Eastern Equine Encephalitis: Symptoms and Prevention

We are well into the rainy season by now and this year we have experienced a larger than usual hatch-out of mosquitos in our area. Bigger mosquito populations can lead to more mosquito-transmitted diseases. Mosquitos can carry diseases such as Zika, Chicungunya and Dengue Fever. Our livestock can also be affected by the overabundance of these pesky insects and this column will be dedicated to one disease that can be transmitted to our horses as well as humans: Eastern Equine Encephalitis (EEE).

Picture Credits: Jonael Bosques, University of Florida
Figure 1. Proactive protection four horses against Eastern Equine Encephalitis involves vaccination and mosquito control strategies. Picture credit: Jonael Bosques, University of Florida.

What is EEE?

This disease is caused by a virus that is transmitted by infected mosquitoes. Birds are virus carriers. When mosquitoes feed on birds, they acquire and transmit the disease agent. The virus is found along the east coast from New England to Florida, the Gulf Coast, and some Midwestern areas, where wading bird populations thrive. The vector mosquito is of the species Culiseta melanura (Figure 2). This mosquito species which does not feed on humans or horses. On occasions this marsh-inhabiting mosquito can transmit the virus to other species of mosquitoes that do feed on humans and horses. It is important to note that sick h

Figure 2. Black-Tailed Mosquito Culliseta melanura is a marsh-inhabiting mosquito that transmits the EEE and West Nile virus. Picture credit: C. Roxanne Connelly, University of Florida.

orses and humans cannot shed this virus further.

This year EEE has been found in horses in Louisiana (1 case), South Carolina (1 case), and Wisconsin (19 cases).


Symptoms to watch out for

In horses, symptoms include: tremulousness, erratic behavior, and loss of coordination. These symptoms develop 10 days after infection. For horses, seizures can occur 48 to 72 hours after initial symptoms are observed. There is no effective treatment for this disease which affects the central nervous system. EEE can kill 90% of exposed unvaccinated horses. Furthermore, horses that recover from EEE might exhibit long-lasting neurological problems.

In humans the symptoms present themselves as of a mild flu accompanied by fever, headache and sore throat. In severe cases, fever and headache can be followed by seizures and coma which often result in permanent brain damage or death.



Vaccination is available for horses, but not for humans. To prevent mosquito diseases in humans, wear appropriate clothing as well as use DEET repellents, particularly around active feeding times (dusk and dawn) for mosquitoes.

Place mosquito larvicide on standing water around your farm or house. Mosquito larvicide such as Bacillus thuringiensis israelensis or BTI comes in different products from donut-shaped “dunks” to pellets which can be safely applied to water troughs, birdbaths and other water features. These products contain bacteria that produce an enzyme that only affects insect digestive tracts (which are alkaline).

Eastern Gambusia (Gambusia holbrooki) or mosquitofish (Figure 3 and 4). can be released in ponds and water troughs around the farm to eat mosquito larvae as well. This live-bearer fish can consume impressive amounts of mosquito larvae. They are native to Florida and the Southern States, inhabiting streams and rivers as well as ponds and lakes prone to flooding.

Eastern Gambusia (Gambusia holbrooki) – female. Picture credit: Jonael Bosques, University of Florida.
Eastern Gambusia (Gambusia holbrooki) – male. Picture credit: Jonael Bosques, University of Florida.


For More Information

Eastern Equine Encephalitis can be prevented with regular horse vaccinations, sound management practices as well as active prevention strategies. If you would like to know more on this topic, please contact the UF/IFAS Extension Hardee County office at 863-773-2164.


Further Reading:



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Posted: August 1, 2017

Category: Agriculture, Livestock, Pests & Disease
Tags: Encephalitis, Equine, Gambusia, Horse, Mosquito


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March 8, 2022

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February 18, 2022

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Jose Zayas

March 29, 2019

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