Eastern and Western Equine Encephalomyelitis

virusesinfectious diseasescore vaccines
Written By Anvia Hampton
Dr. Bruce vaccinating a horse

Learn about EEE and WEE and why vaccination is your horse’s best protection from these serious diseases.

Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE) are mosquito-borne diseases with catastrophic consequences for infected horses. These viral diseases affect the central nervous system and cause both encephalitis (inflammation of the brain) and encephalomyelitis (inflammation of the brain and spinal cord).

Horses contract these viruses when bitten by mosquitoes that have fed on infected birds, the most common reservoirs for EEE/WEE. They’re zoonotic diseases — they affect humans and animals — but both species are dead-end hosts: Equine and human patients cannot spread disease once infected.

As their names suggest, EEE is most prevalent east of the Mississippi River and in the Midwest and Southeast (including Oklahoma and Texas) while WEE is most prevalent in Western states.

A reportable disease in New York (veterinarians must inform officials when cases are diagnosed), the state confirmed five equine EEE cases in 2022, all in unvaccinated horses. While there have been no reported equine WEE cases in the U.S. since 2004, the virus remains prevalent in mosquitoes and birds in Western states.

EEE has a high mortality rate (up to 90%) in horses and can prove fatal within two to three days of a horse developing clinical signs. WEE has a lower mortality rate (40 to 50%), but both often result in long-lasting neurologic impairments in survivors.

Clinical Signs

Both diseases have similar clinical signs, including:

  • Fever;

  • Anorexia;

  • Lethargy and/or depression;

  • Behavioral changes;

  • Ataxia (incoordination);

  • Muscle twitches;

  • Inability to swallow;

  • Central nervous system dysfunction, including impaired vision, compulsive circling, head pressing, and seizures; and

  • Recumbency (the inability to stand).

Diagnosis

Veterinarians typically use blood tests to diagnose mosquito-borne encephalitis, but they can also use cerebrospinal fluid samples.

Treatment

EEE and WEE have no cure. Still, veterinarians can use supportive care — including administering intravenous fluids and anti-inflammatories — to alleviate clinical signs and make horses as comfortable as possible in hopes they will recover.

Prevention

Since mortality rates are high and treatment options are limited, prevention is key and starts with vaccination.

The American Association of Equine Practitioners (AAEP) considers EEE/WEE core vaccines, meaning all horses should receive boosters at least annually (they recommend additional boosters in areas with a longer mosquito season). For previously unvaccinated equids or those with unknown histories, they recommend starting fresh: Administer the initial two-dose series over three to six weeks before starting annual boosters.

There are special considerations for vaccinating foals and pregnant mares, which we are happy to discuss with you.

Additionally, owners can reduce horses’ mosquito exposure by:

  • Emptying, cleaning, and refilling water buckets on a regular basis and eliminating other standing water sources (prime mosquito breeding ground);

  • Using equine fly control, from mosquito repellant sprays to and fly sheets, masks, and boots;

  • Bringing horses inside during mosquitoes’ most active periods (generally around dawn and dusk); and

  • Using fans to keep the air moving, as mosquitoes aren’t strong fliers.

Please contact us to schedule an appointment if your horse needs an EEE/WEE vaccination or booster.

Anvia Hampton
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