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West Nile Virus Illness

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Reporting Obligations

All suspected and confirmed cases must be reported immediately to the local Health Unit.

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Aetiologic Agent

West Nile Virus (WNV) is a mosquito-borne virus of the genus Flavivirus.

Clinical Presentation

There are three clinical manifestations of WNV; asymptomatic, non-neurological and neurological. The majority of WNV cases are asymptomatic. About 20% of infected persons develop the usually less severe symptom complex known as WNV non-neurological syndrome. This presents with 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 symptoms can present as an encephalitis illness as well as conditions similar to acute flaccid paralysis, and parkinsons disease. Fewer than 1% of infected people will develop neurological symptoms.

Modes of Transmission

WNV is transmitted to humans primarily through bites of infected Culex mosquitoes. In Ontario, the main vectors of concern are Culex pipiens and Culex restuans.

Person-to-person WNV transmission can occur through blood transfusion and solid organ transplantation. Intrauterine transmission and probable transmission via human milk also have been described but appear to be uncommon.

Incubation Period

Usually 2–6 days but ranges from 2–14 days and can be as long as 21 in immunocompromised people.

Period of Communicability

Viraemia in humans usually last fewer than 7 days in immunocompetent persons.

Risk Factors/Susceptibility

Dependent on many factors including environmental conditions, season and human activities. Once infected, older age, chronic renal disease, immune suppression, history of alcohol abuse, diabetes and hypertension have been associated with higher risk of severe disease.

Diagnosis and Laboratory Testing

Diagnosis is based on clinical presentation and serological test results. Any of the following serological test results will constitute a confirmed case of West Nile virus:

  • Positive West Nile virus culture
  • Positive for West Nile virus antigen in tissue
  • Positive for West Nile virus-specific nucleic acid
  • Positive for West Nile virus-specific antibody
  • Diagnostic rise in West Nile virus antibody titre
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Treatment and Case Management

There is no specific treatment for this virus, but the symptoms and complications of the disease can be treated. Provide education regarding: the use of insect repellent when outdoors; wearing long sleeve shirts and long pants and light coloured clothes; cleaning up mosquito-friendly areas around your home regularly such as standing water.

Current labels on DEET-containing repellants state that they should not be applied to children under 2 years of age. Health Canada, however, recommends that “DEET may be considered for children aged 6 months to 2 years, where there is a high risk of complications to the child from insect bites. Use sparingly, avoid the face and hands, apply only once a day, and use only the least concentrated product (10% or less)”

Patient Information

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Additional Resources

Public Health Agency of Canada. “West Nile Virus Monitor, Maps & Stats.”

Centers for Disease Control and Prevention – Division of Vector-Borne Diseases. “West Nile Virus”

Public Health Ontario. “West Nile Virus.”


Ministry of Health and Long-Term Care, Infectious Diseases Protocol, 2022.

Public Health Agency of Canada, West Nile virus – protect yourself.