Dr. Sarah Southard

by Sarah Southard, DVM

West Nile

Virus (in horses)

Queens, New York, August 1999 Sarah Southard, DVM Twenty-two years ago next month, an apparently new neurologic disease emerged in a few folks in Queens, New York. It presented with acute, severe symptoms such as paralysis and disorientation. At the same time, caretakers at the Bronx Zoo/ Wildlife Conservation Park had noticed an unusual pattern in bird illness and deaths in and around the zoo. Upon further investigation by New York public health officials and Centers for Disease Control researchers, it was determined that West Nile Virus had made its way to the United States. West Nile Virus is spread by mosquitoes and can affect birds (its reservoir and host species), horses, and humans. The virus can cause inflammation of the brain and spinal cord, resulting in central nervous system dysfunction. When a horse becomes infected, it may show signs of disease within 15 days of the infective bite. While morbidity rates are relatively low at about 10%, mortality rates are quite high, at about 35%. Some infected horses will not develop disease at all. They may remain completely healthy while some may die unexpectedly without showing any clinical signs of disease. Others may show only mild symptoms including lethargy, muscle tremors, and lowgrade fever. Generally these horses will recover rather quickly when appropriate supportive care is administered in a timely manner. A significant number of infected horses will develop severe neurologic disease resulting in disorientation, vision deficits, incoordination, inability to swallow, paralysis, seizures, and death. Even with aggressive supportive care, these horses have a guarded prognosis. There are no direct treatment options available for West Nile Virus. The only option is to provide supportive symptomatic care until the horse’s immune system gets the virus under control. According to data reported by the Pennsylvania State University College of Agricultural Sciences Extension Service, 33% of horses which show clinical signs of West Nile Virus encephalomyelitis will die. About 50% will recover fully, and 17% will either have a disease relapse or will not fully recover to their pre-infection normals. As many as 40% of infected horses will still show signs of changes in behavior or gait at six-months post-infection. While West Nile Virus encephalomyelitis is a very serious disease and all horses are at risk, the good news is that there are effective vaccines available for horses, which when administered correctly, usually provide protective immunity against clinical disease. Discuss with your veterinarian which vaccine would be most appropriate for your horse and on what schedule it should be given. Depending on individual health considerations and risk factors, some horses may need to be revaccinated more often than others to maintain protective levels of antibodies. In addition to vaccination, horse owners and farm managers should practice good environmental hygiene to decrease the mosquito population. Other considerations to decrease the possibility of exposure include bringing horses into the barn at dawn and dusk when mosquitoes are most active; providing fans in barn stalls to discourage mosquitoes and other flying insects; using appropriate insect deterrents and insecticides.