Toronto Public Health reports first probable human case of West Nile virus in 2023

2023 August 4

Toronto Public Health (TPH) has reported its first probable human case of West Nile virus (WNV) in an adult resident in 2023. WNV is an infection transmitted to people through the bite of an infected mosquito.

While the risk of getting infected in Toronto is currently low, TPH encourages residents ahead of the August long weekend to take the following precautions to avoid bites from infected mosquitos:

  • Wear light-coloured clothing, long pants and long-sleeved shirts when outdoors.
  • Apply insect repellent containing N,N-diethyl-meta-toluamide (DEET) or icaridin, following the manufacturer’s instructions.
  • Take extra care during peak mosquito-biting hours, dusk and dawn, by using repellent and covering up.
    Make sure your home has tight-fitting screens on windows and doors.
  • Remove standing water from properties where mosquitos can breed. Standing water includes any water that collects in items such as pool covers, buckets, planters, toys and garbage/recycling containers.

WNV symptoms usually start to show between two and 14 days after a person is bitten by an infected mosquito. Symptoms may include fever, headache, nausea, vomiting, body aches, skin rash and swollen lymph glands. Older individuals or people with compromised immune systems are at higher risk of severe illness. Anyone concerned about any symptoms should contact their health care provider.

TPH conducts mosquito surveillance and monitoring from mid-June until mid-September every year. Once a week, 22 mosquito traps are set across Toronto to collect mosquitos which are then submitted to a laboratory for identification and grouped by the lab into batches of mosquitos to test for WNV. So far in 2023, 20 batches – also known as pools – of mosquitos have been found to be positive for WNV. With the increased hot weather in Toronto lately, there is a higher risk of mosquitos that can transmit the virus once they are infected. In 2022, a total of 14 batches of mosquitos were confirmed as positive for the virus, in addition to 14 laboratory-confirmed human cases of WNV.