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What is Eastern/Western Equine Encephalomyelitis?

EEE and WEE both produce severe, life-threatening neurologic diseases; however, EEE is the more virulent of the two for horses, with mortality approaching 90% (vs. 50% for WEE).

EEE and WEE cause brain and spinal cord swelling (encephalomyelitis) in horses.

The two viruses are separated geographically:

  • EEE most commonly occurs in the southern and eastern parts of North America.
  • WEE occurs more in the west, although both can occur outside their normal geographical ranges.

Both EEE and WEE occur most frequently in late summer to early fall, with overall incidence rates varying from year to year.

What is Eastern/Western Equine Encephalomyelitis?

How is Eastern/Western Equine Encephalomyelitis contracted?


Similar to the West Nile Virus (WNV), EEE and WEE are carried by birds and transmitted to horses (and humans) via the bite of a mosquito, tick, or other bloodsucking insect after it has fed on an infected bird. Direct transmission between horses, or between horses and humans, is unlikely.

Clinical signs and symptoms of Eastern/Western Equine Encephalomyelitis

The incubation period of EEE and WEE viruses is approximately 5–10 days. Clinical signs and symptoms of EEE/WEE in horses include:

Behavioural changes, listlessness, depression

Fever (moderate to high)

Loss or lack of appetite

Sensitivity to stimuli (sound, touch)

Teeth grinding

Central nervous system signs such as head pressing, dementia, circling, and blindness

Prevention and treatment of Eastern/Western Equine Encephalomyelitis

EEE is extremely virulent and kills most—nearly 90%—of infected horses within days. WEE has a lower mortality rate, with approximately 50% of infected horses dying from the disease. Horses that do survive EEE or WEE infection are often left with long-term neurological deficits.

There is no antiviral drug available for EEE or WEE. Treatment for infected horses involves symptom management and supportive care, such as the administration of IV fluids and/or corticosteroids to minimize swelling in the central nervous system.

Vaccination is the most effective way to prevent EEE and WEE in horses. The EEE/WEE vaccine is an AAEP-recommended core vaccine that protects against both diseases.

Other preventative measures to help minimize the risk of EEE or WEE infection include:

Reducing or eliminating the presence of mosquito-breeding habitats, particularly standing water

Eliminating equine access to natural water sources, or careful management of natural water sources to minimize mosquito populations

Using fly sheets and horse-safe insect repellent on horses outside

Minimizing or eliminating turnout during peak mosquito hours (dusk/dawn)

Deterring mosquitoes from the barn (with screens or fans, for example)

Learn more about how

VETERA® vaccines can help protect your horse.

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