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November 2, 2001 State confirms West Nile equine case, Purdue vets offer adviceWEST LAFAYETTE, Ind. – Purdue University veterinarians advise vaccinating horses for West Nile virus in the wake of Indiana’s first confirmed equine case of the illness. The horse, a 7-year-old mare in the north central part of the state, apparently became ill about two weeks ago and was too sick to save. It was euthanized and necropsy and lab tests were performed to confirm the diagnosis. Laurent Couetil, associate professor of veterinary clinical sciences, said if a horse has not yet been vaccinated for West Nile it could still be done this year. Administering the West Nile vaccine, Couetil said, not only provides protection now, but vaccinated horses also will begin building antibodies, or resistance, to the virus faster when they receive a booster shot in the spring when mosquitoes reappear. "Horses should definitely be vaccinated before mosquito season next spring," he said. The initial shot must be followed three to six weeks later by a booster of the vaccine. Couetil said that horse owners should contact their local veterinarian to schedule the shots or they can make an appointment to bring the horse to Purdue’s Large Animal Clinic in West Lafayette. Currently the vaccine is not available through farm supply companies. USDA Animal and Plant Health Inspection Service veterinarian Randy Crom said the risk of a horse in the northern region of the country contracting the disease now "has dropped dramatically" because some frost has been recorded. However, he and Couetil said mosquitoes don’t completely disappear until a hard, killing frost. Crom said as of Nov. 1, the USDA has confirmed 347 West Nile cases in horses in the United States – 22 percent have proved fatal, some outright and some requiring euthanasia. Although the USDA has conditionally approved the vaccine as an annual shot for horses, Couetil said that its effectiveness is uncertain. Horse owners may need to give two to three shots each year, especially in the southern United States where mosquitoes are particularly abundant, he said. Conditional approval means the vaccine is safe, but the level of immunity it provides is unknown. Couetil and Gregory Stevenson, professor of veterinary pathobiology and pathologist with the Indiana Animal Disease Diagnostic Laboratory at Purdue, both said the percentage of horses that die of West Nile virus has declined since the first case was diagnosed two years ago. Most horses now recover from the illness, compared to the previous fatality rate of close to 40 percent. "Horses that have developed West Nile do not have as high a level of virus as found in birds," Stevenson said. Mosquitoes transmit West Nile by biting infected birds, then biting horses. This sends virus-laden saliva into the animal’s bloodstream. According to the USDA, there is approximately a one- or two-week lag time between when a horse is bitten and when it may develop the sickness. Some horses will test positive for the disease but never become ill. Indiana has recorded about 50 cases of birds that died from the illness. No evidence exists that the disease can be passed from horse to horse, horse to human or vice versa. "West Nile is tricky to diagnose because there are several diseases that have similar symptoms," Stevenson said. Veterinarians can detect exposure to West Nile through a blood test, making it important to contact a veterinarian to get a prompt and proper diagnosis, he said. Couetil said most horses that become ill can be successfully treated through support therapy similar to that used for any virus. "We treat the consequences of the disease by providing good nursing care and anti-inflammatory drugs as appropriate," he said. Severe cases of the disease cause encephalitis, or swelling of the spinal cord and brain, and can lead to permanent neurological damage or death. Initially, horses exhibit an abnormal, wobbly, unsteady gait due to loss of muscle control, lethargy, and later, partial paralysis, however, their body temperature usually remains normal. Couetil and Crom said the risk is very low of a horse being infected with the disease during the winter. They recommend owners have their horses revaccinated in the spring about two weeks prior to mosquito season or, if it’s the first shot, about one month before. Couetil and other experts advise horse owners to control mosquito proliferation by keeping horses inside during periods when the insects are most likely to bite, putting screens on stall openings and using insect repellent. At least 14 mosquito species found in the United States carry the virus. Their populations and biting season tend to peak at the end of the summer into mid-October, said Ralph Williams, Purdue entomology professor. Williams said it’s important to drain sources of stagnant water such as drainage ditches, clogged gutters, unused swimming pools, birdbaths, water troughs, pet bowls and even discarded tires. It’s also important to regularly and thoroughly clean water receptacles such as troughs. "These are typical habitats where the types of mosquitoes found in Indiana breed," Williams said. In addition to their adaptability at proliferating, these mosquitoes survive through the winter to present another more widespread onslaught of disease. "We expect this to spread across the country," said Couetil. "It is progressing rapidly." West Nile virus first appeared in the United States in late 1999, although it has been known for hundreds of years in the Middle East and Africa. The name comes from the first time the disease was isolated. That was a 1937 case of a Ugandan woman in the West Nile District. The first equine cases recorded were in Egypt and France in the 1960s. States where horses have tested positive include Connecticut, Florida, Georgia, Kentucky, Louisiana, New York, Pennsylvania, Illinois, Iowa, Virginia, Alabama, Maine, North Carolina and Mississippi. More than 75 percent of the recorded cases have been in Florida, according to the USDA. For more information and background on West Nile virus go to the web site. Writer: Susan A. Steeves, (765) 496-7481, ssteeves@aes.purdue.edu Sources: Laurent Couetil, (765) 494-8548 Ralph Williams, (765) 494-4560 Gregory Stevenson, (765) 494-7473 Randy Crom, USDA, Animal and Plant Health Inspection Service, (800) 940-6524 Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; https://www.agriculture.purdue.edu/AgComm/public/agnews/ Related Web sites: Purdue News Service: (765) 494-2096; purduenews@purdue.edu
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