There are several neurological diseases in horses that can cause long term symptoms even after they seem to have recovered from the disease. West Nile Virus is a viral infection that crosses into the brain and spinal cord causing inflammation that can lead to signs resembling colic, lameness, a decrease in appetite, and fever.  It can cause severe disease in some horses, leading to inability to stand, paralysis of the facial muscles and the tongue, behavioral changes, seizures, and even death.  Dr. Stacy Oke says that most horses with West Nile infection recover in a few weeks, but about 10% of the infected horses continue to have some neurological deficits that can affect their ability to walk and perform. Some can also have residual behavioral abnormalities. 

Eastern Equine Encephalitis is another disease that affects the neurological system and is even more serious than West Nile Virus.  Symptoms of Eastern Equine Encephalitis are similar to West Nile Virus including fever, depression and behavioral changes, circling and head pressing, muscle twitching and paralysis of the cranial nerves, and seizure. Death occurs in 50-90% of cases.  Dr. Maureen Long says most horses that do survive Eastern Equine Encephalitis have long term neurological dysfunction.  Vaccines are available for both West Nile Virus and Eastern Equine Encephalitis and are very effective in most horses.  Both diseases are transmitted by mosquitoes, and both can be transmitted to humans as well as horses, although Eastern Equine Encephalitis is fairly rare in humans.  The diseases are not transmitted directly from horses to people.