WNV has been present in Peel since 2001. Activity varies each year, largely influenced by weather conditions. Peel Public Health monitors WNV through 33 mosquito traps collected weekly from mid-June through September. In 2025, the first positive mosquito pools were detected in late July. Human cases in Ontario are most common between late July and September.
For current provincial surveillance data, visit PHO Vector-borne Disease Tool.
Transmission and clinical presentation
WNV is transmitted to humans primarily through mosquito bites. The incubation period ranges from 2-15 days or up to 21 days in immunocompromised individuals. Most people infected with WNV are asymptomatic. Approximately 20% develop a mild, flu-like illness which can include:
- Fever, headache, and body aches
- Fatigue
- Rash on the chest, stomach, or back
- Swollen lymph nodes
- Nausea, vomiting, or photophobia
Fewer than 1% of infections progress to neuroinvasive disease, which is more severe and can present as:
- Meningitis
- Encephalitis
- Acute flaccid paralysis
Additional neurological features such as tremor, myoclonus, or Parkinsonian-like symptoms may also occur. Individuals over 50 years of age and those who are immunocompromised are at greatest risk for severe illness.
Testing
As per Public Health Ontario (PHO) recommendations, cerebrospinal fluid (CSF) serology is the preferred method for suspected neuroinvasive disease; a paired serum specimen must also be submitted. Consider WNV neuroinvasive disease for patients with fever and acute onset of encephalitis, viral meningitis, acute flaccid paralysis, tremor, myoclonus, or Parkinsonian-like condition.
For patients with non-neurological illness, acute and convalescent serum specimens should be collected 2–3 weeks apart. Consider ordering WNV serology for patients who have at least two symptoms with no apparent alternative cause and a recent exposure to mosquitoes.
Testing is ordered using the PHO General Test Requisition and should indicate “West Nile Virus – Serology” along with onset date, symptoms, exposure history, travel history, and whether the specimen is acute or convalescent. Consultation with PHO or Peel Public Health may be required for test interpretation. For more information, see: PHO Test Information Index: West Nile Virus Serology and PCR.
Reporting:
WNV disease is reportable. All suspect or confirmed cases must be reported to Peel Public Health by fax at 289-801-0257.
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