These viral disorders, usually transmitted by mosquitoes or other blood-feeding insects, involve central nervous system dysfunction and moderate to high mortality. West Nile encephalomyelitis, caused by infection with the West Nile virus, is currently the best-known encephalitis of this type (see Brain, Spinal Cord, and Nerve Disorders of Horses: West Nile Encephalomyelitis in Horses). Other viruses that cause encephalomyelitis are Eastern equine encephalitis (found primarily in eastern Canada, US states east of the Mississippi river, and the Caribbean islands), Western equine encephalitis (found in western Canada, US states west of the Mississippi river, Mexico, and South America), and Venezuelan equine encephalitis (found in Mexico, and Central and South America).
The severity of the disease depends on the individual virus. Infections with Eastern equine encephalitis are generally severe and can progress to death over a short period of time. West Nile virus and Western equine encephalomyelitis are less severe. Not all infected animals develop signs of disease; a horse may be infected with the virus, develop antibodies to it, and eliminate the virus without showing any obvious signs of illness.
Nervous system signs may vary, depending on the type of virus involved. Observable signs can include impaired vision, aimless wandering, head pressing, circling, inability to swallow, irregular gait, weakness and paralysis, convulsions, and death. Many horses have a fever. Horses with West Nile encephalomyelitis may have spinal cord incoordination, muscle contractions, and an abnormal sensitivity to touch. Nearly half of affected horses have a distinctive twitching of the muzzle early in the course of the disease.
A tentative diagnosis is made based on signs and by comparing the location of the affected horse(s) and the season of the year with known data on the insects that carry the disease. Detection of certain blood antibodies (IgM) against the causative virus provides additional support for the diagnosis.
There is no specific treatment for viral encephalitis. Supportive care includes intravenous fluids if the horse is unable to drink, use of appropriate anti-inflammatory agents, and anticonvulsants if necessary. Good nursing care is essential.
Vaccines are currently available for Eastern equine encephalitis, Western equine encephalitis, Venezuelan encephalitis, and West Nile encephalomyelitis. Your veterinarian can recommend the appropriate vaccine and booster schedule for your horse depending on the geographic location and current guidelines.
The outlook depends greatly on the specific viral cause. Horses that recover may or may not have permanent neurologic damage. The disorder can be fatal. Deaths occur within 2 to 3 days after signs appear with Eastern equine encephalitis. Signs of West Nile virus infection may wax and wane over several days to weeks.
Risk to People
People can be infected by all of the viruses that commonly cause viral encephalitis in horses. Signs in people vary from mild flu-like symptoms to death. Children, the elderly, and people whose immune systems are suppressed are the most susceptible. People usually develop permanent neurologic impairment. Human disease is reported infrequently and generally follows equine infections by about 2 weeks.
Last full review/revision July 2011 by William B. Thomas, DVM, MS, DACVIM (Neurology); Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Kyle G. Braund, BVSc, MVSc, PhD, FRCVS, DACVIM (Neurology); Cheryl L. Chrisman, DVM, MS, EDS, DACVIM (Neurology); Caroline N. Hahn, DVM, MSc, PhD, DECEIM, DECVN, MRCVS; Charles M. Hendrix, DVM, PhD; Maureen T. Long, DVM, PhD, DACVIM; Robert J. Mackay, BVSc, PhD; Karen R. Munana, DVM, MS, DACVIM (Neurology); Charles E. Rupprecht, VMD, MS, PhD; Josie L. Traub-Dargatz, DVM, MS, DACVIM; Susan L. White, DVM, MS, DACVIM