Conditions Basics
This information has been translated into other languages – see the links at the bottom of this page.
What is West Nile virus?
West Nile virus is a type of virus that is spread by mosquitoes. The infection it causes may be so mild that people don't even know they have it. But in rare cases, West Nile leads to severe illness that affects the brain or spinal cord. People older than 50 are at the highest risk for serious problems from West Nile.
Most people fully recover from West Nile. But some people who get a severe infection have permanent problems such as seizures, memory loss, and brain damage. A few people die from it.
How is it spread?
Most often, mosquitoes spread the virus by biting birds infected with the virus and then biting people.
Mosquitoes can also spread the virus to other animals, such as horses. But you can't get West Nile from these animals or from touching or kissing an infected person.
West Nile can spread through an organ transplant or a blood transfusion. That rarely happens in Canada and the United States, though, because all donated blood and organs for transplant are screened to see if the virus is present.
What are the symptoms?
Most people who have West Nile have no symptoms. When symptoms do appear, they start 2 to 14 days after the mosquito bite. Mild symptoms may include:
- Fever.
- Headache.
- Feeling very tired and less hungry than usual.
- Body aches.
- A rash, usually on the chest, and swollen glands (lymph nodes).
Most people who have the mild form of West Nile have a fever for 5 days, have a headache for 10 days, and feel tired for more than a month.
West Nile causes serious illness in some people who get infected. It can lead to swelling of the brain (encephalitis), the spinal cord (myelitis), or the tissues around the brain and spinal cord (meningitis). Symptoms of these diseases may include:
- Severe headache.
- High fever.
- Stiff neck.
- Confusion or reduced attention to surroundings.
- Tremors or convulsions.
- Muscle weakness or paralysis.
- Coma.
Call your doctor right away if you or someone you know has symptoms like these.
If you have a severe case of West Nile, symptoms can last for weeks or months, especially if the infection has spread to your brain.
How is it diagnosed?
If your doctor thinks that you may have West Nile, they will ask questions to find out when you were bitten by a mosquito and what symptoms you have.
The doctor may also test your blood for antibodies to the virus. The antibodies can show if you have had a recent West Nile infection. The antibodies don't always appear right away, so your doctor may test your blood again in a couple of weeks.
You may also have other tests, such as:
- A spinal tap (lumbar puncture) to look for antibodies or other signs of the virus in the fluid that surrounds your brain and spinal cord.
- An MRI scan, which makes pictures of your brain. This scan is done to find out if you have encephalitis.
How is West Nile treated?
There is no treatment for West Nile. Your body just has to fight the infection on its own. If you have a mild case, you can recover at home. Be sure to drink enough fluids and get lots of rest. You may also want to take medicine to reduce pain or fever. You may feel well enough to keep doing your normal activities. Ask your doctor if you need to stay home.
If you have severe West Nile, you may need to stay in a hospital so you can get treatment to help your body fight the illness. You may get fluids given through a vein (intravenous, or I.V.) and get help preventing other illnesses such as pneumonia. If you have severe trouble breathing, a machine called a ventilator may be used to help you breathe. You also may be given medicine to help with pain or fever.
How can you prevent it?
You can contact your local health unit for the latest information on West Nile virus in your area. It's also a good idea to take steps to lower your risk of getting mosquito bites:
- Use insect repellent when you go outdoors in the late spring, summer, and early fall.
- Wear long-sleeved shirts and long pants if you know that you will be in areas with lots of mosquitoes or where you know West Nile virus has been found.
- Do not leave puddles or open containers of water near your house. Mosquitoes breed in standing water.
- Stay indoors at dawn, at dusk, and in the early evening when mosquitoes are the most active.
There is no vaccine to prevent West Nile virus in humans, but researchers are working to develop one.
Other languages
West Nile Virus
