06/23/2026
DO YOU KNOW THE DIFFERENCES IN EQUINE ENCEPHALITIS?
Eastern (EEE), Western (WEE), and Venezuelan (VEE). All are mosquito-borne viruses that can cause brain inflammation and severe neurologic disease.
Where Do They Occur?
• EEE (Eastern Equine Encephalitis): Historically in the U.S., it has occurred in all states east of the Mississippi River, with particular prevalence in the Southeastern U.S. More recently, cases have been identified as far west as Texas and north into eastern Canada.
• WEE (Western Equine Encephalitis): Recorded more commonly in the Western U.S.; historically had large outbreaks, but no equine cases have been reported in the last two decades.
• VEE (Venezuelan Equine Encephalitis): Causes outbreaks in Central and South America and Mexico, and only occasionally in the southern U.S.; last occurred in the U.S. (Texas) in 1971, but there is continuing concern about re-introduction.
How Do They Spread?
• EEE & WEE: Spread primarily by mosquitoes, with birds (and some snakes) helping keep the virus cycling in nature. Horses are “dead-end hosts”—they typically do not develop enough virus in their blood to infect mosquitoes.
• VEE: Also spread by mosquitoes, but horses can develop a high enough viremia to infect mosquitoes, meaning horses can help amplify outbreaks. This is a major difference from EEE/WEE.
All three can cause fever and neurologic signs. Signs can vary, but may include behavior changes, incoordination, blindness, seizures, and coma.
Mortality Rates in Unvaccinated Horses?
• EEE: 75–95% (often within 2–3 days of signs starting)
• WEE: 20–40% (generally less severe than EEE)
• VEE: 40–90%
For surviving patients, any of these infections can leave long-term neurologic deficits; this is especially noted with EEE and VEE.
Reportability:
All three (EEE, WEE, VEE) are reportable diseases—your veterinarian will notify animal health officials of suspected and confirmed cases.
How Can We Help Prevent Disease?
• EEE and WEE vaccines are considered “core” for horses living in or traveling to North America, because these diseases are severe and mosquito-borne.
• VEE vaccination is risk-based (depends on likely exposure).
• Reducing mosquito exposure matters because these viruses are primarily mosquito transmitted.
If you have questions or concerns about Equine Encephalitis, contact your veterinarian.
