Most horse owners regularly vaccinate against equine encephalomyelitis.  Many horse owners today do not realize how important it is that they vaccinate their horses for this disease.  This disease first caught public attention because it is one of the rare equine diseases that can also be spread to humans.  This is because mosquitoes, similar to that of West Nile Virus, spread the disease.  Other insects such as bloodsucking bugs, ticks, mites and lice may also transmit the disease, although the mosquito is the most common transmitter.  The disease, like West Nile, is also carried by birds, which tend to be excellent reservoirs for the disease.

Equine encephalomyelitis is broken down into geographical locations and is currently only present on the American continents.  Western and Eastern are the most common in the United States, although horses that live in the southern states are often vaccinated for the Venezuelan strand as well.  The Venezuelan strand can be spread via the insects, but also through the air when infected animals are in close contact with uninfected animals.  All of the viruses have been isolated into specific species in natural conditions.  These include horses, humans, donkeys, mules, monkeys, snakes and frogs.  In the laboratory, calves, dogs, mice and guinea pigs have also been infected. 

The Western strain of the disease tends to have the lowest death rates among infected animals.  Approximately 20 to 30 percent of infected animals will die from the Western strain.  The Eastern and Venezuelan strains have higher death rates reaching 70 to 90 percent.  Those animals who recover from the disease tend to have an immunity that lasts for about 2 years after infection, however these horses often suffer permanent brain damage that keeps them from returning to their active and useful lives.  Young horses tend to be the most susceptible to the diseases. 

Most horses will incubate the virus for one to three weeks.  During this period of time most horse owners will not know that their horses are infected.  The first sign is usually a fever that may reach up to 106 degrees Fahrenheit.  The fever is often short lived, lasting only 24 to 28 hours, this often means that this first sign is missed, especially if horses are out to pasture and not used on a regular basis.  The next signs that appear are often nervous signs such as hypersensitivity to sound, excitement and restlessness.  These tend to peak with the fever.  Shortly afterwards the horses will usually begin to show signs that are associated with the brain lesions that develop.  These include drowsiness, drooping ears, abnormal gaits and circling.  The hose may also stand with their head held very low and with food just hanging out of their mouths.  The symptoms only progress from here and the next stage is paralysis.  The hose will lose the ability to raise its head, their lower lip drops and their tongue may hang from their mouth.  They have difficulty moving, urinating and defecating.  The last stage is death.  These symptoms progress very quickly.  Many horses may die within 4 days after the first symptoms appear.

The good news is that most horses are vaccinated against these diseases.  There are also combination vaccinations that allow you to vaccinate for EEE, WEE, and VEE in one shot.  These are given in two doses and protect the horse for about one year.