Eastern equine encephalitis (EEE), otherwise known as Triple E or sleeping sickness, is a rare and lethal virus in horses that causes inflammation of the brain.

EEE is transmitted through mosquito bites and attacks the central nervous system (CNS). It is most prevalent in the eastern regions of North America. [1]

The prognosis for this encephalitis is very poor, especially once neurological symptoms have developed. While there is no cure for EEE, supportive treatment may be administered to make the horse comfortable and help them fight infection. [2]

If you think your horse may have contracted the EEE virus, contact your veterinarian immediately for examination and diagnosis. Prompt intervention is critical for survival and for protecting other horses in the area.

Eastern Equine Encephalitis (EEE)

The terms encephalitis and encephalomyelitis are often used interchangeably when referring to the Eastern Equine Encephalitis virus.

Encephalitis refers to inflammation of the brain, whereas encephalomyelitis refers to inflammation of the brain and spinal cord. [3]

Horses are affected by several encephalitis viruses transmitted by insects that can lead to nervous system dysfunction. These viruses include West Nile, Western Equine Encephalitis (WEE) and Venezuelan Equine Encephalitis (VEE).

The clinical signs of EEE and VEE are typically very severe compared to the other viral encephalitides. [4]

Clinical Signs

During a viral epidemic of EEE, horses tend to be the first animals to show signs of infection.

After a horse becomes infected with EEE, there is a brief incubation period lasting two to three weeks before symptoms begin to appear. These symptoms are usually severe and often result in death within a few days. [4][5][6][7]

Clinical signs of EEE include:

  • Neurological signs (impaired vision, wandering, head pressing)
  • Ataxia (poor coordination and irregular gait)
  • Mild fever
  • Dullness and depression
  • Tremors
  • Inability to urinate or defecate
  • Seizures
  • Paralysis
  • Death

In the event of an encephalitis outbreak among horses or livestock, a viral infection should be investigated as the primary cause. The presence of mosquitoes in the area impacts both the investigation of the outbreak and the diagnosis of the infection. [3]

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Where is the EEE Virus Found?

The EEE virus is predominantly found in the Great Lakes regions and the Atlantic coast states of North America. There have been documented cases in the Caribbean islands as well. Transmission of the virus primarily takes place in regions with freshwater swamps. [6][8]

Cases of of EEE are commonly reported in regions of the United States, including Florida, Georgia, New Jersey, New Hampshire, Maine, and Massachusetts.

In Canada, equine fatalities caused by the EEE virus have been reported in Alberta; however, they are more prevalent in Ontario and the eastern provinces.

Viral Transmission

The EEE virus circulates between avian species and mosquitoes, whereby the latter become infected when they feed on birds carrying EEE. Horses, classified as ‘dead-end hosts’, do not harbor high levels of the virus in their blood and are unable to transmit it to other animals or insects. [9]

The mosquito species typically responsible for outbreaks in North America is Culiseta melanura, although there are more than 20 mosquito species that may transmit the virus during the spring and summer seasons. [6]

During winter, the mosquito’s inactive season, birds, reptiles, and amphibians serve as reservoir hosts. However, most of these animals are not highly susceptible to the EEE virus, and they may not develop an infection. [8]

The C. Melanura mosquito is believed to sustain the circulation of the EEE virus among wild birds until it lands and feeds on a susceptible dead-end host, such as a horse.

EEE Virus in Humans

While rare, humans can contract the EEE virus from mosquitoes. Both humans and horses are dead-end hosts, so they cannot transmit the virus to other mosquitoes or animals. In humans, the infection carries a significant risk of mortality, and there is no licensed vaccine available. [6][8]

Symptoms of EEE infection in humans resemble those of the flu, including fever, chills, muscle aches, and vomiting. In severe cas