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Eastern Equine Encephalitis

How is the virus transmitted to horses?

Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then transmit the virus animals, including horses and humans, with a bite. The disease is not directly transmitted between horses, from birds to horses or from horses to humans. For additional information on EEE transmission and disease by the Center of Disease Control EEE Transmission and Disease.

What symptoms can you see in horses?

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. Blood samples should be collected by a veterinarian and sent for diagnostic testing.

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.

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.-5 p.m., Monday-Friday. For after-hours reporting, call 1-800-342-5869 or email:

Is there a vaccine?

A vaccine is available through your local veterinarian. The vaccine consists of a two-dose series given 3 to (no more than) 6 weeks apart. Horses should be vaccinated with both vaccine doses at least 3 weeks prior to the start of mosquito season (June to December). After your horses receive their first two-dose vaccine, you need to have booster shots given 2 to 3 times per year thereafter. Please consult with your veterinarian.

How can horses be protected?

Yes, 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.            

For additional information visit the Florida Department of Agriculture and Consumer Services website on EEE.

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