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Western Equine Encephalitis (WEE)


  • Western equine encephalitis (WEE) is a viral, vector-borne disease that can cause inflammation of the brain.
  • Mosquitoes transmit the virus from birds to horses and humans.
  • There is no cure for WEE, but the prognosis is fair.
  • Vaccination and good vector management practices are important for protecting horses from WEE.

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What is Western equine encephalitis?

Western equine encephalitis (WEE), is a viral, vector-borne disease that circulates in horses and humans in North and South America and causes inflammation of the brain. The WEE virus is related to the viruses that cause Eastern and Venezuelan equine encephalitis.

Outbreaks of WEE virus have resulted in significant numbers of deaths in horses and humans. Survivors can have permanent neurological impairment. The virus circulates naturally in birds and is transmitted to horses and people primarily by the mosquito vector Culex tarsalis. Horses and humans infected with WEE are not contagious to other animals or people.

What are the clinical signs of Western equine encephalitis?

The clinical signs of WEE include depression, anorexia, lethargy, and fever. Neurologic signs can include tremors, convulsions, paralysis, weakness, ataxia, incoordination, and recumbency.

How is Western equine encephalitis diagnosed?

Western equine encephalitis is diagnosed through bloodwork. Testing can include enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR) on cerebrospinal fluid (CSF) samples to identify the WEE virus.

How is Western equine encephalitis treated?

There is no cure for WEE. Treatment largely consists of supportive care for horses with clinical signs, including intravenous (IV) fluids and anti-inflammatory medications.

What is the prognosis for Western equine encephalitis?

The prognosis for horses with WEE is fair. The mortality rate is 20-40%, significantly lower than for Eastern equine encephalitis (EEE). Long-term deficits may occur in horses that recover from WEE.

How can Western equine encephalitis be prevented?

The American Association of Equine Practitioners (AAEP) recommends the WEE vaccine as a core vaccine. Horses should be kept up to date on vaccinations, which are usually administered yearly. Work with your veterinarian to determine an appropriate vaccination schedule for your horse(s).

Good vector control is also important to protect your horse from WEE. Use insect repellents and keep horses in at night when mosquitoes are most active. Eliminate mosquito breeding grounds such as standing water, brush piles, and old tires. Stock tanks and water troughs with mosquito-feeding fish.

For more information

AAEP Disease Fact Sheet - WEE