West Nile Virus

West Nile Virus

Dr. Zach 2019

What is it?

How do you catch it?

What are the signs and symptoms?

What is the big deal?

How is it treated?

How to prevent it?

A total of 76 people contracted West Nile virus in the Monteregie on Montreal’s South Shore in 2018, making it the most affected part of the province.  In all, 201 Quebecers were found to be infected last summer, and 15 people died as a result, according to public health officials.

CDC Info:

West Nile virus (WNV) is the leading cause of mosquito-borne disease in north america.  It is most commonly spread to people by the bite of an infected mosquito. Cases of WNV occur during mosquito season, which starts in the summer and continues through fall. There are no vaccines to prevent or medications to treat WNV in people. Fortunately, most people infected with WNV do not feel sick. About 1 in 5 people who are infected develop a fever and other symptoms. About 1 out of 150 infected people develop a serious, sometimes fatal, illness. You can reduce your risk of WNV by using insect repellent and wearing long-sleeved shirts and long pants to prevent mosquito bites.


Transmission Cycle

West Nile Virus Transmission Cycle 

West Nile virus is most commonly spread to people by the bite of an infected mosquito.

Mosquitoes become infected when they feed on infected birds. Infected mosquitoes then spread West Nile virus to people and other animals by biting them.

In a very small number of cases, West Nile virus has been spread through:

West Nile virus is not spread:

  • Through coughing, sneezing, or touching

  • By touching live animals

  • From handling live or dead infected birds. Avoid bare-handed contact when handling any dead animal. If you are disposing of a dead bird, use gloves or double plastic bags to place the carcass in a garbage can.

  • Through eating infected birds or animals. Always follow instructions for fully cooking meat from either birds or mammals.



No symptoms in most people. Most people  (8 out of 10) infected with West Nile virus do not develop any symptoms.

Febrile illness (fever) in some people. About 1 in 5 people who are infected develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash. Most people with this type of West Nile virus disease recover completely, but fatigue and weakness can last for weeks or months.

Serious symptoms in a few people. About 1 in 150 people who are infected develop a severe illness affecting the central nervous system such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord).

  • Symptoms of severe illness include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis.

  • Severe illness can occur in people of any age; however, people over 60 years of age are at greater risk. People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk.

  • Recovery from severe illness might take several weeks or months. Some effects to the central nervous system might be permanent.

  • About 1 out of 10  people who develop severe illness affecting the central nervous system die.


  • See your healthcare provider if you develop the symptoms described above.

  • Your healthcare provider can order tests to look for West Nile virus infection.

  • To learn more about testing, visit our Healthcare Providers page.


  • No vaccine or specific antiviral treatments for West Nile virus infection are available.

  • Over-the-counter pain relievers can be used to reduce fever and relieve some symptoms

  • In severe cases, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

  • If you think you or a family member might have West Nile virus disease, talk with your health care provider.

  • Treatment

  • There is no specific treatment for WNV disease; clinical management is supportive. Patients with severe meningeal symptoms often require pain control for headaches and antiemetic therapy and rehydration for associated nausea and vomiting. Patients with encephalitis require close monitoring for the development of elevated intracranial pressure and seizures. Patients with encephalitis or poliomyelitis should be monitored for inability to protect their airway. Acute neuromuscular respiratory failure may develop rapidly and prolonged ventilatory support may be required.

  • Various drugs have been evaluated or empirically used for WNV disease, as described in a review of the literature for health care providers:



No WNV vaccines are licensed for use in humans. In the absence of a vaccine, prevention of WNV disease depends on community-level mosquito control programs to reduce vector densities, personal protective measures to decrease exposure to infected mosquitoes, and screening of blood and organ donors. Personal protective measures include use of mosquito repellents, wearing long-sleeved shirts and long pants, and limiting outdoor exposure from dusk to dawn. Using air conditioning, installing window and door screens, and reducing peridomestic mosquito breeding sites, can further decrease the risk for WNV exposure.


The most effective way to prevent infection from ­­­West Nile virus is to prevent mosquito bites. Mosquitoes bite during the day and night. Use insect repellent, wear long-sleeved shirts and pants, treat clothing and gear, and take steps to control mosquitoes indoors and outdoors.

Protect yourself and your family from mosquito bites

Use Insect Repellent

Use Environmental Protection Agency (EPA)-registered insect repellents


 with one of the active ingredients below. When used as directed, EPA-registered insect repellents are proven safe and effective, even for pregnant and breastfeeding women.

  • DEET

  • Picaridin (known as KBR 3023 and icaridin outside the US)

  • IR3535

  • Oil of lemon eucalyptus (OLE)

  • Para-menthane-diol (PMD)

  • 2-undecanone

Find the right insect repellent for you by using EPA’s search tool



Tips for babies and children

  • Always follow instructions when applying insect repellent to children.

  • Do not use insect repellent on babies younger than 2 months old.

    • Instead, dress your child in clothing that covers arms and legs.

    • Cover strollers and baby carriers with mosquito netting.

  • Do not use products containing oil of lemon eucalyptus (OLE) or para-menthane-diol (PMD) on children under 3 years old.

  • Do not apply insect repellent to a child’s hands, eyes, mouth, cuts, or irritated skin.

    • Adults: Spray insect repellent onto your hands and then apply to a child’s face.

Tips for Everyone

  • Always follow the product label instructions.

  • Reapply insect repellent as directed.

    • Do not spray repellent on the skin under clothing.

    • If you are also using sunscreen, apply sunscreen first and insect repellent second.

Natural insect repellents (repellents not registered with EPA)

  • We do not know the effectiveness of non-EPA registered insect repellents, including some natural repellents.

  • To protect yourself against diseases spread by mosquitoes, CDC and EPA recommend using an EPA-registered insect repellent.

  • Choosing an EPA-registered repellent ensures the EPA has evaluated the product for effectiveness.

  • Visit the EPA website to learn more.

  • External

Wear long-sleeved shirts and long pants

Treat clothing and gear

  • Use permethrin to treat clothing and gear (such as boots, pants, socks, and tents) or buy permethrin-treated clothing and gear.

    • Permethrin is an insecticide that kills or repels mosquitoes.

    • Permethrin-treated clothing provides protection after multiple washings.

    • Read product information to find out how long the protection will last.

  • If treating items yourself, follow the product instructions.

  • Do not use permethrin products directly on skin.

Take steps to control mosquitoes indoors and outdoors

  • Use screens on windows and doors. Repair holes in screens to keep mosquitoes outdoors.

  • Use air conditioning, if available.

  • Stop mosquitoes from laying eggs in or near water.

    • Once a week, empty and scrub, turn over, cover, or throw out items that hold water, such as tires, buckets, planters, toys, pools, birdbaths, flowerpots, or trash containers.

    • Check indoors and outdoors.

Prevent mosquito bites when traveling overseas

  • Choose a hotel or lodging with air conditioning or screens on windows and doors.

  • Sleep under a mosquito bed net if you are outside or in a room that does not have screens.

    • Buy a bed net at your local outdoor store or online before traveling overseas.

    • Choose a WHOPES-approved bed net: compact, white, rectangular, with 156 holes per square inch, and long enough to tuck under the mattress.

    • Permethrin-treated bed nets provide more protection than untreated nets.

      • Do not wash bed nets or expose them to sunlight. This will break down the insecticide more quickly.

    • For more information on bed nets, visit CDC’s page on insecticide-treated bed nets.

    • For more information on traveling overseas, visit Travelers’ Health.


Health Canada surveillance:

Human surveillance

During the West Nile virus season (mid-April to October), Canada conducts ongoing human case surveillance across the country.

Monitoring West Nile virus nationally is a joint effort between the Government and its partners, including:

  • provincial and territorial ministries of health

  • First Nations authorities

  • blood supply agencies

The Government relies on the provinces and territories to report the number of West Nile virus cases.

To accurately reflect the annual occurrence of West Nile virus cases in Canada, health professionals need to remain vigilant in:

  • diagnosing West Nile virus

  • reporting cases to their public health regional authorities

Surveillance information is reported to the Government by its partners.

View the latest case numbers and weekly analysis

How many human cases of West Nile virus are reported annually?

The first human case of West Nile virus infection in Canada was reported in Ontario in 2002. The Government currently has data for West Nile virus human clinical cases reported between 2002 and 2017:

  • 2017: 200 cases

  • 2016: 104 cases

  • 2015: 80 cases

  • 2014: 21 cases

  • 2013: 115 cases

  • 2012: 428 cases

  • 2011: 101 cases

  • 2010: 5 cases

  • 2009: 13 cases

  • 2008: 36 cases

  • 2007: 2215 cases

  • 2006: 151 cases

  • 2005: 225 cases

  • 2004: 25 cases

  • 2003: 1481 cases

  • 2002: 414 cases

These numbers may change slightly as provincial or territorial public health organizations can sometimes retroactively identify cases.

Surveillance detects only a portion of the West Nile virus cases in Canada. The true number of West Nile virus infections in Canada is likely greater.

West Nile virus surveillance map

Figure 1. Human cases in Canada, December 15, 2018

These are preliminary data, under validation.

Figure 1. Human cases in Canada, December 15, 2018 – Text description


  • N.L. – Newfoundland and Labrador

  • P.E.I. – Prince Edward Island

  • N.B. – New Brunswick

  • N.S. – Nova Scotia

  • Que – Quebec

  • Ont. – Ontario

  • Man. – Manitoba

  • Sask – Saskatchewan

  • Alta. – Alberta

  • B.C. – British Columbia

  • Y.T. – Yukon

  • N.W.T. – Northwest Territories

  • NU – Nunavut

Mosquito surveillance

Mosquito surveillance depends on the level of anticipated or current West Nile virus activity in a particular area.

In areas where West Nile virus has not been found, surveillance of mosquitoes focuses on:

  • which kinds of mosquitoes are present

  • how abundant the mosquitoes are

  • whether West Nile virus has entered the mosquito population

In areas where West Nile virus has already been found, mosquitoes may still be tested for West Nile virus. This information can help to:

  • anticipate new outbreaks

  • monitor ongoing outbreaks

  • identify different kinds of mosquitoes that could spread the virus to:

  • birds

  • animals

  • people

  • It can also be used to decide if, when, where and how to act to reduce the risk of infection by applying mosquito control measures

Control of mosquito populations

Provinces and municipalities are responsible for control of mosquito populations.

Bird surveillance

When West Nile virus first came to North America, bird surveillance was used as an early indicator of the virus in animals.

Experience from past outbreaks showed the following birds were highly susceptible to West Nile virus:

  • crows

  • jays

  • magpies

  • ravens

Infected dead birds are a good indicator to determine whether people in particular areas are at risk.

Some provinces and territories no longer conduct dead bird surveillance. Instead, they focus efforts on other surveillance activities.

The Canadian Wildlife Health Cooperative tests dead birds for West Nile virus in geographical areas with a sufficiently high risk. They do this in collaboration with:

Testing focuses on the detection of West Nile virus in the early season (late April to July).

Report dead birds

Dead birds in your community may be a sign of the presence of West Nile virus. Contact the Canadian Wildlife Health Cooperative to report dead birds that you suspect have West Nile virus.

Horse surveillance

Among large animals, horses are especially at risk for West Nile virus infection and disease. Horses can also be early indicators of West Nile virus in a region. The Government monitors West Nile virus in horses with the help of:

  • provincial veterinary laboratories

  • veterinarians

  • other members of the animal health community

Vaccination may be used to prevent infection in horses. Currently, there are three West Nile virus vaccines registered for use in horses in Canada. Local veterinarians should be contacted for guidance on appropriate vaccination products and strategies.

Veterinary laboratories

All veterinary laboratories in Canada are required to report suspicion or diagnosis of West Nile virus in all domestic animal species to the CFIA.

Surveillance in First Nations communities

In First Nations communities, First Nations and Inuit Health regional staff advise:

  • Chiefs

  • councils

  • federal government departments

They are advised on emerging needs for West Nile virus public health control measures.

To get information about specific West Nile activity in their community, First Nations residents can contact their:

  • Environmental Health Officer

  • Community Health Centre

  • Nursing Station