CEPS/Veterinary Extension
2938 Vet. Med. Basic Sciences Bldg.
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907

By Joseph Hahn
Information Specialist
University of Illinois
College of Veterinary Medicine

Equine encephalitis, also called sleeping sickness, is a viral disease which affects parts of a horse's central nervous system. This virus is attracted to nervous tissue which leads to inflammation in and damage to the brain.

"This is a disease of the summer and fall primarily because it is carried by mosquitoes," states Dr. R.D. Scoggins, equine Extension veterinarian at the University of Illinois College of Veterinary Medicine at Urbana. "Mosquitoes pick up the disease from birds, principally pheasants. Mosquitoes can then transmit the virus to horses and humans." He stressed that while humans can develop viral encephalitis, the disease is transmitted from mosquitos that have bitten birds, not from mosquitoes that have bitten horses.

There are three main types of equine encephalitis.

1.Western Equine Encephalitis -- most common form in the Midwest and Western states and the least fatal.
2.Eastern Equine Encephalitis -- seldom seen in the Midwest and much more deadly. Primarily in the eastern states.
3.Venezuelan Equine Encephalitis -- most lethal and rarest form in the United States. The southern tier of states is at the greatest risk.

This disease is called sleeping sickness because horses show signs of weakness and depression. Other symptoms may include fever, walking in circles or into obstacles, refusal to eat, and standing with its head lowered. This disease incubates in the horse for one to three weeks before signs are seen and the horse may be sick for up to three to six weeks.

Those horses that recover from encephalitis are often called dummy horses. "Mares will not be good mothers and they are often not safe to ride or to compete if they have been through a severe bout of the disease, particularly the Eastern form," says Dr. Scoggins. This is due to the permanent damage that has occurred to the brain.

"Because it is a viral disease, we do not have any anti-viral drugs that are effective in treating it," states Dr. Scoggins. "Treatment is all symptomatic, making sure horses get the nutrition they need and using anti-inflammatory drugs to control fever."

"Horse owners need to be aware that the vaccines are highly effective. If there is a problem with the vaccines it is that they are not long lasting," remarks Dr. Scoggins. "Horses that are vaccinated prior to the May 1 in a summer with an excessive amount of mosquitoes need to think about revaccination in August or September." It is important to use at least a bi-valent vaccine (one with protection against Western and Eastern forms) that will protect against both Western and Eastern encephalitis. Horses traveling to places where Venezuelan encephalitis has occurred, should use a tri-valent vaccine (protection against all three forms). The vaccines are safe to use on all horses at all ages. Foals should be vaccinated at three to four months of age.

Besides vaccination, mosquito control is another important method in preventing this disease. Some recommendations are to eliminate standing water when possible, placing fish in water tanks to eat mosquito larvae, keeping pastures cut short, and using citronella to help ward off mosquitoes.

Dr. Scoggins concludes, "since there are other things that may be causing similar central nervous system signs, it is important to have your local veterinarian confirm a diagnosis of equine encephalitis."

If you have any questions, or would like further information, contact your local veterinarian or contact Dr. Scoggins at the University of Illinois College of Veterinary Medicine (217/333-2907).