Venezuelan Equine Encephalitis (VEE)

horse grazing at dusk

What is Venezuelan equine encephalitis?

Venezuelan equine encephalitis (VEE) is a viral, vector-borne disease in horses and humans in Central America, South America, Mexico and occasionally the southern United States that causes inflammation of the brain. The VEE virus is related to the viruses that cause Eastern and Western equine encephalitis.

Eastern and Western equine encephalitis are typically spread from birds to horses by mosquito vectors, but are not transmitted between horses by mosquitoes. Conversely, if a mosquito bites a horse infected with VEE, there is enough virus in the blood that it can be transmitted to another horse that the mosquito bites. Rodents are the natural hosts for the VEE virus, but birds may be involved in some cases.

Since aerosolized VEE can cause fatal encephalitis in humans, it is considered a possible bio-warfare agent.

What are the clinical signs of Venezuelan equine encephalitis?

The clinical signs of VEE include fever, anorexia, depression, weight loss, dehydration, weakness, paralysis, difficulty swallowing, gait abnormalities, seizures, and other neurological signs.

How is Venezuelan equine encephalitis diagnosed?

Venezuelan equine encephalitis is diagnosed through bloodwork. Testing can include enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR) on cerebrospinal fluid (CSF) samples to identify the virus.

How is Venezuelan equine encephalitis treated?

Treatment for VEE primarily includes supportive care such as intravenous (IV) fluids and anti-inflammatories.

What is the prognosis for Venezuelan equine encephalitis?

Horses that are infected with certain subtypes of VEE virus do not develop serious disease. Other subtypes can cause significant illness and death, with mortality rates as high as 90%.

How can Venezuelan equine encephalitis be prevented?

The American Association of Equine Practitioners (AAEP) recommends the VEE vaccine based on a horse’s risk of exposure. Venezuelan equine encephalitis is a reportable animal disease in the United States and cases should be handled accordingly.

Good vector control with respect to VEE prevention can be challenging, as the disease is prevalent in areas with high rainfall and those susceptible to flooding, making it difficult to eliminate mosquito breeding areas of standing water. Aerosol insecticide applications are recommended when feasible.

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu

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