Medical Advisory: First Human Case of West Nile Virus (WNV) 2025

Issued by: Dr. Hamidah Meghani, MPH, MMgt, FRCPC, Public Health Physician
Date: August 21, 2025

Niagara Region Public Health (NRPH) has reported its first laboratory-confirmed human case of West Nile virus (WNV) infection of 2025. NRPH requests that health care professionals consider WNV illness in patients that present with compatible symptoms and recent exposure to mosquitos.

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. Specimens received without the appropriate forms are subject to cancellation.
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