Florida Department of Agriculture and Consumer Services

Eastern Equine Encephalitis

Program Resources

The arthropod-borne viruses, also known as arboviruses, include a group of viruses that can be transmitted to humans and horses by infected mosquitoes.

The Eastern equine encephalomyelitis (EEE) virus 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. Arboviruses may cause encephalomyelitis (inflammation of the brain and spinal cord) and even death.


Mosquitoes become infected when they feed on infected birds, which circulate high levels of the virus in their blood for a period of time. Transmission to humans requires mosquito species capable of creating a "bridge" between infected birds and uninfected mammals. These mosquito species include some Aedes, Coquillettidia and Culex species.

Infected mosquitoes can then transmit the virus to humans and animals while biting to take a blood meal. The virus is located in the mosquito’s salivary glands. During blood feeding, the virus may be injected into humans or animals where it may multiply, causing illness.

The disease is not directly transmitted between horses, from birds to horses, or from horses to humans.

Clinical Signs

The incubation period for EEE is approximately one week. Early signs are generally subtle and are often undetected; they involve fever (mild or severe), depression, stiffness and lack of appetite. These signs may last up to five days during which viremia (presence of the virus in the blood) occurs. Signs of neurological disease usually follow.

Horses may become profoundly depressed (therefore the name "sleeping sickness") or exhibit abnormal behavior such as propulsive walking, head-pressing, aggressiveness, circling or hyperexcitability. Some horses show signs of facial nerve dysfunction such as blindness, head tilt, and paralysis of the muscles of the face, mouth and throat. Some horses become comatose, seizure or die suddenly.

Equines infected with EEE may also show one or more of the following signs: fever, depression, loss of appetite, weakness, central nervous system disorders (lack of coordination, chewing movements, head pressing, "sawhorse" stance, circling, paddling motion of the limbs, and convulsions), irritability and aggressiveness toward handlers, blindness, excitability, and abnormal sensitivity to light and sound in some cases, although some horses infected with EEE may show no clinical signs before dying.


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.

Blood samples should be collected by a veterinarian and sent for diagnostic testing. The EEE IgM capture ELISA test is a specific test that detects acute EEE infection in animals, is usually positive within six days postinfection and can remain positive for up to two months postinfection. A positive EEE IgM capture ELISA test is indicative of recent infection. Unexposed animals vaccinated for West Nile virus will test negative on the EEE IgM capture ELISA test.


Since EEE is a viral disease, antibiotics are ineffective and there are no effective antiviral drugs available for treatment. Therefore, the only treatment available is supportive care with documented mortality rates between 70 and 95 percent.


Vaccines are available for EEE and are highly effective when administered correctly.

Current prevention relies on vaccination of horses and minimizing horse exposure to mosquitoes during the peak mosquito feeding periods at dawn and dusk.

Application of mosquito repellant can also effectively reduce the number of mosquito bites on horses.

Mosquito control efforts to eliminate mosquito breeding sites are also important. Methods to reduce mosquito breeding sites include:

  • Draining unnecessary standing water found in wheelbarrows, tires, etc.;
  • Cleaning water containers (e.g., birdbaths, plant saucers) at least weekly;
  • Scheduling pasture irrigation to minimize standing water;
  • Keeping swimming pools optimally chlorinated and draining water from pool covers; and 
  • Stocking water tanks with fish that consume mosquito larvae (contact local mosquito control for assistance) or using a mosquito “dunk” available at hardware stores.

Regulatory Considerations

Eastern equine encephalitis is a reportable disease. State and local agencies monitor arboviral activity in mosquitoes, sentinel chicken flocks, horses and dead birds.

The Florida Department of Health, in cooperation with several other agencies including the Florida Department of Agriculture and Consumer Services, has implemented an Arboviral Surveillance and Response Plan to monitor arboviral activity throughout the state. The plan provides for a tiered level of response aimed at preventing and controlling certain mosquito populations during times of increased arboviral activity when the risk to human exposure is enhanced.

Suspected or known cases of EEE must be reported to the State Veterinarian’s Office. To report EEE or other reportable diseases, call (850) 410-0900 from 8 a.m. to 5 p.m., Monday through Friday. For after-hours reporting, call 1-800-342-5869 or email  RAD@FreshFromFlorida.com.