Equine encephalomyelitis

By Jennifer Kunz, AnimalForum.com staff

Equine encephalomyelitis is an inflammation of the brain and spinal cord. This disease not only affects horses, but is also deadly for humans.

The virus was isolated, characterized, and vaccines were produced in the 1930s. The two forms currently active in the United States are Eastern equine encephalomyelitis (EEE) and Western equine encephalomyelitis (WEE). The viruses responsible for causing these diseases are members of a family of viruses called the alphavirus.

WEE is most active in the western United States and parts of the Midwest. WEE was responsible for the death of 180,000 horses between 1931 and 1938 in the San Joaquin Valley, Calif. There have been isolated cases in every state west of the Appalachians, parts of western Canada, and in Central and South America.

EEE has an eastern United States distribution. There have been isolated cases in Michigan and Wisconsin as well as outbreaks in the Caribbean, Dominican Republic, Haiti, Panama, and Central and South America.

The mosquito transmits the virus from small infected animals such as birds and rodents to horses. The warm, humid weather of the summer is good for mosquito breeding and this is when outbreaks are more common. Transmission of EEE and WEE is not horse to human, but bird or rodent to human via the mosquito.

In contrast, Venezuelan equine encephalomyelitis can be transmitted directly from an infected horse to a human via a mosquito. In countries where Venezuelan equine encephalomyelitis occurs, the epidemics are considered explosive. A 1969 outbreak near Ecuador caused 31,000 human cases. The last known outbreak of Venezuelan equine encephalomyelitis in the United States occurred in Texas in 1971.

Approximately two days after equine infection with encephalomyelitis, there is an inapparent infection and low-grade fever. The first apparent signs are at four to five days. At that time, the animal generally has a fever and rapid heart rate, is showing signs of anorexia, depression, and variable other neurological signs.

As the illness progresses the brain stem and spinal cord are affected. Muscle weakness becomes apparent and there are behavioral changes and dementia. Notable symptoms include aggression, head pressing, wall leaning, compulsive circling, and blindness. Other signs might include uncontrolled twitching of the eyeball, and facial muscle paralysis.

As the disease progresses, a semi-comatose and convulsive state occurs. Death usually follows two or three days later. If the animal survives, residual nervous system problems result. The reported mortality rate for EEE is 75-90 percent, and it's 20-50 percent for WEE.

Encephalomyelitis vaccines are available for horses from several different companies. They are packaged as single or combination vaccines. The combination commonly referred to as a "three-way shot" includes a vaccination for EEE, WEE, and tetanus. "Four-way shots" include EEE, WEE, tetanus, and influenza.

All unimmunized horses are at risk regarding this disease. Speak to your vet today if your are unsure about your horse's vaccination history!