West Nile Virus

Description

West Nile Virus (WNV) was first isolated in humans in 1937, in Uganda. From there, it spread throughout Africa, West Asia, the Middle East, and Europe, finally reaching the United States in 1999.

Harbored in birds and spread by mosquitoes, horses are affected by WNV more than other domestic animals. In an outbreak in 2001, 416 horses from 19 states were diagnosed with the disease and 24 percent of those horses died or were euthanized.

Horses and humans cannot spread the disease. The virus is carried from infected birds by mosquitoes and enters the body through mosquito bites, where it causes inflammation of the brain, leading to a number of symptoms related to loss of muscular and neurological control.

Symptoms

• Stumbling and lack of coordination
• Weakness of limbs, partial paralysis, and inability to stand
• Muscle twitching
• Depression and apprehension
• Diarrhea
• Personality changes, with hyperexcitability, irritability, and refusal of food and water
• Signs of central nervous system damage, including paralysis, convulsions, coma, and, eventually, death

Causes

West Nile virus is caused by a virus that is harbored in birds and spread via mosquitoes to other birds, people, and horses. Most birds do not develop the disease, although crows and blue jays often die as a result of infection with the virus. Horses and people are known as dead-end hosts because no evidence shows cases of the disease being spread to other humans or animals.

Prevention

The best prevention is controlling mosquitoes to prevent infected mosquitoes from biting the horse. Mosquito breeding areas should be removed by clearing away any potential sources of water in which mosquitoes can breed. This includes old tires, clogged roof drains, wading pools, wheelbarrows, bird baths, empty containers of all kinds, unused or neglected swimming pools and spas, puddles left after rainfall, neglected water troughs, or any other items or areas where water might become trapped.

Steps should be taken to limit the horses' exposure to mosquitoes by keeping them indoors during peak mosquito activity periods, such as dawn and dusk. Stalls should be screened whenever possible and fans installed over horses to help deter mosquitos.

Since mosquitoes are attracted to light, avoid turning lights on in the stalls during the evening or overnight. Placing incandescent bulbs around the perimeter of the stable or use of electric bug zappers will draw mosquitoes away from the horses. Also, birds, including chickens, should not be living in or close to the stable.

In addition, any dead birds should be reported to local health officials so they can be tested for the virus.

Treatment

At the first signs of symptoms of the disease, a veterinarian should be called in to accurately diagnose the illness and prescribe treatment. Since antibiotics are not effective in treating viral infections, such as WNV, treatment is aimed at supporting the horse through the acute phase of the illness.

The horse should be confined to prevent injury and the legs should be wrapped. Access to clean water is important. Veterinarians recommend intravenous fluids and anti-inflammatory drugs to decrease brain and spinal cord swelling. Valium or pentobarbital may be prescribed if seizures occur. If necessary, feed and water may be administered by stomach tube.

In 2002, a vaccine intended to aid in the prevention of WNV in horses was licensed by the U. S. Department of Health. It should be administered in two intramuscular doses, three to six weeks apart. The vaccine differs from the vaccine that protects horses from equine encephalitis and that vaccine is not effective against WNV. It is recommended that all horses in areas where mosquitoes are found should be vaccinated.