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Middlesex-London Health Unit

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Eastern Equine Encephalitis Virus

Eastern Equine Encephalitis Virus (EEEV) is a vector-borne disease that can be spread to humans or horses through the bite of an infected mosquito.1 In the past, EEEV has mainly affected horses; however, in recent years there have been a few EEEV-positive mosquitoes identified in Ontario.1 EEEV-positive means that the mosquito was found to carry Eastern Equine Encephalitis Virus. To date, there have been no human cases of EEEV reported in Ontario or Canada.1


How does Eastern Equine Encephalitis Virus affect humans?

Eastern Equine Encephalitis Virus (EEEV) is a virus and is typically found in wild birds.2 Humans or horses bitten by an infected mosquito can get EEEV. The transmission cycle (how the virus is passed) begins when mosquitoes feed on the blood of an infected bird. Once a mosquito bites an infected bird, it becomes infected with EEEV and could possibly transmit the virus to humans or horses through a bite.2

  • Learn how you can protect yourself from getting Eastern Equine Encephalitis Virus by reading about prevention and personal protection.
  • By vaccinating horses, these animals will be protected and the hope is that, as a result, there will be less spread of EEEV to humans.1

What are the signs and symptoms of Eastern Equine Encephalitis Virus?1

If a human becomes ill with Eastern Equine Encephalitis Virus (EEEV), the person will typically not experience any signs or symptoms. If symptoms do occur, they will be very general and may include fever, chills, muscle pain, or weakness. General symptoms do not involve the nervous system and can last for two weeks.

Some people infected with EEEV can develop a more severe form of illness called encephalitis (swelling of the brain), but this type of illness is rare. Symptoms can include: sudden onset of severe headache, fever, drowsiness, loss of appetite, vomiting, or coma. About one third of all patients who develop encephalitis will die from the disease or suffer permanent mental and physical side effects. If you experience any of these symptoms and think you might have Eastern Equine Encephalitis Virus, go to see your doctor right away.

Can all mosquitoes transmit Eastern Equine Encephalitis Virus?

A 'vector' is the term used to describe a mosquito species that can transmit (pass) disease through a mosquito bite. Not all mosquito species are vectors. A ‘non-vector’ or ‘nuisance mosquito’ cannot carry a disease, and therefore cannot transmit disease to humans. Vector mosquito species must first bite an infected bird or animal in order to become infected themselves and able to pass the disease on through a mosquito bite.

What is being done about Eastern Equine Encephalitis Virus in Middlesex-London?

The Middlesex-London Health Unit monitors and controls vector mosquito species in order to reduce the spread of Eastern Equine Encephalitis Virus and other Vector-Borne Diseases. The Health Unit performs surveillance and sampling at about 250 standing water sites located on public property throughout Middlesex-London. Staff monitor standing water sites and collect mosquito larvae samples. If the larvae collected are identified as vector species, then licensed staff will perform a larvicide treatment on the area of standing water. Learn more by reading about mosquito surveillance, control and standing water,

Additional Information 

For more information on the activities carried out by the Vector-Borne Disease Team, please please contact the Vector-Borne Disease team at:

  • 519-663-5317


Please note: Where indicated, the source of the information on this web page is the Ministry of Health and Long-Term Care. © Queen’s Printer for Ontario, 2011.

Date of creation: November 1, 2012
Last modified on: March 11, 2024


1Ontario. Ministry of Health and Long-Term Care. (2011, April). Eastern Equine Encephalitis virus surveillance and management guidelines. Toronto, ON: Queen’s Printer for Ontario.
2Ontario Agency for Health Protection and Promotion (Public Health Ontario). (2014). Eastern equine encephalitis virus: History and enhanced surveillance in Ontario. Retrieved from