Encephalomyelitis is inflammation in the brain and/or spinal cord. West Nile virus, the cause of West Nile encephalomyelitis, was first found in North America in 1999. This viral disease, transmitted primarily by mosquitoes, is characterized by central nervous system dysfunction and can be fatal or (if the horse survives) can result in prolonged signs of disease.

Encephalomyelitis in horses caused by infection with the West Nile virus has been called Near Eastern equine encephalitis or lordige in France. In the United States, 10 to 39% of infected horses show signs. Once signs appear, the death rate is approximately 30%. The number of equine cases of West Nile virus infection in the US has been declining since it hit a high of more than 15,000 cases in 2002. This decline is thought to be because of availability and increased use of vaccines, increased awareness of the need to control mosquito populations, and increased immunity to the virus within wild bird populations (the reservoir hosts).

All horses are susceptible to West Nile virus encephalomyelitis if not vaccinated against the disease. Any age horse can be affected, but adults are affected most commonly, and older horses develop more severe disease.

The signs and course of the disease are highly variable. Not all horses infected with the virus develop the disease, and some horses can die without showing signs of illness prior to death. The first signs of the disease may be general and include signs of discomfort or anxiety, lameness, low-grade fever, or malaise. Neurologic signs can include muzzle twitching, impaired vision, aimless wandering, head pressing, circling, inability to swallow, irregular gait, trembling, lack of coordination, weakness, muscle contractions, paralysis, convulsions, and death. Horses may become abnormally sensitive to sights, sounds, or touch. Once complete paralysis has developed in one or more legs, the death rate can reach 60 to 80%. A presumptive diagnosis is made by blood test.

Serum or plasma products containing antibodies to the virus can be given as part of the treatment; however, treatment of signs and supportive care to keep the horse comfortable and safe are the most commonly administered treatments. Anti-inflammatory drugs, including nonsteroidal drugs and/or corticosteroids, are often prescribed, but do not always result in improvement. Several different therapies may need to be tried to find one that helps. Good supportive care is important. In horses that recover, most return to normal functioning within 1 to 6 months, but at least 10% of owners of horses with West Nile virus infection have reported some longterm effects.

Vaccination helps protect against West Nile virus encephalomyelitis. Most horses given one of the available vaccines according to the manufacturer's recommendations will be fully protected from disease. In the unlikely event that a horse vaccinated against West Nile virus contracts the disease, the signs tend to be less severe and recovery better. Your veterinarian can recommend an appropriate vaccination schedule for your horse based on current guidelines.

The transmission cycle of West Nile virus. Mosquitoes are the disease vector, and birds are the reservoir hosts. Horses and people are incidentally infected.

Good environmental management can also help prevent West Nile virus infection. Because mosquitoes breed in standing, stagnant water, all water tanks and buckets should be cleaned at least once a week, and areas where stagnant water can collect, such as empty flower pots and used tires, should be eliminated. Fans that blow over horses in the barn area can reduce mosquitoes and other flying insects. (Be sure that all electrical cords for fans are out of the horses' reach to avoid electrical shock.) Appropriate insecticides should be used according to label directions in the summer and other times when mosquitoes are common.

Transmission to People

Mosquitoes can also transmit the West Nile virus to people. Most people who contract the virus never develop symptoms. However, about 1 in 150 people exposed to the virus develop the severe form of the disease. Infection is more common in the elderly. West Nile fever in people is a syndrome characterized by fever, headache, and weakness.

Horses do not act as a source of West Nile virus for mosquitoes, and infected horses do not pose a risk to humans who care for them. Special precautions should be taken if people will be involved in assisting with a necropsy (animal autopsy) or when handling blood products from infected horses.

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