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Medical Advisories and Memos

Medical Advisory: Heightened Surveillance for West Nile Virus (WNV)

Issued by: Dr. Azim Kasmani, MD, MSc, FRCPC, Medical Officer of Health and Commissioner
Date: July 29, 2025

Niagara Region Public Health and Emergency Services' mosquito trapping and testing has confirmed the first West Nile Virus (WNV) positive mosquito pool of the year by lab result. This is the first positive testing pool since the annual monitoring season began on June 3, 2025. While there has not yet been any reported human WNV cases in Niagara this year, health care professionals should be on heightened surveillance during active mosquito season.

Early recognition and prompt testing are critical as West Nile Virus activity increases province-wide. Please remain vigilant, test, and notify public health immediately. Prompt collaboration between clinicians and public-health partners enables timely case management and coordinated community-level interventions.

Clinical Presentation

  • Average incubation period ranges from three to 14 days after infected mosquito bite
  • Most WNV infections are asymptomatic. Only 20 percent of people will develop symptoms, but the risk of severe illness increases with age and those with weakened immune systems.
  • Symptomatic individuals may develop a mild febrile illness up to 14 days following exposure (e.g. fever, headache, myalgias/arthralgias, etc). Some individuals may develop neuroinvasive disease accompanied by neurological symptoms (e.g. meningitis, encephalitis, paralysis).

Testing and Management

  • Testing for WNV infection is indicated in individuals with clinically compatible signs/symptoms of infection AND relevant exposures (e.g. noted mosquito bites, travel to or residence in an endemic area with ongoing WNV transmission) especially during the summer months
  • Testing information can be found on the Public Health Ontario West Nile Virus – Serology and PCR webpage
    • Effective July 2, 2025, submission of the new Vector-borne and Zoonotic Virus Testing Intake Form is mandatory, along with the General Test Requisition when requesting specific vector-borne or zoonotic virus tests.
  • Management is primarily supportive, there is no specific treatment, medication, or vaccine for WNV

Reporting

WNV is reportable to the Medical Officer of Health under Ontario Regulation 135/18 and amendments of the Health Protection and Promotion Act. If you have a suspect or confirmed case of WNV, contact Public Health by the next business day:

  • Call 905-688-8248 ext. 7330 (Mon-Fri, 8:30 a.m. to 4:30 p.m.)
  • Toll free: 1-888-505-6074
  • After-hours: 905-984-3690

Prevention Advice for Patients

  • Wear long-sleeved shirts, long pants, socks and shoes when outdoors 
  • Use a Health Canada-approved mosquito repellent containing DEET or Icaridin
  • Drain areas of standing or stagnant water regularly, including bird baths, old tires, pails, toys and wheelbarrows
  • Ensure all windows and doors in your home have screens that are in good condition

More Information

  • West Nile Virus – Niagara Region Public Health
  • Ontario Vector-Borne Disease Tool - Public Health Ontario
  • West Nile Virus - For Health Professionals - Government of Canada
 
 
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Niagara Region Public Health
1815 Sir Isaac Brock Way, Thorold, L2V 4T7
Tel: 905-688-8248 • Toll Free: 1-888-505-6074
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