APHIS Factsheet

Veterinary Services January 2008

Eastern Equine 

Encephalomyelitis

Eastern equine encephalomyelitis (EEE) is a 

mosquitoBborne viral disease of all equine species 

such as horses, asses, and zebras. After infection, 

equines may suddenly die or show progressive central 

nervous system disorders. The rapidity of deterioration and eventual outcome of infection vary among 

individual horses. The equine mortality rate due to 

EEE ranges from 75 to 90 percent. 

 Humans can also contract this disease. Healthy 

adults who become infected by the virus may experience fluBlike symptoms such as high fevers and 

headaches. The young, the elderly, and people with 

weakened immune systems can become severely ill or 

die from this disease. 

 More information regarding the human form of 

EEE can be found on the Centers for Disease Control 

and Prevention Web site at www.cdc.gov

History

Eastern equine encyphalomyelitis virus (EEEV) is 

thought to have been the cause of EEE in North 

American horses since 1831. However, the virus 

itself may have been present in its endemic form long 

before that. The virus did not receive its name until 

a major outbreak occurred in horses in coastal areas 

of Delaware, Maryland, New Jersey, and Virginia in 

1933.

 Additional outbreaks occurred in Virginia and 

North Carolina in 1934 and 1935 respectively. Mosquitoes were first determined to be potential carriers of 

EEEV in 1934. Since then, a number of studies have 

shown that various mosquito species of Aedes and

Culex could become infected with and transmit EEEV 

from one vertebrate to another. 

 It was during the 1935 outbreak that birds were 

considered to be a possible reservoir host for the 

virus, but it was not until 1950 that the first virus 

isolation was made from a wild bird proving that to 

be true. Subsequent studies have shown that many 

birds, including almost all passerine species (i.e., jays, 

blackbirds, warblers, finches, and sparrows), are 

susceptible to EEEV infection. 

 Because outbreaks of EEE are infrequent, the 

disease has a significant economic and social impact 

once a specific area has been identified. When the 

disease appears in an area for the first time, there is a 

loss of horses and/or poultry. The area may also

experience an increase in human morbidity and 

mortality. 

Clinical Signs

Equines infected with EEE may show one or more of 

the following signs:

• Fever 

• Depression 

• Loss of appetite 

• Weakness 

• Central nervous system disorders (lack of coordi- 

 nation, chewing movements, head pressing, 

 "sawhorse" stance, circling, paddling motion of the 

 limbs, and convulsions)

• Irritability and aggressiveness towards handlers

• Blindness

• Excitability

• Abnormal sensitivity to light and sound 

 In some cases, horses infected with EEE may 

show no clinical signs before dying. 

Confusion with Other Diseases

The clinical signs of EEE can be confused with those of 

other diseases that affect the central nervous system. 

These include Venezuelan equine encephalitis, Western equine encephalitis, West Nile equine encephalitis, 

African horse sickness, rabies, tetanus, and bacterial 

meningitis. EEE might also be mistaken for toxic poisoning. Definitive diagnosis can be made by isolating 

the virus in a laboratory or by testing blood for the 

presence of antibodies to the virus. 

How it Spreads

The virus that causes EEE is transmitted primarily by 

mosquitoes that bite an infected animal and then bite 

and feed on another animal or human. The speed with 

which the disease spreads depends on the 

density of mosquito populations.

Control Measures

EEE probably will never be eradicated from the United 

States because of the reservoirs that exist in so many 

areas throughout the country. Therefore, continual 

vigilance and conscientious immunization programs will 

always be necessary. 

 Monovalent, bivalent, or trivalent horse vaccines 

containing EEE are available. Horse owners should 

choose such vaccines depending on the prevalence or 

likely occurrence of the disease.

 General control measures aimed at reducing 

mosquitoBcarrying populations will significantly 

diminish but not eliminate the risk of equine infection. 
United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, 

national origin, age, disability, and where applicable, sex, marital 

status, familial status, parental status, religion, sexual orientation, 

genetic information, political beliefs, reprisal, or because all or part 

of an individual’s income is derived from any public assistance 

program. (Not all prohibited bases apply to all programs.) Persons 

with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) 

should contact USDA’s TARGET Center at (202) 720–2600 (voice 

and TDD). To file a complaint of discrimination, write to USDA, 

Director, Office of Civil Rights, 1400 Independence Avenue, S.W. , 

Washington , D.C. 20250–9410 , or call (800) 795–3272 (voice) or 

(202) 720–6382 (TDD). USDA is an equal opportunity provider and 

employer. 

Report Suspicious Signs

Owners of equines have an essential role in preventing EEE from spreading. Horse owners are encouraged to vaccinate their animals and put safeguards in 

place that prevent animals= exposure to mosquitoes, 

as well as report any suspicious signs of EEE in 

animals to a veterinarian.

Additional Information

For more information, contact

USDA, APHIS, Veterinary Services

4700 River Road, Unit 41

Riverdale, MD 20737–1231

Telephone (301) 734–8093

Fax (301) 734–7817

or visit our Web site at 

www.aphis.usda.gov/animal_health/index.shtml