West Nile Virus in Washington: What you need to know this season
West Nile virus (WNV) isn’t new to Washington, but it returns each year with mosquito season and brings renewed risks to both people and animals. Recently, the Washington State Department of Agriculture (WSDA) announced the first equine case of WNV in Yakima County this year, reminding us that even late summer carries the potential for virus activity.
What is West Nile Virus?
West Nile virus is a mosquito-borne disease that primarily cycles between birds and mosquitoes. Infected mosquitoes can also transmit the virus to horses, humans, and occasionally other animals—but horses account for about 96% of all reported non-human cases.
Infected animals may show signs of illness including fever, weakness, stumbling, behavioral changes, or difficulty standing. Horses are particularly vulnerable and can develop serious neurological symptoms, sometimes resulting in death or long-term issues like coordination problems. While effective vaccines are available for horses, unvaccinated animals or those that have missed a booster may face significant risks.
Symptoms usually appear within two to fourteen days of being bitten by an infected mosquito.
Where and when WNV appears in Washington
West Nile virus was first detected in Washington mosquitos in 2002, with the first confirmed human cases in 2006. The virus typically emerges during mosquito season, which runs from late spring through early fall. It is most often detected in the eastern and south-central parts of the state.
This year, mosquito pools testing positive for WNV have already been confirmed in Benton, Grant, and Yakima counties. These detections serve as an early warning, even if no human cases have been reported yet in 2025. Since mosquitoes remain active until the first hard freeze, it’s important to stay vigilant into the fall.
Recent case in Yakima County horse
On September 11, 2025, the Washington Animal Disease Diagnostic Laboratory (WADDL) confirmed a case of West Nile virus in a yearling thoroughbred colt in Yakima County. The horse developed neurological symptoms before testing positive. As horses lack natural immunity to WNV, they are at high risk for severe illness without vaccination. The American Association of Equine Practitioners considers WNV to be a core vaccine requiring an initial vaccination with a booster 3-6 weeks later and then annual vaccines thereafter. In this recent case, the booster wasn’t administered in the recommended time frame, likely decreasing vaccine efficacy. In cases where clinical signs develop, the fatality rate can reach up to 33 percent, and survivors may continue to suffer from coordination and mobility issues for months.
Dr. Amber Itle, Washington’s State Veterinarian, encourages animal owners to remain cautious:
“Even as temperatures begin to cool in the evenings, mosquitoes that carry WNV and other mosquito-borne diseases remain active until a hard freeze occurs. It is important for all animal owners to continue taking precautions to protect animals and themselves from mosquito exposure.”
What animal owners should do
Vaccination is the most effective way to protect horses from West Nile virus. Farm owners should also remain alert for signs of illness in other livestock during mosquito season. In addition to vaccination, animal owners should:
• Keep horses indoors or use fans during dusk and dawn, when mosquitoes are most active
• Eliminate standing water in troughs, buckets, and containers to reduce mosquito breeding
• Use insect repellents approved for animals and appropriate for the species
• Monitor animals for signs such as stumbling, weakness, or sudden behavioral changes, and contact a veterinarian immediately if symptoms arise
Protecting people: Human health matters too
While animals can be early indicators of West Nile Virus (WNV) in an area, people can be affected too. The good news is that most people who get WNV don’t feel sick at all. If symptoms do occur, they’re usually mild—like the flu.
Serious illness is rare, but in a small number of cases, WNV can cause brain or spinal cord swelling. This can lead to confusion, tremors, paralysis, or—very rarely—death. Older adults and those with weakened immune systems are more at risk. Some people may have lasting symptoms like fatigue or memory problems after recovering.
There’s no vaccine or specific treatment for WNV in people, so prevention is key. To protect yourself:
- Use EPA-registered insect repellent
- Wear long sleeves and pants, especially at dawn and dusk
- Make sure window and door screens are secure
- Dump standing water around your home
- Stay updated on local mosquito activity and health alerts
WNV is not spread person to person—only through mosquito bites. If you feel sick after being outdoors, contact your doctor and mention your exposure. Early care can help manage symptoms and prevent complications.
Birds and WNV
Birds are a key part of the WNV cycle. While most birds infected with WNV survive, some species—like crows, magpies, and jays—are more vulnerable and may die from the virus. Dead birds can serve as early indicators of local virus activity. If you find a dead bird that appears otherwise uninjured, contact your local health department to ask whether it should be reported or tested. These reports help officials track virus movement and guide mosquito control efforts.
Image courtesy San Gabriel Valley Mosqito & Vector Control District; https://www.sgvmosquito.org/west-nile-virus-9a35cae
The bigger picture: Why it matters
West Nile virus is a powerful example of the link between animal and human health. The virus often circulates in birds and mosquitoes long before it reaches people or livestock. This makes animal illness and insect surveillance important early warning systems for communities.
As climate patterns shift and warm seasons extend, vectors like mosquitos can become established in new geographic areas allowing mosquito-borne illnesses like WNV to becoming more widespread. Washington’s veterinary and public health teams continue to monitor virus activity, respond to outbreaks, and provide guidance to help reduce risk.
Stay updated
For current information on WNV in Washington: