First Ottawa human case of West Nile Virus in 2025

To: Ottawa Physicians and Nurse Practitioners
Date: Aug 27, 2025
From: Dr. Robin Taylor, Associate Medical Officer of Health

Dear Colleagues,

Ottawa Public Health (OPH) is writing to heighten your awareness of the possibility of illness from West Nile virus (WNV) infection in Ottawa, as the first human case of WNV has been reported in an Ottawa resident for the 2025 season.

The main vectors of WNV, Culex pipiens mosquitoes, reproduce and feed around human habitation making people at risk around their homes and in urban areas. Even the smallest amounts of standing water with organic material lasting at least one week can serve as breeding habitat for the mosquito.

This serves as a reminder to keep WNV in mind for the differential diagnosis of:

• fever with symptoms that can include headache, weakness, myalgia or arthralgia, GI symptoms, and a possible transient maculopapular rash
• meningitis
• encephalitis, and/or
• acute flaccid paralysis

Reporting to Public Health

If you have a confirmed or suspected case of West Nile Virus, please report by the next business day: Monday to Friday from 8:30 am to 4:30 pm: Call 613-580-2424, extension 24224 (leave a confidential voicemail) or fax 613-580-9640. For more details on how to report, please visit Reporting a communicable disease.

Laboratory Testing

Detailed information on laboratory testing for human WNV infection may be found at the Public Health Ontario (PHO) website at Publichealthontario.ca/westnilevirus; look for laboratory testing.

West Nile Virus Disease

WNV disease has an incubation that is typically 2 to 6 days (range 2 to 14 days) but can be several weeks in immunocompromised persons. Subclinical or asymptomatic infections account for 70 to 80% of human WNV infections, WN febrile illness for approximately 20%, and neuroinvasive disease for less than 1% (in whom there is a fatality rate of about 10%). Risk for serious illness increases with age, with people over 60 years at greatest risk along with persons with certain medical conditions such as cancer, diabetes, hypertension, renal disease, and organ transplants.

August and the first half of September are typically the highest risk periods for human WNV illness in eastern and other parts of southern Ontario. Risk persists until the first hard frosts of the fall. Getting rid of standing water and protecting oneself against mosquito bites are important prevention measures.

Further information on WNV may be found on the OPH website:

• For physicians
• For the public