Eastern equine encephalitis virus (EEE), also known as sleeping sickness, and Western Equine Encephalitis (WEE) are viral diseases that cause inflammation of the brain and spinal cord. Wild birds are a natural reservoir for EEE and WEE viruses, and mosquitoes that feed on these birds can transmit the virus to mammals, including horses and humans.

Cases of EEE are most commonly seen in the southern and eastern parts of North America. The mosquito Culiseta melanura is the primary vector (although other species of mosquito can also transmit the virus). The Canada Food Inspection Agency (CFIA) reported eight horses testing positive for EEE in Ontario in 2020. No human cases were reported in the 2020 season.

Mosquito larva hatching in water. Photo: Shutterstock/Pernsanitfoto

WEE virus is distributed more in the west, although both WEE and EEE can occur outside their normal geographical areas. Outbreaks of WEE virus have resulted in significant numbers of deaths in horses and humans. Survivors can have permanent neurological impairment. The virus circulates naturally in birds and is transmitted to horses and people primarily by the mosquito vector Culex tarsalis. 

Incidence rates vary each year, and both EEE and WEE occur most frequently in late summer to early fall. In addition to mosquitoes, ticks and other bloodsucking insects can also transmit the virus after feeding on an infected bird. The incubation period ranges from two days to three weeks. Direct transmission between horses, or between horses and humans, is not likely.

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EEE is more virulent, although both EEE and WEE are both life-threatening neurological diseases. 

The clinical signs of EEE often come on abruptly and signs of both viruses can include depression, anorexia, fever, and lethargy. Neurological signs may include tremors, weakness, ataxia, paralysis, seizures, decreased awareness of surroundings, and recumbency.

Keep horses in the barn at night when mosquitoes are active. Photo: Canstock/Stokkete

Both EEE and WEE viruses are diagnosed through bloodwork. Testing can include enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR) on cerebrospinal fluid (CSF) samples to identify the WEE or EEE virus.

Treatment largely consists of supportive care for horses with clinical signs, including intravenous (IV) fluids and anti-inflammatory medications. There is no cure for EEE or WEE.

The prognosis for horses with WEE is fair. The disease has a 20 to 40 percent mortality rate, and long-term deficits may occur in horses that recover from WEE. The prognosis for horses diagnosed with EEE is very poor, with up to 90 percent of infected horses dying from the disease. Horses with neurological signs that become unable to stand on their own have the poorest prognosis for survival. Horses that do survive can have permanent brain damage.

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The American Association of Equine Practitioners (AAEP) recommends the EEE and WEE vaccines as core vaccines. Horses should be kept up-to-date on vaccinations, which are usually administered yearly (boosters may be recommended more frequently in warm climates with year-round mosquito activity). Boosters may be advised when outbreaks occur. Work with your veterinarian to establish an appropriate vaccination schedule for your horse and situation.

In addition to vaccination, it is important to practice good vector control. Use insect repellents and keep horses in at night when mosquitoes are most active. Eliminate mosquito breeding grounds such as standing water, brush piles, and old tires. Stock tanks and water troughs with mosquito-feeding fish.

With files from UC Davis Center for Equine Health and Boehringer Ingelheim.

Main Photo: Petri Volanen