Eastern equine encephalomyelitis


Eastern equine encephalomyelitis is a viral disease caused by an arbovirus Read the content of the note 1 . Infected mosquitoes can spread the virus to other animals and, in rare cases, to people.

The virus that causes Eastern equine encephalomyelitis is maintained in nature in a transmission cycle between birds and mosquitoes that act as reservoirs for the virus. In Québec, cases are typically seen from June to October.

At-risk animals

Equids (such as horses) and humans are the species most commonly affected by Eastern equine encephalomyelitis.

Many bird, mammal and reptile species, both domesticated and wild, can also be infected. Most will show no signs of illness.

Clinical cases of the disease have been reported in pigs, rodents and white-tailed deer.

Signs of the disease

Animals infected with Eastern equine encephalomyelitis may show signs of the disease.

In horses

Horses that are infected may die suddenly with no clinical signs.

Early symptoms include fever and anorexia and may be followed by progressive neurological symptoms:

  • Depression
  • Lethargy, drowsiness
  • Hyperexcitability
  • Aggression
  • Blindness
  • Weakness
  • Paralysis of the face and/or tongue
  • Spinning, circling
  • Lack of coordination, followed by paralysis of one or more limbs
  • Inability to stand up, lying on its side
  • Convulsions
  • Coma
  • Death

Between 75% and 95% of infected horses die within two to three days after the onset of symptoms. Many survivors will exhibit residual neurological signs.

In birds

Most birds will show no clinical signs of the disease.

However, the virus can cause mortality in some species of birds such as pigeons, pheasants and emus. Symptoms may include:

  • Severe diarrhea
  • Fever
  • Exhaustion
  • Partial or total paralysis of one or both legs
  • Involuntary circular movements
  • Death

In other animals

The virus can also infect other species of animals, but the vast majority will fight off illness and show no symptoms.

Clinical signs of the disease in these other animals can be similar to those of many other diseases and include fever, weakness, lack of coordination, muscle spasms, convulsions and behavioural changes.

Transmission and incubation period

Eastern equine encephalomyelitis is spread by the bite of a mosquito carrying the virus. Mosquitoes typically become virus carriers by feeding on infected birds, and can transmit the virus by biting birds, humans and other animals.

If an animal is infected, that can indicate that there are mosquitoes carrying the virus in the immediate environment, assuming the animal has not travelled out of the area.

In rare cases, the virus can also be spread through contact with sick or dead animals. Humans can be infected if they handle the internal organs, brain or cerebrospinal fluid of an infected animal without proper protection.

Infected horses do not develop a high enough level of virus in their bloodstream to transmit the virus to mosquitoes. This means they cannot transmit the virus to other horses around them or to humans.

Time between the virus entering the animal’s body and the appearance of symptoms

The incubation period for the disease is three to seven days.


There is no specific treatment for infection with the Eastern equine encephalomyelitis virus. Symptomatic treatment is used to alleviate symptoms.

Protection and prevention

Vaccination is an effective way to prevent Eastern equine encephalomyelitis in horses. In Québec, horses must be vaccinated annually in the spring before the peak risk season, by early May at the latest. Vaccinated horses will be protected until the first hard frost. If a horse is being vaccinated for the first time, the protection period is calculated from the time it receives its booster. Always follow the manufacturer’s instructions.

It is critical to control insects that serve as disease vectors.

Remove any stagnant water from your property, and remove any items where stagnant water can accumulate, such as containers, buckets, old tires and garbage, as these are ideal breeding sites for mosquitoes.

Troughs should be cleaned weekly.

Keep animals indoors during peak periods of mosquito activity, particularly dusk and dawn. Stables should be covered in mosquito netting to prevent insects from coming in.

Use appropriate insecticides or insect repellents. Always follow the manufacturer’s instructions.

In humans

Humans can contract the disease if they are bitten by a mosquito carrying the virus.

The incubation period is 4 to 10 days. Most people show no clinical signs of illness.

Some people may experience flu-like symptoms such as fever, chills and muscle aches. The illness generally lasts one to two weeks, and most people make a full recovery.

In some cases, infected people may develop meningitis or encephalitis and exhibit the following clinical signs:

  • Headache
  • Vomiting
  • Diarrhea
  • Disorientation
  • Convulsions
  • Drowsiness
  • Coma

Approximately one third of people who develop encephalitis die within 2 to 10 days of the onset of symptoms. Many survivors of the brain infection suffer permanent neurological damage.

The best way to protect yourself from diseases spread by mosquito bites is to avoid being bitten. Visit the Protecting yourself from mosquito and tick bites page to learn more about preventing bug bites.


In Québec, the réseau équin (French only) conducts surveillance of the Eastern equine encephalomyelitis virus to reduce the risk of infection in horses and humans.

For up-to-date information about cases of the disease in Québec and across Canada, visit the Canadian Animal Health Surveillance System This hyperlink will open in a new window.’s Equine Diseases Dashboard.

Last update: January 8, 2024


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