Keeping Venezuelan Equine Encephalomyelitis 'Foreign'

Most horse owners are familiar with Eastern and Western equine encephalomyelitis, if only through the statement they receive from their veterinarian following their horses' annual vaccinations. But there's another "EE" that, while long absent from the United States, should not be forgotten: Venezuelan equine encephalomyelitis, or VEE.

The zoonotic disease recently made headlines when the Army announced it was investigating the whereabouts of three vials containing samples of VEE. Caree Vander Linden, a spokeswoman for the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick, Md., told the Associated Press that the vials were discovered missing last year during an inventory of a group of samples left by a departing researcher. According to reports, investigators are trying to determine if these samples were among those destroyed when a freezer malfunctioned.

VEE is a non-contagious viral infection of horses and other equids that can cause a severe and typically fatal encephalitis/encephalomyelitis, which is defined as an inflammation of the brain and spinal cord.

"Increased international movement of horses and the accidental introduction of VEE-harboring mosquitoes are factors that if they came together could result in a potential calamitous situation for the U.S. equid population."
--Dr. Peter Timoney
"The last reported case of Venezuelan equine encephalomyelitis in North America occurred in 1972; however, our currently unprotected horse population for VEE would facilitate widespread dissemination of the virus if it were to be re-introduced," explained Peter Timoney, FRCVS, PhD, former head of the Gluck Equine Research Center in Kentucky and one of the world's leading authorities on equine infectious diseases.

In addition to the potential negative impact on the equine industry, this disease is of considerable public health significance. Affected humans typically develop flu-like symptoms, but some individuals can become severely ill. Young children are particularly susceptible and can present with malaise, severe headache, fever, photophobia, vomiting, diarrhea, convulsions, coma, and paralysis.

Unlike West Nile virus and Eastern and Western encephalomyelitis viruses, VEE can be spread directly from horses to humans via the bite of infected mosquitoes.

VEE horse

This horse died of Venezuelan equine encephalomyelitis (VEE) during an outbreak in Texas.

"The virus load in the blood of horses infected with VEE is high enough that a competent mosquito species could become infected by feeding on such an animal and later, transmit the disease to a human," explained Timoney. Outbreaks of VEE in equids reported in the Western hemisphere are very often accompanied by outbreaks of VEE in humans.

Given the availability of competent mosquito species and a naive equid population, VEE could occur anywhere in the country once the causal virus has been introduced.

According to Timoney, the key factors involved in maintaining a VEE-free country include:

  • Appropriate import control measures to restrict the entry of VEE exposed equids;
  • Rapid identification of VEE if it were introduced and implementation of a vaccination program to contain the disease, and;
  • An integrated pest management mosquito control program.

The American Association of Equine Practitioners (AAEP) recommends vaccinating against VEE only in the face of the threat of exposure to the disease. Traditionally, few of our horses are vaccinated (typically in the states adjoining the Mexican border).

When asked if we should be vaccinating against VEE, Timoney explained that there are presently no grounds for implementing a widespread policy of vaccinating against VEE.

"Increased international movement of horses and the accidental introduction of VEE-harboring mosquitoes are factors that if they came together could result in a potential calamitous situation for the U.S. equid population," he added.

Despite the theoretical risk of VEE re-emergence, Timoney did emphasize, "At the present time there is no evidence that VEE poses an imminent threat of re-introduction into the United States. There have been no indications in recent years that the epidemic subtypes of the causal virus (1AB and 1C) have been active in Venezuela or Colombia. Similarly, no activity with VEE subtype 1E virus has been reported in horses in Mexico."

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Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.