antibodies and signs of infection, and can also be used to diagnose meningitis (inflammation of the membranes that surround the brain and spinal cord).

Imaging scans

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

Prevention

If you are travelling to areas where West Nile virus (WNV) is present, you should be aware of the risks and take appropriate precautions.

Before you go

Make sure you are aware of potential risks, particularly if you are over 50 years of age, and take steps to protect yourself against mosquito bites.

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

Travel insurance and EHIC

Before travelling, take out adequate travel insurance for countries you will be visiting. Check your insurance policy gives information about what to do if you become seriously ill, and that it covers you for repatriation on medical grounds.

While you are away

You will need to ensure you are protected from biting mosquitoes at all times as there is no treatment for WNV.

If the virus is reported to be in or near the area where you are travelling, be particularly careful and pay close attention to local news about the movement of the virus.

You should follow the advice listed below while you are away.

  • The mosquitoes that carry WNV are most active around dusk and dawn, but some also bite during the day. You will therefore 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 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 (plugs) can be burned in enclosed areas to repel mosquitoes.
  • Avoid areas where there are likely to be large numbers of mosquitoes, such as near water.
Content supplied by NHS Choices