On July 16, 2026, TPH received confirmation that one batch of mosquitoes has tested positive for West Nile virus (WNV) in Toronto. In general, the first positive mosquito pool tends to coincide with the first transmissions of WNV to humans.
Human Infection with WNV:
• Most common in Toronto from July to end of September and can occur outside this time period.
• While approximately 75% of human WNV infections are asymptomatic, most symptomatic patients present with a febrile illness characterized by headache, myalgia, arthralgia, gastrointestinal symptoms or a maculopapular rash.
• People > 50 years of age and those who are immunocompromised are at increased risk of WNV neuroinvasive disease, which presents as encephalitis or viral meningitis with risk of permanent neurologic damage or death.
• Up to 2% of infected older adults develop neuroinvasive disease, compared with a 0.7% risk across all ages.
• Though there is no specific treatment for neuroinvasive disease, diagnostic suspicion can help prevent misattribution to other causes of altered mental status, such as dementia or stroke or non-WNV causes of infection/sepsis.
• Annual reported human cases are likely an underestimate of the true burden of disease.
TPH recommends the following during the summer months:
• Test for WNV infection in individuals with:
o Clinically compatible signs/symptoms of infection, AND
o Relevant exposures (e.g. noted mosquito bites, travel to or residence in an endemic area with ongoing WNV transmission, among others), OR
o New onset of compatible signs/symptoms following an alternative exposure route (e.g. blood transfusion, organ transplantation, infant born to a mother with a confirmed WNV infection during pregnancy or while breastfeeding, among others)
NOTE: Fever and rash in adults during the summer is highly suggestive of WNV infection.
• Suspect WNV neuroinvasive disease for patients with fever and acute onset of encephalitis, viral meningitis, acute flaccid paralysis, tremor, myoclonus, or Parkinsonian-like conditions.
• Consider WNV for cases of acute confusion or altered mental status in older adults.
• Counsel patients to take effective preventive measures.
Report all suspect and confirmed cases of WNV illness to TPH by phone: 416-392-7411 OR Fax: 416-392-0047, Monday to Friday, 8:30 am – 4:30 pm. After hours: 3-1-1 or 416-392-2489 for callers from outside of Toronto.
When considering WNV infection, health care providers should note the following:
• Routine bloodwork may reveal leukocytosis and other non-specific findings typically associated with viral infections.
• Head imaging is non-diagnostic.
• Consultation with medical microbiology or TPH may be necessary to interpret results of WNV serology or PCR.
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