Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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West Nile Disease: A Threat To Midwest Horses


Pet Column for the week of August 19, 2002


Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Jennifer Stone
Information Specialist

By now, most people have heard of West Nile virus. Extensive news coverage has tracked its spread across the midwestern United States and the infection of several people. While this disease does affect humans, it can pose an equally serious health threat for horses.

The West Nile virus entered the United States in the fall of 1999 and has steadily spread westward. It entered Illinois in the summer of 2001, killing countless birds and two horses in the fall of 2001. This year birds infected by West Nile virus have been identified in nearly all Illinois counties, and several human cases and multiple horse cases are currently diagnosed in Illinois.

Dr. R. Dean Scoggins, an Extension veterinarian at the University of Illinois College of Veterinary Medicine, warns that this disease can be very serious for horses and that precautions should be taken to prevent exposure. He says, "This disease is spread by mosquitoes that feed on infected birds. The infected mosquito then feeds on another animal and the virus enters that animal's bloodstream through the saliva of the mosquito."

Preventing your horse from being exposed to mosquitoes may seem impossible, but you can take steps to reduce the risk for your horse. "Mosquitoes commonly feed at dusk and dawn, so it is important for horse owners to use mosquito repellant on their horses both indoors and outdoors during these times. Using fans in the stalls can also reduce the likelihood of exposure," says Dr. Scoggins. "These precautions should be maintained until after a killing frost in the fall."

In spite of the best precautions, infection can still occur. Dr. Scoggins suggests that horse owners to be on the lookout for this serious disease. "Signs of the disease in horses include depression, loss of appetite, fever, incoordination, and weakness," says Dr. Scoggins. The virus causes inflammation of the brain, which is often accompanied by neurological problems, including difficulty walking, tremors, abnormal head posture, circling, and convulsions.

Once the disease has been contracted, there is no specific treatment available. Many horses have been saved by supportive care after the horse begins to show signs, however. "Be sure to call your veterinarian at the first sign of a problem! The earlier you can begin supportive therapy, the better," says Dr. Scoggins.

A vaccine for the virus was introduced in August 2001. Although there is still limited information on the effectiveness of the vaccine, preliminary evidence suggests that it is effective, and few problems have been reported. "This vaccine is given in a series of three shots and protection from the virus occurs about 2 weeks following the booster vaccination," says Dr. Scoggins "Maximum protection occurs about 6 to 8 weeks after the first injection."

Foals can be vaccinated at 6 to 8 weeks of age. If the mare was vaccinated for West Nile before foaling, then you can wait until the foal is 3 to 4 months of age to give a booster.

"There have been a few cases in which the horse was infected with the virus in spite of being vaccinated for the disease, but usually these horses had either not received one of their boosters or were infected before the booster had time to take effect," says Dr. Scoggins "I recommend that all horse owners discuss the vaccine with their veterinarian. It is also important that people control mosquito breeding in their area by removing any standing water on their property and keeping tall grass mowed."

Discuss prevention options with your equine veterinarian, and if you suspect that your horse may already have the disease, contact your equine veterinarian immediately.