TORONTO – The Public Health Agency of Canada confirmed the first human case of West Nile virus acquired in Canada for the year, coinciding with the August long weekend. An adult in Toronto, who had no travel history, was reported to be infected with the mosquito-borne virus. Prior to this, there had been two reported cases of West Nile virus in Canada, but both individuals contracted the virus while traveling abroad.
West Nile virus was first identified in Canada in 2002 and originated in the West Nile region of Uganda. Dr. Isaac Bogoch, an infectious diseases specialist at the University Health Network in Toronto, explained that the virus spreads through migratory birds, which carry the virus. When mosquitoes bite these infected birds, they can then transmit the virus to humans and other animals through their bites.
Generally, the type of mosquitoes that are known to carry and transmit West Nile virus are most active at dusk and during the night. Human infections are typically reported in mid to late summer, with cases declining as the weather cools.
Humans do not transmit the virus to each other except in rare instances such as through blood transfusions, organ transplants, or maternal transmission during pregnancy or breastfeeding, according to the Public Health Agency of Canada.
The majority of individuals infected by the virus will not exhibit any symptoms. Dr. Bogoch indicates that roughly 20 to 30 percent of those infected may develop symptoms that usually resolve on their own within a few days. Symptoms typically present between two and 14 days after a mosquito bite and may include fever, muscle aches, headaches, fatigue, nausea, vomiting, skin rash, swollen lymph nodes, and a stiff neck.
Severe outcomes from West Nile virus can occur in up to 2 percent of infected individuals, leading to neuroinvasive diseases such as encephalitis or meningitis—conditions characterized by the inflammation of the brain and surrounding areas, respectively. While anyone can potentially suffer from severe illness due to West Nile virus, older adults are at increased risk.
Currently, there is no antiviral medication available for West Nile virus. Most patients can manage their symptoms with over-the-counter medications like acetaminophen, along with rest and hydration. For rare cases of neuroinvasive disease, supportive care is provided in hospitals, which may include fluids and electrolytes, as well as rehabilitation if necessary.
As there is no vaccine for West Nile virus, preventative measures focus on avoiding mosquito bites. Effective methods include using insect repellents containing DEET or Icaridine. However, these products should not be applied to infants under six months old; instead, parents may consider using mosquito nets over cribs or strollers for babies outdoors.
Additionally, wearing long sleeves, long pants, socks, and a hat can help minimize mosquito bites, while choosing lighter-colored clothing is advisable as dark colors attract mosquitoes. Installing screens on open windows and eliminating standing water in residential areas also contribute to reducing mosquito populations, as stagnant water serves as breeding grounds.
The number of reported cases of West Nile virus in Canada varies significantly each year, with cases ranging from a few to a peak of 2,401 cases in 2007 since the Public Health Agency of Canada began monitoring domestic infections in 2003. Preliminary data for 2024 indicated 166 reported cases.
According to Mark Johnson, a spokesperson for the Public Health Agency of Canada, the risk of infection for most Canadians remains low, as a relatively small number of mosquitoes across the country carry the West Nile virus.
