HealthWatch: EEE, WEE, & VEE

Eastern, Western, and Venezuelan equine encephalitis

Eastern, Western, and Venezuelan equine encephalitis (EEE, WEE, and VEE) are arboviruses—meaning they are transmitted by blood-feeding insects or ticks—and these pathogens can cause fatal neurologic disease in horses.

All of these viruses have public health implications because they are zoonotic, meaning they can be communicable from animals to humans under natural conditions. EEE and WEE, like West Nile virus, can be spread from birds to humans via mosquitoes. Infected horses cannot pass these diseases to other horses, animals, or humans, however, so equine infection is generally indicative of a virus being active in a geographic area. EEE is a reportable disease in most states for this reason. VEE, however, is even more serious, as it can be transmitted from horses to mosquitoes to humans. Veterinarians see VEE predominantly in South and Central America.

There are two variants of EEE. The one in North America is more pathogenic to horses than the one in South and Central America. EEE in North America occurs primarily in eastern parts of the United States and Canada, but it has been seen as far west as South Dakota and Texas.

The fatality rate for EEE in horses is 75-95%, compared with a 20-40% fatality rate for WEE. The course of EEE can be swift, with death occurring two to three days after onset of clinical signs, despite intensive care. Horses that survive might have long-lasting impairments and neurologic problems. The case fatality rate for VEE in horses is 40-90%.

There are effective equine vaccines for every one of these viruses. “EEE, WEE, and WNV are three of the five core vaccines for horses recommended by the American  Association of Equine Practitioners (AAEP,,” said Dr. Kathy Seino, an assistant professor of equine medicine at Washington State University. “After the primary series, you need to annually revaccinate one month before mosquito season starts in your area. In regions with a long mosquito season, you may need to revaccinate once or twice. In Florida, many horsemen vaccinate every four months.”

Oral Health
Bridles and bits should be custom-designed or fitted in order to minimize the occurrences of oral ulcers, according to a group of Swedish researchers led by Dr. Ove Wattle of the Department of Clinical Sciences at the Swedish University of Agricultural Science.

To assess the prevalence, location, and type of oral ulcers in horses better, Wattle and colleagues performed oral examinations on 113 horses and ponies. All horses included in the study had routine dental care.

Wattle and colleagues found that horses ridden with bit and bridle had a significantly higher prevalence of large and acute cheek ulcers compared to horses not in work.

The authors concluded a bit and bridle can cause oral ulcers despite regular dental floating, and they suggested a more individual approach to the choice and fitting of bits and bridles to prevent the formation of oral ulcers.

Riding Position Affects the Horse’s Back
Austrian researchers have reported that the stability of a rider’s seat affects the forces acting on a horse’s back. Using an electronic pressure mat placed under a dressage saddle, scientists with the University of Veterinary Medicine Vienna’s Movement Science Group measured the forces created when an experienced rider rode 10 different sound horses at the sitting trot, rising trot, and in the two-point (jumping) position.

The sitting trot created the highest load, followed by the rising trot and the two-point seat. In the two-point position, the rider’s back is most stable, placing the least amount of load on the horse’s back. In all positions movement on the y-axis accounted for the differences in load. The rider’s transverse movement had no effect on the horse’s back.

For young horses, or horses recovering from back problems, a combination of the rising trot and two-point positions provides optimal training conditions without overloading the horse’s back.

Excerpted from The Horse: Your Guide to Equine Health Care. Free weekly newsletters at