Health Care Provider Information on West Nile Virus

Key Messages

West Nile virus (WNV) is a vector-borne disease transmitted by infected mosquitoes.

  • In Ontario, locally acquired WNV occurs in the summer months, with the majority of cases occurring in August and September.
  • Prevention of WNV infection is primarily through mosquito control and personal protection from mosquito bites. There is no human vaccine for WNV infection.
  • Approximately 20% of humans with WNV infection will develop symptoms of usually mild (through to sometimes debilitating) febrile illness, which may include headache, fatigue, myalgia, rash, nausea and vomiting. Less than 1% of those infected may develop severe neurological illness, including meningitis, encephalitis, and acute flaccid paralysis (poliomyelitis-like syndrome).
  • Serology is the preferred laboratory testing method for WNV. All confirmed or suspected cases of WNV disease must be reported to Ottawa Public Health.
  • Treatment is primarily supportive.

West Nile virus (WNV) is a virus of the family Flaviviridae which was originally identified in Africa in 1937. The first recorded outbreak in North America happened in New York City in 1999. In Canada, the virus was first confirmed in birds in 2001 and the first human case was confirmed in Ontario in September 2002.

It is most commonly transmitted to humans by Culex mosquitoes, but also may be transmitted by blood transfusions, organ transplants, laboratory exposure, and perinatal transmission in pregnancy, delivery, or breastfeeding.

Local Epidemiology

Mosquitoes responsible for spreading West Nile Virus, primarily those of the Culex genus, are present in Ontario. Seasonal outbreaks tend to occur in summer and early fall.The first human cases of WNV disease in Ontario were reported in 2002. Since then, activity has varied from year to year, with noteworthy outbreak years in 2002, 2005, 2012, and 2017 (395, 101, 271, and 154 human cases respectively). There were 13 reported cases in Ontario in 2014, 34 in 2015, 55 in 2016,147 in 2017, 137 in 2018, 19 in 2019, 77 in 2020, 23 in 2021, 23 in 2022, and 54 in 2023.  

The year-to-year variability of human cases has been reflected in Ottawa as well, being dependent on mosquito populations and WNV mosquito-bird-mosquito amplification, which in turn are influenced by variations in temperature and precipitation. Respectively from 2002 to 2022 inclusive, total confirmed and probable reported human cases of WNV disease were 0, 4, 1, 3, 2, 0, 1, 0, 0, 0, 8, 4 ,2, 0, 2, 20, 7, 1, 12, 0 and 1.

Changes in annual human case incidence mirrors identification of WNV in mosquitoes, which Ottawa Public Health traps throughout Ottawa during the WNV season to monitor WNV activity and to estimate risk to humans. Further information regarding cases and incidence of WNV disease in Ottawa can be found under Reportable Infectious Diseases Statistics. For detailed information on WNV activity in Ontario, please see Public Health Ontario’s West Nile Virus Surveillance Tool, which summarizes Ontario mosquito, human, and weather data and allows users to view current and historic trends of West Nile virus seasons in Ontario. 

Signs and Symptoms

About 80% of humans infected with WNV are asymptomatic, though mounting an immune response. An average incubation period of 2 to 6 days (range 2 to 14 days; up to 21 in immunocompromised) follows the bite by an infected mosquito before the onset of symptoms. Of those who develop symptomatic WNV infection, the majority have a mild to moderate illness, called 'WNV fever'; less than 1% develop more severe manifestations called 'WNV neurological syndrome' or 'WNV neuroinvasive disease'.

WNV Fever

A majority of symptomatic cases experience a mild flu-like illness with fever, headache, and body aches, occasionally with a skin rash and swollen lymph nodes or other non-specific symptoms that last several days. Other symptoms may include nausea, vomiting, eye pain or photophobia.

WNV Neurological Syndrome

Less than 1% of people with WNV infection experience severe central nervous system illness. WNV neurological symptoms can present as a meningitis or encephalitis illness as well as conditions similar to acute flaccid paralysis, and Parkinson's disease. The risk of severe illness increases with age (neuroinvasive disease incidence increases by about 1.5 times for each decade of life) and immunocompromised status (such as transplant recipients, or patients with cancer, diabetes, hypertension, or kidney disease).


Of those who are symptomatic with WNV fever, most recover spontaneously, although fatigue and weakness can last for weeks to months. Patients with WNV neurological syndrome have a case-fatality rate of 10%, with patients who have WNV encephalitis or acute flaccid paralysis having a worse prognosis than those with WNV meningitis. WNV infection typically results in lifelong immunity.

Pregnancy and Children

In pregnancy, most women infected with WNV have delivered infants without evidence of infection or clinical abnormalities. In children with WNV infection, the most common presenting symptom is fever.

Diagnosis / Laboratory testing

Contact Ottawa Public Health if you suspect WNV disease - we can assist you in ensuring that the appropriate test is done in a timely fashion and you will have thereby also reported the suspected case to us. Alternatively, please refer to Public Health Ontario Laboratory Services testing information below:  

Lab Testing 

Serology is the preferred testing method. For serology, a 5.0 mL blood or 1.0 mL serum sample is required. In the event of suspicion of WNV neuroinvasive disease, a CSF sample for serological testing (WNV IgM) may also be valuable.

Molecular testing, which because of the brief viremia in humans is less sensitive than CSF IgM ELISA, must be approved by speaking to a microbiologist through Public Health Ontario Laboratory Customer Service at 416-235-6556 or 1-877-604-4567 (toll-free). Molecular testing requires 400 µL of CSF, EDTA blood, serum or plasma.

The laboratory requisition form requires the following information:

Specimens should be stored at 2-8ºC and shipped to Public Health Ontario Laboratory on dry ice.

Reporting Requirements

Confirmed and suspected cases of WNV disease must be reported to Ottawa Public Health as per the Health Protection and Promotion Act.

Call 613-580-6744, ext 24224 or fax 613-580-9640 (Monday to Friday from 8:30 am to 4:30 pm) to report. After hours, on weekends, and on holidays: call 3-1-1 to report.

Communicable Diseases Reporting Page, Ottawa Public Health


The primary treatment for WNV disease is supportive. Depending on the clinical picture, this may include analgesia, antiemetics, rehydration therapy, and monitoring for neurological dysfunction.

Standard blood and body substance precautions are sufficient to limit spread, as the virus is not usually found in blood or secretions during clinical illness.


At the population level

Ottawa Public Health has an integrated WNV surveillance and control program, which includes testing and larviciding mosquito breeding sites, surveillance of mosquito populations including testing for WNV infection, surveillance of WNV disease in the human population, and public education about elimination of mosquito breeding sites around the home and personal protective measures.

At the individual level

Applying bug spray to protect against mosquito bites

Members of the public can minimize their exposure to WNV-infected mosquitoes by:

  • Eliminating mosquito breeding sites from their properties: emptying outdoor containers, cleaning gutters, covering rain barrels, keeping swimming pools clean and chlorinated, eliminating standing water sites on private and public property, and
  • Taking personal protective measures to guard against mosquito bites: applying mosquito repellent to skin and clothing, covering exposed skin, wearing light coloured, loose-fitting, tightly woven clothing, avoiding hours between dusk and dawn and, at any time, shady, wooded areas, fitting windows and doors with screens.
Public Health Role

Prevention of WNV disease

Ottawa public health has a mosquito control program that involves the application of biological and chemical larvicides to surface waters and catch basins, respectively. Mosquito control using ultra-low volume aerial or ground spray application of adulticides, a common practice in the United States, has not been necessary to date from a local risk management perspective. Ottawa Public Health also conducts public education on mosquito-bite prevention.


Standing water with mosquitos

Mosquito vectors

Ottawa Public Health conducts surveillance of mosquito vectors through the use of mosquito traps placed throughout the City, as well as larval surveillance of water sites on City property. Mosquitoes are regularly tested for WNV infection during the WNV season. 

Human cases

Suspected and confirmed human cases of WNV disease are to be reported by physicians and laboratories to Ottawa Public Health under the Health Protection and Promotion Act. Ottawa Public Health investigates every case of WNV to identify the likely source of infection (e.g., standing water) and, where appropriate, takes action to minimize further risk to the population. Ottawa Public Health also generates weekly reports of WNV case counts throughout the WNV season, and promptly communicates changes in the local epidemiology of WNV to physicians to aid in clinical diagnosis. To receive these communications, sign up for OPH's Physician Health Alerts and Physicians' Update e-newsletter.

Patient Information

Information for the public can be found on the OPH website (Printable PDF) and the Health Canada website.

For questions about OPH's WNV mosquito control program or complaints about standing water please contact OPH at 613-580-6744.

Physician Resources

Lab Testing


General Information on West Nile Virus

International Resources on West Nile Virus

Contact Us

If you have questions regarding West Nile virus human illness, or want to notify Ottawa Public Health about a case of West Nile virus, please contact us at 613-580-2424, ext. 24224. For questions concerning OPH's WNV mosquito control program or complaints about standing water please contact us at 613-580-6744.

Communicable Disease Reporting Form


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