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Vector-Borne Equine Encephalitides

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Equine encephalitis viruses—Eastern, Western, and Venezuelan equine encephalitis

(EEE, WEE, VEE) and West Nile virus (WNV)—are spread by infected mosquitos and

can cause severe brain inflammation in equids (such as horses and donkeys) and

people. These viruses are widespread in birds and rodents, making them reservoirs

for disease. Unvaccinated horses are particularly susceptible.
EEE is commonly found in the Eastern United States and Canada, although cases

have been identified further west in recent years. WEE has not been detected in the

United States since the late 1990s. VEE is found in Central and South America and

occasionally in Mexico. The last outbreak in the United States was in 1971. WNV was

introduced into the United States in 1999 and is now widespread in certain parts of

the country.

What To Look For

Eastern, Western, and Venezuelan equine encephalitis viruses often affect the

nervous system. Clinical signs can be non-specific. Look for:

Fever

Severe depression, known as “sleeping sickness”

Behavior changes

Impaired vision

Circling or head pressing

Muscle twitches

Inability to swallow

Paralysis

Convulsions

Death

Signs of West Nile virus infection vary and can include:

Fever

Incoordination

Hind-end weakness

Depression

Anorexia

Muscle tremors

Teeth grinding

Inability to swallow

Head pressing

Excessive sweating

Behavior changes
Down and unable to rise

How To Prevent This Disease

To help prevent this disease from spreading, here's what you can do:

Vaccinate your horses every year, more frequently in high-risk areas.

Vaccination, repeated at least annually, effectively prevents disease. Ask your

veterinarian about boosters if your horses live in or are traveling to an area

with year-round mosquito populations or an area that has previously reported

cases of Eastern or Western equine encephalitis. Vaccination for Venezuelan

equine encephalitis may be necessary for horses in higher-risk areas, such as

near the Mexican border. Consult with your veterinarian to make risk-based

decisions.

Reduce your horse’s exposure to mosquitoes. Eliminate standing water on

your property and place fans inside where horses rest. Limit outdoor activities

at dawn and dusk and use equine-approved mosquito repellent.

Report dead wild birds, especially crows, jays, magpies, and ravens, to your

local health department.

How It Is Treated

Horses should be given supportive care based on the severity of their symptoms.

Report Signs of Animal Disease

Producers or owners who suspect an animal disease should contact their

veterinarian to evaluate the animal or herd. Find an accredited veterinarian.

Animal health professionals (veterinarians; diagnostic laboratories; public health,

zoo, or wildlife personnel; and others) report diagnosed or suspected cases of

nationally listed reportable animal diseases to APHIS District Offices and to the State

animal health official as applicable under State reporting regulations. 
Controlling Equine Encephalitis

Vaccination Guidelines for Veterinarians

Core Vaccination Guidelines Eastern Western (American Association of Equine

Practitioners)

AAEP Core Vaccination Guidelines West Nile Virus (American Association of

Equine Practitioners)

Annual Testing and Case Summary Reports

Each year, APHIS prepares a summary of equine encephalitis cases in horses by

State and county.

Eastern, Western, and Venezuelan Equine Encephalitis

2022 Reported Equine Cases of Eastern Equine Encephalitis as of January 11,

2022

2020 Summary of Eastern Equine Encephalitis Cases in the United States

2019 Summary of Eastern Equine Encephalitis Cases in the United States

2018 Summary of Eastern Equine Encephalitis Cases in the United States

2017 Summary of Eastern Equine Encephalitis Cases in the United States

2016 Summary of Eastern Equine Encephalitis Cases in the United States

West Nile Virus

2022 Reported Equine Cases of West Nile Virus as of January 11, 2022

2020 Summary of Equine West Nile Virus Cases in the United States

2019 Summary of Equine West Nile Virus Cases in the United States

2018 Summary of Equine West Nile Virus Cases in the United States

2017 Summary of Equine West Nile Virus Cases in the United States

2016 Summary of Equine West Nile Virus Cases in the United States

To request copies of previous years' reports, send an email to aphisweb@usda.gov.

More Information
Fast Facts: Equine Encephalitis (The Center for Food Security and Public Health)