West Nile Virus

West Nile Virus is transmitted through the bite of an infected mosquito. It primarily affects humans, horses, and birds, but can also infect other animals.

Please consider vaccination or a booster for your horses. The best time to do this is before the start of mosquito season in your area.

Symptoms of the virus in equines

The virus causes an inflammation of the brain. West Nile Disease can cause a range of symptoms, from mild to severe. The most common signs include:  

• Fever
• Problems with coordination, especially in the rear legs
• Weakness, muscle twitching, lying down with the inability to rise, seizures
• Drooping lips and lip smacking, head drooping, grinding teeth
• Hypersensitivity to sound or touch

Currently, there is no specific treatment for West Nile Disease. Your veterinarian can give your horse supportive care. West Nile Disease has a case fatality rate of 33% in horses that are showing clinical signs. Prevention is the best way to avoid it.

Oregon Equine Cases (2025)

• 1 horse in Lake County

Protect your horses through immunization

The virus is endemic in Oregon and it is important to vaccinate your horses. The American Association of Equine Practitioners (AAEP) recommends West Nile Vaccine be incorporated into the core vaccines given to horses annually. Following the initial series, annual boosters—given two to four weeks before the mosquito season begins—will provide adequate protection. Consult your veterinarian for more information on immunizing your horses against West Nile Virus.

If you suspect your horse is infected with West Nile Virus

If your horse is sick or has neurologic problems, it should be evaluated by your veterinarian. Your veterinarian can rule out other diseases that may cause similar signs such as Equine Herpes Virus-1, Equine Protozoal Myeloencephalopathy, Eastern or Western Encephalomyelitis, and Rabies. Your veterinarian can collect a blood sample and submit it for testing to determine if your horse has an infection with West Nile Virus.

West Nile Virus in humans

Humans can only get the virus from the bite of an infected mosquito; the disease does not spread from other animals to humans, or from person to person. The illness causes no symptoms in 80 percent of those who are infected. Of those who develop an infection, most are mild, with fever and flu-like symptoms. About one in 150 develops severe illness, including coma, convulsions, paralysis and vision loss. The most severe cases, as well as most deaths, occur in those over 55 who have conditions such as diabetes or hypertension or chronic illness.

Minimize the threat of exposure to West Nile Virus

• In addition to vaccinating your horses, the best way to minimize the threat of West Nile for your and your horses is to control mosquito populations and prevent exposure to them.
• Eliminate sources of standing water that can serve as a breeding ground for mosquitoes, including children's wading pools, old tires, buckets, and other containers.
• Clean bird baths and change water in them at least weekly, and more often in the summer.
• Consider avoiding outdoor activities from dusk until dawn or take extra care to use repellent and protective clothing during evening and early morning.
• When possible, wear long sleeves, long pants and socks when outdoors.
• Treating clothes with repellents containing permethrin or DEET will provide extra protection, since mosquitoes may bite through thin clothing. Do not apply repellents containing permethrin or DEET directly to skin. Here are some tips for using DEET safely.