West Nile virus confirmed in Ottawa area mosquitoes

Posted Friday September 10, 2021

Posted By: Emily Ngo

Category: General

To: Ottawa Physicians and Nurse Practitioners, Emergency Departments and Walk-in Clinics, IPAC teams
Date: September 9, 2021
From: Dr. Monir Taha, Associate Medical Officer of Health, Ottawa Public Health

Ottawa Public Health (OPH) is writing to heighten your awareness of the possibility of illness from West Nile virus (WNV) infection in Ottawa. OPH has received the first positive results for WNV from Ottawa area mosquito traps for the 2021 season. With more people enjoying the outdoors because of COVID-19, more people may be at risk for mosquito bites around their homes and in urban areas where Culex pipiens mosquitoes thrive.

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

Please report to OPH if you suspect human WNV by calling 613-580-2424, ext. 24224.

Detailed information on laboratory testing for human WNV infection may be found at the Public Health Ontario (PHO) website at www.publichealthontario.ca/westnilevirus; look for laboratory testing and Labstract LAB-SD-011.

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.

Human cases of WNV may occur as soon as within 2 weeks following detection of the WNV in mosquitoes. Typically, August and the first half of September are 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. Hot weather speeds up reproduction of Culex pipiens, the northern house mosquito, which is the primary vector of WNV in the southern and eastern parts of Ontario including Ottawa, as well as the multiplication of the West Nile virus within the mosquito. Standing water, even in small containers, provides mosquito breeding sites.

Further information on WNV may be found on the OPH website: For physicians: http://www.ottawapublichealth.ca/en/professionals-and-partners/hcp-west-nile-virus.aspx

For the public: http://www.ottawapublichealth.ca/en/public-health-topics/west-nile.aspx

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