West Nile Virus

Updated June, 2022

NCCID Disease Debriefs provide Canadian public health practitioners and clinicians with up-to-date reviews of essential information on prominent infectious diseases for Canadian public health practice. While not a formal literature review, information is gathered from key sources including the Public Health Agency of Canada (PHAC), the USA Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO) and peer-reviewed literature.

This disease debrief was prepared by Christine Yanagawa. Questions, comments, and suggestions regarding this disease brief are most welcome and can be sent to nccid@manitoba.ca

What are Disease Debriefs? To find out more about how information is collected, see our page dedicated to the Disease Debriefs.

Questions Addressed in this debrief:

  1. What are important characteristics of West Nile virus?
  2. What is happening with current outbreaks of West Nile virus?
  3. What is the current risk for Canadians from West Nile virus?
  4. What measures should be taken for a suspected West Nile virus case or contact?


West Nile virus (WNV) is a mosquito-borne disease caused by a positive, single-stranded RNA virus belonging to the genus Flavivirus. Found in Africa, Europe, the Middle East, West Asia, and North America, WNV is maintained in an enzootic cycle involving a mosquito-bird-mosquito transmission. While humans may become infected, they are not part of the pathogen’s lifecycle. The spectrum of WNV disease severity in humans ranges from asymptomatic to mild, moderate, severe, and critical. Reducing the risk of infection is best achieved by preventing mosquito bites.

What are important characteristics of West Nile virus?


West Nile virus (WNV) is a mosquito-borne disease (MBD) caused by a positive, single-stranded RNA virus belonging to the genus Flavivirus, in the Flaviviridae family. Other members of the WNV family include the viruses that cause yellow fever, dengue, and Zika. Found in Africa, Europe, the Middle East, West Asia, and North America, including Canada, WNV is maintained in an enzootic cycle involving mosquito-bird-mosquito transmission. Birds of the Corvidae family (jays, crows, magpies, and ravens) and mosquitoes of the Culex spp., are most susceptible. The principal vectors of WNV, mosquitoes, may become infected when taking a blood meal from an infected bird, the reservoir host. Subsequent feedings by the mosquito may transmit the virus to humans and other animals, with the potential to multiply and cause illness.  Vertical transmission (from adult to eggs) may also perpetuate WNV throughout mosquito populations.

While humans, horses, and other mammals may become infected, they are not part of the pathogen’s lifecycle; as such, they are considered incidental, or ‘dead-end’, hosts. In a small number of human infections, WNV has been transmitted through organ transplant, blood transfusion, exposure in a laboratory setting, and from mother to baby through transplacental transmission, childbirth, or breastfeeding. Thus far, no human-to-human transmission through casual contact has been reported. In addition, there is no evidence that WNV can be transmitted to humans by touching infected live animals, such as horses. Handling blood or other tissues of an infected dead animal, however, may spread WNV to humans, particularly through open wounds. 

Signs and Symptoms, and Severity

The spectrum of WNV disease severity ranges from asymptomatic to mild, moderate, severe, and critical. The majority of infected individuals, about 70-80%, are asymptomatic or subclinical, while approximately 20% of infected people experience mild to moderate symptoms of febrile illness. These may include headache, fever, fatigue, malaise, nausea, vomiting, diarrhea, and seldomly, enlarged lymph nodes, and a skin rash on the trunk of the body.

Fewer than 1% of patients develop WNV neurological syndrome, a neuroinvasive conditioninvolving the central nervous system, which typically manifests asencephalitis, meningitis, or poliomyelitis. Symptoms may includehigh fever and altered mental status, neck stiffness, seizures, focal neurological deficits, or movement disorders, and paralysis. Although rare, symptoms have also included rhabdomyolysis, optic neuritis, uveitis, pancreatitis, hepatitis, myocarditis, and cardiac dysrhythmias. The case fatality ratio among patients with WNV neuroinvasive disease is approximately 10%.

The incubation period for WNV disease varies from 2 to 14 days, and up to several weeks in immunocompromised persons. While the majority of individuals with WNV infection will recover completely, the convalescent stage may include several weeks to months of fatigue, malaise, and weakness. Permanent residual neurological deficits often occur in patients recovering from WNV encephalitis or poliomyelitis.



West Nile virus was first isolated and identified in an infected person in Uganda’s West Nile district in 1937. WNV infections have since been reported in numerous countries, with large outbreaks occurring in Greece, Israel, Romania, and Russia. In 1999, WNV was introduced to New York and subsequently became endemic throughout much of the United States. The first human case of WNV in Canada was reported in Ontario in 2002. Because temperature plays a key role in the biology, ecology, and epidemiology of WNV (i.e., viral amplification and development rate in mosquitoes occurs more rapidly in warmer temperatures), the risk of infection in most of Canada begins in mid-April into the first hard frost of the season (late September or October). Thus, most people in Canada are at peak risk between mid-July to early September, and at dawn and dusk when mosquitoes are most active. Along with temperature, other climatic factors such as precipitation and relative humidity, are important variables in the transmission cycle of WNV. Changes in climate have contributed to increased incidences of MBD by 10% in Canada over the past 20 years.

While everyone is susceptible to acquiring WNV disease, host susceptibility is important in the manifestation of symptoms; people over the age of 50, and those with chronic diseases like cancer, diabetes, alcoholism, and cardiopathy, and those on immunosuppressing medication or treatments are at greater risk of developing more serious clinical outcomes. Similarly, children who are immunocompromised are at greater risk of becoming severely ill. Most women with a documented WNV infection during pregnancy have delivered infants without infection or clinical abnormalities.

Clinical and Laboratory Diagnosis

West Nile virus is diagnosed according to patient symptoms, known exposures, and laboratory results. A diagnosis should also be investigated in any infant born to a woman infected by WNV during pregnancy or while breastfeeding.

Laboratory diagnosis is accomplished by Enzyme-Linked Immunosorbent Assays (ELISAs) of blood serum or cerebrospinal fluid to identify WNV-specific IgM and IgG antibodies. IgM antibodies are detectable from 3 to 8 days after symptoms begin and typically persist for 30 to 90 days. However, these immunoglobulins may persist longer, therefore positive results may reflect a past WNV infection. If serum is collected within 8 days of illness onset, virus-specific IgM antibodies may not yet be detectable, requiring repeated testing at a later date.

While a reactive IgM antibody response rarely results from cross-reactive antibodies with other flaviviruses, a plaque reduction neutralization test (PRNT) may be employed to confirm WNV infections for cases emerging outside of the WNV season (of the local health region).

Other testing for WNV disease may include viral culture and reverse transcriptase-polymerase chain reaction (RT-PCR), although test sensitivity for molecular identification is lower than screening performed by ELISA.

Prevention and Control

Given that there are no WNV vaccines for humans, reducing the risk of WNV infection is best achieved by preventing mosquito bites. The Centers for Disease Control and Prevention and the Public Health Agency of Canada recommend people:

  • Cover exposed skin with light-coloured protective clothing, such as loose long-sleeved shirts and pants, socks, and hats.
  • Avoid outdoor activities at peak mosquito biting times (dawn and dusk).
  • Apply insect repellents that contain DEET or Icaridin when outdoors.
  • Cover strollers and baby carriers with mosquito netting.

Furthermore, there are numerous ways recommended to reduce mosquito breeding sites around homes.

Dead birds suspected to have WNV disease should be reported to the Canadian Wildlife Health Cooperative.

As WNV can be transmitted via blood-to-blood contact, when hunting and processing wild game, always:

  • cover any open wounds on hands or arms.
  • wash gloved hands, then wash bare hands after handling dead animals.


There is no vaccine for West Nile virus infections in humans; however, there are three vaccines registered for use in horses in Canada.


There is no specific antiviral treatment for WNV; rather, supportive care is provided according to patient symptoms. Mild to moderate symptoms may be treated with over-the-counter pain relievers to decrease fever and to ease discomfort, while severe symptoms and cases of neuroinvasive WNV may require hospitalization, intravenous fluids, respiratory support, antiemetic therapy, and treatment for secondary infections. While various therapeutic agents have undergone clinical trials, none have provided validated benefits thus far.

What is happening with current outbreaks of West Nile virus?

To date (2021), there have been:

  • 29 human clinical cases reported across Canada in: Ontario (18), Québec (6), and Manitoba (5)
  • 26 positive wild birds reported in: Ontario (8), Québec (15), and Manitoba (3)
  • 20 positive equine cases reported in: Ontario (3), Manitoba (6), Saskatchewan (10), and Alberta (1).

Situation updates, case counts, and surveillance activities within First Nations and Inuit communities can be obtained by contacting the local Environmental Health Officer, Community Health Centre, or nursing station.

What is the current risk for Canadians?

The risk of WNV infection fluctuates depending on seasons and geography. Since initial identification in Canada in 2002, the annual incidence of human cases has fluctuated from 1,481 cases in 2003,  five cases in 2010. These shifts in counts represent, in part, changes in weather that affect mosquito reproduction and virus transmission rates. In 2003 and 2007, the majority of human cases occurred in the Prairies region (i.e., Alberta, Saskatchewan, and Manitoba), while in 2002, 2012, and 2018 most cases were reported in Ontario and Québec, reflecting a geographical variation in risk to Canadians.

The incidence rate of WNV infections (clinical and asymptomatic) in Canada in 2019 was 0.09 per 100,000.

What measures should be taken for a suspected West Nile virus case or contact?

Healthcare providers should notify all probable and confirmed cases of WNV disease, including West Nile Virus Neurological Syndrome (WNNS), West Nile Virus Non-Neurological Syndrome (WN Non-NS), and West Nile Virus Asymptomatic Infection (WNAI), to their respective local health authority.

In First Nations and Inuit communities, regional health staff are recommended to advise Chiefs, councils, or appropriate federal government departments.

For suspected equine cases, reports should be directed to provincial/territorial Chief Veterinarians.