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Eastern Equine Encephalitis
Eastern Equine Encephalitis (EEE) is commonly found along the eastern United States, the Great Lakes region, and the Gulf Coast, as well as in South and Central America and Alberta, Canada. While the exact source is unclear, the spread of the disease begins with the mosquito, Culiseta melanura, in the acidic waters of swamp-like areas. This mosquito, which does not feed on humans or other mammals, infects birds in the surrounding area. Other mosquitoes, however, like the Aedes spp. (“species”) and the Coquillettidia spp. feed on the infected birds, and then transmit the virus to their other prey, which includes mammals, such as horses and humans. Thus begins the cycle of the disease, with a horse’s illness or death from the disease signaling the virus’ presence in the region.  
 
Within one to three weeks of the mosquito bite, an infected horse will begin to show signs of the illness with fever and inappetence. As it progresses, more neurological symptoms appear with compulsive walking or circling, chewing movements, excessive salivation, incoordination, and apparent blindness, often crashing into walls or fences. The horse may also be seen pressing his head or leaning against a wall or fence. As the horse’s condition worsens, he hangs his head low with his tongue hanging out and his ears and lips drooping. His staggering gait, muscle twitching, and weakness gradually worsen to paralysis, seizures, coma, and then death.  
 
Since there is no cure for EEE, the only means of therapy is supportive, through intravenous fluids, non-steroidal anti-inflammatory agents, and antibiotics for secondary infections. Veterinarians may also administer valium and pentobarbital to control the seizures. The horse is essentially a “dead-end” host, meaning the viremia, or presence of virus in the bloodstream, is not sufficient to infect mosquitoes and thereby further transmit the disease. For this reason, it is not necessary to quarantine the animal – a concern that prevents many horse owners from informing the public of an EEE-diagnosed death. It is better, however, to keep the stricken animal in a stall and closely monitored in order to limit his chances of injury to himself.  
 
Veterinarians base their diagnosis of EEE on several factors, including: the history of the animal, a physical examination, and lab tests (analysis of the presence of antibodies in the blood, virus isolation, and a study of the cerebrospinal fluid – the fluid that surrounds the surface of the brain and spinal cord). According to one veterinarian of the Virginia Department of Agriculture and Consumer Services, Dr. Akey, they first rule out rabies as the culprit since that is more common than EEE. Unfortunately, since the disease progresses so rapidly (death can often follow within 48 – 72 hours after the first noticeable signs of illness), a confirmed diagnosis of EEE is often made after the horse’s death, at which point brain tissue may be studied for the characteristic microscopic brain lesions and virus isolation.  
 
The key to protecting your horse is to make sure he is vaccinated. All nine horses that died in Virginia had not been vaccinated. Veterinarians recommend vaccinating a horse once a year, with those in warmer climates receiving their boosters every six months. It is best to time the vaccination approximately one month prior to the start of the mosquito season (from April to August/September). Pregnant mares should be vaccinated with an inactivated (killed) virus three to six weeks before foaling, and foals that have been vaccinated at less than six months should be vaccinated again by the time they are a year old. Since EEE is endemic, in that it is native to this area and always present, everyone should have his or her horse vaccinated.  
 
For more information regarding this disease, go to the Center for Disease Control web site at www.cdc.gov or contact you veterinarian.




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