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Eastern Equine Encephalitis


The Virginia Department of Agriculture and Consumer Services (VDACS) has confirmed the first case of eastern equine encephalitis (EEE) in a Virginia horse this year [2015]. The affected horse had not been vaccinated for EEE. Last year [2014], Virginia had one reported case of equine EEE. The Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) confirmed EEE in two horses in East Texas.

EEE is a viral disease that normally cycles between wild birds and mosquitoes. As the virus infection rate increases in birds it is more likely to be transmitted by an infected mosquito that bites horses and humans. EEE cannot be transmitted from horse to horse, or from a horse to a human. Horses and humans are considered "dead-end" hosts which means if infected they cannot transmit the virus back to feeding mosquitoes.

 EEE abruptly attacks the central nervous system and is transmitted to horses by infected mosquitoes. Of the neurologic diseases that affect horses, EEE has the highest mortality rate. Clinical signs of EEE include moderate to high fever, depression, lack of appetite, cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing), behavioral changes (aggression, self-mutilation, or drowsiness), gait abnormalities, or severe central nervous system signs, such as head-pressing, circling, blindness, and seizures. The course of EEE can be swift, with death occurring 2-3 days after onset of clinical signs despite intensive care; fatality rates reach 75-80 per cent among horses. Horses that survive might have long-lasting impairments and neurologic problems.

Richard Wilkes, DVM, VDACS state veterinarian, encouraged horse owners to work with their veterinarians to plan a vaccination schedule that would protect their horses from EEE and West Nile virus (WNV), another mosquito-borne disease.

“Highly effective vaccines are available for EEE, Western Equine Encephalitis and West Nile virus. These equine neurologic diseases are preventable with proper vaccination. This is why it is so important to keep your horse current on their vaccinations,” said Terry S. Hensley, DVM, MS, TVMDL assistant agency director and Texas AgriLife Extension Service veterinarian. The Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) confirmed West Nile virus (WNV) in two horses in Texas, one early in July and another in late July. [Additional information on WNV here: ]

Available vaccines are generally effective in drastically reducing the incidence of both EEE and WNV in horses. For the vaccine to be effective, it must be handled and administered properly and be given at least two weeks before the horse is exposed to the virus. In addition, to stimulate full immunity, horses must be vaccinated twice, about 30 days apart, the first year of vaccination. The vaccines are effective for 6-12 months, so horses should be revaccinated at least annually. In an area where the disease occurs frequently, most veterinarians recommend vaccination every six months.

In addition to vaccination, it is a good idea to avoid mosquito-infested areas and to take measures to reduce the local mosquito population to minimize the chances of mosquitoes biting your horse.

Message from Dr. Lew Strickland, UTCVM:

“I recommend that horse owners vaccinate for EEE (West Nile and Western Equine Encephalitis, too) well before now in the spring, but we will still have mosquitoes for some time, and VA is not far from us especially those in the north east. If you have any questions please feel free to contact me.

Lew Strickland, DVM, MS, DACT

Extension Veterinarian

UT Extension  |  UT College of Veterinary Medicine
2506 River Drive - Brehm, Room 246
865-974-3538 |

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