Frequently Asked Questions About West Nile Virus

What are the symptoms and how is it transmitted?

What is West Nile Virus?

West Nile virus (WNV) is a virus carried by mosquitoes that can cause illness in humans. It was first found in North America in New York City during the summer of 1999 and has since spread dramatically into other parts of the U.S. and Canada. It was first identified in Waterloo Region in the summer of 2001.

How does it spread?

WNV usually circulates within a bird-mosquito cycle. Birds carry the virus; mosquitoes bite the birds and become infected. Mosquitoes then bite other animals and birds, passing the virus on. People can become infected when they are bitten by an infected mosquito.

Can you catch it from someone who is infected?

There is no evidence to suggest that WNV can spread directly from person to person. There is also no evidence that the virus spreads directly from birds or animals to humans. People become infected from the bite of an infected mosquito.

People can contract WNV following a blood transfusion with infected blood. Canadian Blood Services has put protocols in place to help prevent this from happening.

How would I know if I have it and what should I do?

Most people who become infected with WNV do not experience symptoms. For those who do become ill (about 20%) symptoms can occur within three to 15 days of being bitten by an infected mosquito. Symptoms include mild fever, headache, muscle aches, stiff neck, swollen glands and skin rash.

A much smaller number of people might develop encephalitis, or swelling of the brain, and may experience prolonged muscle weakness and neurological problems. This is a serious illness. Recovery can take up to a year. Death occurs in three to 15% of cases.

Although WNV has been identified in a wide range of ages, people over 50 years or with weakened immune systems are more likely to develop encephalitis. If you are experiencing any of the symptoms noted above, please contact a qualified health practitioner immediately.

Is there a cure?

There is no specific  treatment or cure for WNV, but the symptoms and complications can be treated. Most people recover on their own. It has been shown that 50% of people with encephalitis still have significant symptoms one year after the onset of illness.

Can I develop immunity to the virus once I become infected?

When a person becomes infected with WNV, the body creates antibodies to fight the virus. These antibodies give the person immunity to the virus, which protects them against future infection. It is assumed that immunity will be lifelong, although it may decrease in later years. Since four out of five people infected with WNV do not become sick, some people may not know that they have been infected. Therefore, some people may not even be aware  they are immune to the virus.


How can I "Fight the Bite"?

How can I reduce my chances of exposure?

Several steps can be taken to minimize exposure to mosquitoes. These include eliminating standing water where mosquitoes breed, screening mosquitoes out of your home by fixing window screens, wearing protective clothing, and considering using an insect repellent containing DEET. To learn more about protecting yourself from WNV, see our Protecting Yourself page.

Where do mosquitoes breed?

Mosquitoes need wet areas with standing water to lay their eggs. A larva emerges out of the egg and develops into a pupa, remaining in the water during this entire maturation process until maturing into an adult and flying away.

It takes approximately 4-7 days for an egg to mature into an adult mosquito. By getting rid of stagnant and standing water around your home such as in birdbaths and other containers, you are eliminating their breeding grounds.

Are mosquitoes found in certain locations, such as swamps or birdbaths, more likely to carry the virus?

No. Although only certain species of mosquitoes carry the virus and different species of mosquitoes have preferred breeding sites, mosquitoes' location does not provide a reliable means of determining which ones carry the virus. Caution should be taken against all mosquitoes regardless of where they breed.

How important is it for me to eliminate standing water around my home?

Adult mosquitoes are usually found close to the original breeding sites where they were born. By eliminating standing water around your home, you significantly reduce the risk of getting bitten by an infected mosquito.

How can I prevent mosquitoes from breeding in the water on my pool cover?

Drain any water that accumulates on your pool cover twice a week. A small amount of liquid bleach can also be added to the water on the pool cover. Liquid bleach must not be added to any other standing water that is not completely contained on your property. For example, do not add bleach to ditches or ponds that flow in or out of your property since this can harm the environment.

Do I need to get rid of my rain barrel since it contains standing water?

No. Rain barrels benefit both the community and the environment by reducing demand on our regional water supplies. However, keep a cover and screen on your rain barrel at all times. If the screen becomes damaged, repair or replace it with a standard window screen. All rain barrels distributed by the Region have covers and screens.

Can I keep my birdbath?

Yes. Just make sure you change the water twice a week to remove any larvae that may be breeding there.

Do I have to drain my ornamental pond?

It is not necessary to drain your ornamental pond. However, make sure you turn on your pump and consider installing a waterfall to keep the water flowing. Water in your pond should be moving 24 hours a day or at the least in the early morning and evening. Fish can also be placed in ponds to help, although consumption of mosquito larvae will be minimal.

Is there anything else I can do to eliminate standing water and other mosquito breeding grounds around my home?

There are many places where standing water can collect around your home, such as flower pots, canoes and eaves troughs. To learn more about how to eliminate standing water and other potential mosquito breeding grounds, please read our fact sheet, Personal Protective Measures Fact Sheet.

Now that I have eliminated mosquito breeding grounds around my home, how can I keep mosquitoes out of my home?

To prevent mosquitoes from getting into your home, fix window screens, doors and any other places where they could enter. To learn more about how to keep mosquitoes out of your home, please read our fact sheet, Personal Protective Measures Fact Sheet.

Are there any insect repellents that you recommend I use?

Consider using an insect repellent containing DEET or other registered ingredients.

Note that DEET-based repellents are the most effective repellents available for deterring mosquitoes.

Should I take any precautions when using insect repellents?

Yes. Make sure you read and follow the manufacturer's directions carefully and never use anything labelled 'insecticide' on your body. Remember to use the lowest concentration needed for the time you are going to spend outside and never use DEET on children less than six months of age.

For additional tips, please read our fact sheet, Personal Protective Measures against Mosquitoes.

I am allergic/sensitive to products containing DEET. Are there any alternatives that you recommend?

Anyone who is allergic or sensitive to DEET products should take other precautions whenever possible. These can include wearing protective clothing and using repellents with other approved ingredients such as oil of lemon eucalyptus compound.  See our Protecting Yourself page for full details.

For further information, please read our fact sheet, Personal Protective Measures against Mosquitoes.

What should I do about dead birds?

You do not need to call Public Health. Double bag the bird and place it in your garbage for pick-up or bury the carcass at least two feet deep in an area that will not be disturbed. Do not bury in vegetable gardens. Make sure to wash your hands thoroughly with soap and water after handling a dead bird.

Questions or concerns? Talk to a Public Health Inspector at 519-575-4400

My neighbour has an unused swimming pool in their backyard that is filled with standing water. What can be done about this?

Call Region of Waterloo Public Health at 519-575-4400 and give the location of the property. A letter and educational material will be sent to the property owner outlining the health concerns with standing water and ways to remove the standing water. You may also wish to check with your local municipality to see if they have any by-laws relating to standing water.

Can I apply sunscreen and insect repellent to my skin at the same time?

Yes, but first apply the sunscreen and allow it to absorb for 30 minutes. Then apply the repellent, but remember it shouldn't be applied under clothing.

Can my pet/livestock become infected with West Nile Virus?

WNV infections have been reported in a wide range of animal and bird species, including horses, dogs, cats, deer, crows, chickens, geese, blue jays and gray jays, raptors and owls.

An equine vaccination product for horses is available from veterinarians in North America. The Ontario Ministry of Agriculture and Food encourages the vaccination of horses for WNV.