Eastern Equine Encephalitis

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Also Known As

 Eastern Equine Encephalomyelitis, EEE


Eastern Equine Encephalitis (EEE) virus tends to occur in outbreak form. First identified in mosquitoes in Louisiana in 1951, the disease is transmitted to horses by mosquitoes that have fed on infected wild birds.

The normal geographical distribution of EEE cases in horses occurs east of the Mississippi River, typically including the coastal states of Alabama, Florida, Georgia, Louisiana, Maryland, Mississippi, New Jersey, North Carolina and South Carolina, although cases have occurred in the midwestern states of Indiana, Michigan and Ohio.

At least one case of EEE has occurred in a horse in California, but the origin of the case was not established.

EEE virus attacks the central nervous system and unvaccinated horses are very susceptible to the infection. The disease appears within five days after a mosquito transmits the virus to the horse. Onset of clinical symptoms are abrupt and infected horses often die within three days.

The fatality rate is 90% or higher and an animal that survives the disease may have brain damage.

Other diseases and poisoning may have similar symptoms. It is important that a speedy diagnosis is made. A veterinarian can take blood samples to send to a diagnostic laboratory.


  • Fever
  • Sleepy appearance
  • Muscle twitches in head, neck, shoulder and flanks
  • Staggering gait
  • Recumbency


EEE occurs in a wide variety of songbirds, including the blue jay, chickadee, cardinal, tufted titmouse and catbird. Mosquitoes that inhabit marshes and wooded wetlands, named Culiseta melanura,transmit the virus from bird to bird and also to horses and people.

Horses get Eastern Equine Encephalitis when they are bitten by a mosquito carrying the EEE virus.

During years of outbreaks, mosquito species other than Culiseta melanura become involved in the transmission cycle. EEE cases in horses usually begin to appear in mid- to late-summer and increase in the fall.

Transmission of EEE from birds to horses usually occurs when the mosquito population is high. Horses are usually the first animals to become infected because they are normally housed in rural areas and have a greater exposure to mosquitoes.

The potential for an outbreak of EEE appears to correspond to the number of unvaccinated horses. It is unusual to see the disease in a horse that has received the initial vaccination series and the proper boosters.


Annual vaccination of all horses and other equines is recommended. Horses vaccinated for the first time must receive an injection of EEE-WEE combined vaccine, followed by a second dose three to four weeks later.

The two-dose series is necessary to establish an effective immune response. If an outbreak occurs in the area, a second booster is advised.

Foals should be vaccinated at 4, 5, and 6 months of age for a total of three doses. Another booster should be given in six months and annually from then on.

In addition to making sure that all equines are vaccinated, horses should be taken in at times when the mosquito population is highest. This usually includes the hours around dawn and dusk. In some cases, horses can be blanketed or covered with protective gear to prevent bites. Insect repellant can be used on horses to prevent bites from mosquitoes.

In all cases, the population of mosquitoes should be controlled as much as possible by following the recommendations of mosquito-control personnel and the agencies in the area where horses are kept.

Any bodies of stagnant water should be drained and water-holding containers such as buckets, tires and other receptacles where water might accumulate should be emptied. Horse troughs and waterers should be flushed often to reduce mosquitoes near paddock areas.

Insecticides for treating mosquito nesting areas around homes and livestock premises should be used on a regular basis.


There is no specific treatment for EEE. Antibiotics are not effective against viruses, and no effective anti-viral drugs have been developed.

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