In 1999, West Nile virus was first discovered in the northeastern United States. Clinical disease caused by the virus was first identified in birds, followed by humans and equines. By 2002, over 15,000 horses were diagnosed in more than 41 states.
Birds are the primary reservoir for WNV and, in birds, the disease is usually fatal. In the U.S., crows, blue jays and magpies are the most prevalent carriers. West Nile virus is transferred between birds by mosquitoes. When a bird is infected, the virus replicates quickly, resulting in large amounts of the virus in the bloodstream. The virus spreads to horses, humans and other mammals when a mosquito takes blood from an infected bird and transmits blood to an uninfected animal. WNV infection in mammals does not result in a large amount of the virus in the bloodstream. Because of this, mammals are considered dead-end hosts, and mosquitoes are unable to transmit WNV from an infected mammal to an uninfected mammal.
Horses may exhibit clinical signs when infected by WNV. As with many viral infections, horses do not experience any clinical illness after becoming infected. However, WNV can cross the blood-brain barrier and cause disease in the brain and spinal cord. This is referred to as WNV encephalomyelitis and varies in range and severity. The most common signs of disease include ataxia — loss of full control of movements — in the hind limbs as well as twitching of the muzzle and neck muscles. Other symptoms include fever, wandering, impaired vision and generalized weakness. More severe signs can include depression, stumbling, paralysis, recumbency and death. Horses that become recumbent — not able to get up — have a higher risk of dying.
West Nile virus also can effect humans. Local health organizations monitor bird and mosquito populations to predict when WNV cases will begin to show up. Diagnosis of WNV is made by the presence of clinical signs plus the detection of antibodies in the bloodstream. The clinical signs of WNV cannot be distinguished from other equine neurological diseases, so a blood test is required to look for specific WNV antibodies. If horses have any clinical signs, call a veterinarian immediately.
Treatment of WNV encephalomyelitis is supportive as there is no known cure. Controlling pain and inflammation with anti-inflammatories and fluid therapy is the common treatment. Initiating supportive care at early onset of the disease leads to a more favorable outcome. The mortality rate for infected horses is estimated to be 35 percent. Approximately two-thirds of infected horses will recover. However, 40 percent of horses that recover will continue to show clinical signs.
Prevention is key to reducing the risk of contracting WNV. Foals should first be vaccinated for WNV at 6 months of age if the mare has been previously vaccinated. The initial vaccination should be given in a three-shot series given three to six weeks apart. In heavily affected areas, the vaccine can be administered as often as every four months. It is vital to visit with your veterinarian when determining a vaccination schedule.
The American Association of Equine Practitioners has given recommendations to help eliminate mosquitoes from horse areas. These include getting rid of areas of standing water, cleaning water troughs, using larvicides to control mosquito populations, keeping horses inside during the peak mosquito times of dusk to dawn, using insect repellants specific for mosquitoes on horses, removing birds from stall areas and protecting yourself with proper clothing and mosquito repellent.