Introduction/Causative Agent: Equine encephalomyelitis is a term used to describe inflammation of the brain and spinal cord. It is sometimes called "sleeping sickness" by many horse people. Equine encephalomyelitis is often caused by viruses of the Togaviridae family. The most common ones here in the United States are the eastern equine encephalomyelitis (EEE) and western equine encephalomyelitis (WEE) viruses. Venezuelan equine encephalomyelitis (VEE) is considered an exotic disease here in the United States; however, outbreaks have occurred in the past (1971). VEE is more commonly found in Central and South America. Because these viruses can infect people causing significant problems and death, they are considered zoonotic diseases.
Clinical Signs: Starting about 5 days after infection, the horse goes off feed, has a fever, and may be stiff. As signs progress, the horse may act sleepy, with the head held low, lips relaxed and the eyes closed. Additional signs may include lack of coordination, muscle twitching, head tilt, blindness, inability to eat or chew, and overexcitement when stressed. Over time the animal may become comatose and eventually die. Horses infected with VEE will often have diarrhea and die before any neurological signs develop. VEE can also cause ulcers to form on the lips, mouth, tongue and gums. Abortions and bleeding from the lungs can also occur. For non-vaccinated animals, death rates for EEE range from 75-90%, for WEE they range from 19-50%, and for VEE they range from 40-80%.
Disease Transmission: These viruses are transmitted from one animal to another by mosquitoes and ticks. Rodents and birds can act as reservoirs for the disease. A reservoir, in this instance, is an animal that can harbor the virus. These rodents and birds, however, do not spread the disease directly to horses. The virus requires a mosquito or tick bite for transmission. Because viremia (viruses in the blood) from WEE does not occur in horse infections, the horse is considered a "dead-end" host. This means that one horse cannot spread a WEE infection to another. However, VEE does cause significant viremia. This means that a horse can act as a carrier of the VEE virus and potentially spread the virus to other horses. This spread can occur through mosquitoes and ticks, and through discharge from the nose and eyes. EEE is thought to have certain times during the infection where, if situations are ideal, the virus could spread from horse to horse through insect feeding.
Treatment/Prevention: There is no treatment for these diseases. Use of routine anti-seizure medications may be beneficial in some cases. Vaccination is the only way to prevent this problem, but it is important to read vaccine labels. For example, the typical 4-way vaccine protects against EEE and WEE but not VEE. See the recommended vaccination program on page A905. It is also important to control insect populations that may spread disease. VEE is a reportable disease in the United States. Horses infected with VEE should also be quarantined to at least 3 weeks to help prevent viral spread.