Human West Nile virus case and a horse with EEEV in Ottawa

Ottawa Public Health (OPH) is recommending that residents protect themselves against mosquito bites to reduce the risk from West Nile virus (WNV) and Eastern Equine Encephalitis virus (EEEV). Ottawa Public Health is reporting the first human case of WNV infection in an Ottawa resident for the 2025 season and has confirmed that a horse in Ottawa recently tested positive for EEEV.

Both diseases spread to humans through mosquito bites and anyone who lives, works, or participates in outdoor activities where mosquitoes are present can be at risk. Protecting yourself from mosquito bites is the best way to prevent WNV and EEEV.

Ottawa Public Health urges residents to protect themselves and their families from mosquito bites when spending time outside at home or in the woods by:

• Applying a Health Canada-approved mosquito repellent containing DEET or icaridin to exposed skin and clothing 

• Wearing light-coloured, tightly woven, loose-fitting clothing, such as long pants, a long-sleeved shirt, shoes and socks, to protect exposed skin 

• Making sure all windows and doors in your home have screens that are in good condition 

• Removing, or emptying once per week, standing-water sites around your home, such as bird baths, toys, flower-pot saucers, swimming-pool covers, old tires, wheelbarrows, buckets, and cans 

• Keeping all openings to rain barrels covered

West Nile virus is an infection spread primarily by the northern house mosquito (Culex pipiens) that, in a small number of cases, can cause serious illness. Most people will not develop any symptoms if infected, but about 20 per cent may experience flu-like symptoms, such as:

• Fever
• Headache
• Muscle aches
• Rash

The risk of more serious illness - occurring in less than one per cent of infections, in which WNV impacts the central nervous system - increases with age. Older adults, the elderly and people with weakened immune systems are at higher risk.

In Ontario, as of August 20, 2025 (including data up to August 17), there have been 11 human cases of WNV. In 2024, there were 13 human cases of WNV in Ottawa and 87 in Ontario.

Eastern Equine Encephalitis virus is normally transmitted between wild birds and mosquitoes but can occasionally infect horses and, rarely, humans through infected mosquitoes’ bites, similar to the transmission of WNV. Humans do not get infected from a horse or another human. Whereas mosquitoes that carry WNV are most commonly found around homes in urbanized areas, the mosquitoes that carry EEEV are usually found close to swampy hardwood forests.

Human infections with EEEV have been rare in Ontario. Historically, there have been four human cases of EEEV in Ontario, the most recent reported in 2024 in Ottawa. Although rare, the disease is of concern because, like WNV, it can cause a serious neurological disease and even death.

Visit OttawaPublicHealth.ca/WestNileVirus to learn more about mosquito-borne diseases. Residents can also connect with OPH on Facebook, Instagram, Bluesky and YouTube.