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Incurable Horse Virus, also referred to as Eastern Equine Encephalitis (or EEE) is a deadly disease caused by an agent alphavirus called Eastern Equine Encephalitis Virus (or EEEV). Incurable Horse Virus is spread by a mosquito species called Culiseta melanura, which typically feed on birds and reptiles. On occasion, one of these mosquitos will infect a horse, or even more rarely, a human.

What does EEE do?

Unfortunately, the onset of EEE is almost guaranteed to be fatal in horses, with survival rates reported to as low as 5%. The virus causes the horses brain to swell severely (encephalitis), which then creates pressure between the brain and the skull, damaging the brain. Because the skull limits the direction in which a brain can swell, the brain is pressed towards the brain stem and spinal cord.

The brain stem is responsible for most vital, motor functions. This kind of pressure limits, and eventually stops, the vital functions of the animal.

Seizures that result in death will occur 48 to 72 hours after the first physical signs of symptoms.

Symptoms of EEE in Horses:

  • Lack of appetite
  • Drooping eyelids
  • Drooping lower lip
  • Depression
  • Blindness
  • Aimlessly wandering and circling
  • Staggering
  • Uncoordinated gate
  • Pressing head into corns
  • Paralysis (sometimes, but not always)
  • Seizures
  • Recumbency
  • Erratic behavior
  • Fever higher than 103 degrees Fahrenheit

EEE in Humans

Luckily for humans, the species of mosquito that carries EEEV tend to prefer the taste of birds, and therefore seldomly bite humans. Among those humans bitten, the large majority may experience flu-like symptoms, and only 5% are likely to develop the associated encephalitis (swelling of the brain).

The prognosis for those who experience encephalitis is poor: one third of these patients dies, and the majority of survivors experience ongoing neurological problems.

Children infected with EEEV are more likely to recover from the neurological problems than adults. Immunocompromised populations, such as young children and the elderly, are more likely to experience death after infection.

Preventing EEEV

There is no EEEV vaccine for humans, but there is one for horses! Veterinarians recommend vaccinating your horse against EEEV. The first vaccination requires a series of two shots, and annual booster shots are required to ensure effectiveness.

You can also make a habit of washing out your horse’s trough regularly, and eliminate other potential breeding ground resources such as standing water (rain gutters, buckets around the barn, old tires sitting around, etc).

For humans, the best prevention method is to avoid infection by avoiding mosquito bites, either by using insect repellant or protective clothing while outside during mosquito season, especially during dawn/dusk, when mosquitos are most active.

How Rare Is it?

On the bright side, this disease is exceedingly rare. To put things into perspective, 2019 had the largest EEE outbreak in half a century, in which the CDC only reported 38 cases. During an average year, the CDC typically reports 4-5 human cases, and 18 veterinary cases, which are mostly horses.

Here at BRD, we hope to help keep you informed and your horses healthy! For additional, helpful information about horse care, join our newsletter.

Williams, C. and Fonseca, D. M. (October, 2020). Questions Regarding Eastern Equine Encephalitis and Horses. Rutgers: New Jersey Agricultural Experiment Station, Horses Publications. Cooperative Extension Fact Sheet FS737.

Centers for Disease Control and Prevention. (December 14, 2021). Eastern Equine Encephalitis Virus: Statistics & Maps. Centers for Disease Control and Prevention.

Minnesota Department of Health. (March, 2018). Eastern Equine Encephalitis Fact Sheet.

Mckay, R. (n.d.) Eastern Equine Encephalitis. University of Florida Health: Large Animal Hospital, College of Veterinary Medicine.

Brennan-Krohn, T. (Oct. 7, 2019). Eastern Equine Encephalitis Virus (EEEV): the Role of Diagnostics. American Society for Microbiology.

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