In 2002, West Nile Virus (WNV) exploded in the U.S. horse population, resulting in 15,257 documented equine cases of the mosquito-borne virus than can cause encephalitis, or inflammation of the brain, in susceptible animals and people. In 2003, however, the number of reported equine cases fell to 5,181, a fact that might have some horse owners thinking WNV is under control.

That's hardly the case. In fact, WNV will be a threat to horses as long as a virus reservoir remains in the bird population, as is the case with Eastern, Western and Venezuelan equine encephalitis. Now's the time to strategize on the use of vaccines, mosquito sprays and reducing exposure to mosquitoes in order to prevent WNV from infecting your ranch horses.

WNV was first detected in the U.S. in August 1999. Previously, it only had been found in parts of Europe, Asia and Africa. Probably introduced into the U.S by an infected bird or mosquito, WNV is now in 46 states and parts of Canada and Mexico.

The first U.S. WNV outbreak occurred in New York City, where 62 people were diagnosed with the disease, and seven died. In October 1999, 25 horses, all from Long Island, NY, were confirmed with WNV encephalitis, of which nine died or were euthanized. Horses can be exposed to WNV and not show signs but of those infected and that become ill, almost one-third die or are euthanized.

WNV Transmission Cycle

In the U.S., WNV is transmitted by 43 different mosquito species, primarily members of the Culex species. In taking a blood meal, an infected mosquito can infect a bird. Other mosquitoes can then pick up the virus from the infected bird, and pass it on to horses, as well as people. No evidence suggests that horses can transmit WNV to other horses, birds or humans.

WNV also has been diagnosed in dogs, cats, alpacas, mules, sheep and goats. In October 2002, WNV-positive cattle were reported in Nebraska. Visit the National Wildlife Health Center Web site listed below.

But, birds and horses are the animals most likely to develop illness from WNV infection. Crows are especially susceptible, and those infected become sick and die. WNV infection doesn't always lead to signs of illness in people or animals, however. Horses may become ill after the virus infects the central nervous system, causing symptoms of encephalitis.

Clinical signs of encephalitis in horses may include general loss of appetite and depression, in addition to any combination of weakness, incoordination, muscle twitching or tremors; altered mental state, hypersensitivity to touch or sound, cataplexy (emotional stimulus) or narcolepsy (sleeping); seizures, blindness, facial paralysis, tongue weakness, difficulty swallowing, recumbency and fever.

Don't presume horses with clinical signs of encephalitis have WNV. Diagnosis requires ruling out other diseases with similar neurological signs, such as rabies, botulism, equine protozoal myeloencephalitis, and Eastern (EEE), Western (WEE) and Venezuelan (VEE) equine encephalitis.

A positive diagnosis of WNV can be made only via a blood sample, and determining the number of horses exposed is difficult as some exposed horses may not show any clinical signs or exhibit only a few signs. Among clinically ill horses that recover from WNV encephalitis, up to 40% may have residual effects, such as gait abnormalities and a change in temperament or behavior. Such effects can be significant and affect the owner's use or enjoyment of the horse.

Treating Horses With WNV

In August 2003, an antiserum was conditionally licensed for use in horses to aid in the control of signs and progression of WNV. Affected horses should be treated based on symptoms with the focus on reducing the disease severity. Weakened, impaired animals should be protected from self-injury. Fluid and nutritional supportive therapy also may be required, either intravenously or by stomach tube. Consult your veterinarian for recommendations.

In 2002, Fort Dodge Animal Health released West Nile -Innovator equine vaccine. Only sold to veterinarians, its recommendations call for two initial doses intramuscularly, 3-6 weeks apart, followed by a yearly booster. Additional doses are at the discretion of the veterinarian and client.

It's recommended the two initial doses be completed at least one month before the horse will be exposed to mosquitoes. Positive over-wintering mosquitoes have already been found. A large number of adult mosquitoes are expected this summer with warmer weather. For more information on the vaccine, visit the Fort Dodge Web site at www.equinewestnile.com/.

For 2004, vets and horse owners have two other choices. Fort Dodge has released an equine combination vaccine that includes WNV. The vaccine combines WNV killed-virus vaccine with protection against EEE, WEE and VEE, as well as tetanus.

Merial also has released a recombinant vaccine, RECOMBITEK WNV. Merial says the product utilizes recombinant DNA technology to stimulate a fast, multifaceted and long-lasting immune response. For more information, go to www.merial.com.

It may be wise to start vaccinations and boosters earlier this year, as positive mosquitoes and birds are being found earlier every season. Check with your local veterinarian or Extension horse specialist for your state's WNV counts.

Be sure to keep vaccination records on your animals because the testing methods currently used can't distinguish between vaccinated and infected horses. This could also affect international shipping of horses. Remember, vaccination against EEE, WEE and VEE doesn't protect horses against WNV.

There has been concern regarding the vaccine's safety in pregnant mares. But, researchers at Colorado State University and the University of California-Davis found that didn't appear to be the case.

Reducing Exposure

The best method of prevention is to reduce exposure to the virus. This means eliminating mosquitoes and their hatch sites by minimizing standing water. Other steps include:

  • Avoid turning on lights inside the stable after daylight. Mosquitoes are attracted to yellow incandescent bulbs.

  • If light is needed near the stable, place incandescent bulbs outside the stable to attract mosquitoes away from the horses. Black lights (bug zappers) don't attract mosquitoes well.

  • Reduce bird numbers in and around the stable area. Eliminate roosting areas in the stable rafters. Certain species of wild birds are thought to be the main reservoir for the virus. (While pigeons can become infected with WNV, they don't appear to transmit the virus to mosquitoes.)

  • Look around your property for dead birds, such as crows, and report any suspicious birds to your health department. Use gloves to handle dead birds as directed by your health department officials.

  • Topical preparations of mosquito repellents are available for horses, but read the product label before use.

  • For treating standing water, Bti is a natural-occurring bacteria long used to kill mosquito larvae. Its small, donut-shaped forms, often called “mosquito dunks,” are designed for use in small areas of standing water. A granular is available and effective for larger areas such as ponds.

For help in assessing mosquito exposure risks on your property and for suggested control practices, contact your county Extension office, county department of environmental protection, county health department, or mosquito and pest control company.

Research is underway to learn more about WNV encephalitis. Stay informed, especially if WNV has been detected in your area.

Ann Swinker is an Extension horse specialist and an associate professor of dairy and animal science at Pennsylvania State University in University Park.

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