Purdue News
Purdue News
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October 1, 2001 Purdue experts recommend West Nile vaccination for horsesWEST LAFAYETTE, Ind. – Although it’s officially fall, experts say the next few weeks to mid-October are the height of biting season for West Nile virus-carrying mosquitoes. Because of this, Purdue University veterinarians recommend that horse owners have their animals vaccinated to provide protection now and build up resistance for later. "We have an Indian summer here that can get pretty hot, so it’s best to get the first two shots now," said Laurent Couetil, Purdue associate professor of clinical sciences. The initial shot must be followed three to six weeks later by a booster of the vaccine. "I would certainly recommend vaccination of horses in any state that has had cases of West Nile virus in birds," Couetil said. Indiana has recorded 27 cases of birds that died from the illness. No Indiana horses have tested positive for the disease, but 137 equine cases have occurred in the United States this year, according to the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service. Administering the West Nile vaccine now, Couetil said, not only provides protection for the rest of this mosquito season, 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. 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 virulent, he said. Conditional approval means the vaccine is safe, but the level of immunity it provides is unknown. Couetil and H. Leon Thacker, director of the Indiana Animal Disease Diagnostic Laboratory and head of the Purdue Department of Pathobiology, 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 more than 30 percent. "Horses that have developed West Nile do not have as high a level of virus as found in the birds," Thacker said. He also said the horses that succumbed to the virus were elderly or suffered from compromised health. Couetil said there is no reason to euthanize a stricken animal. Those 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. 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. No evidence exists that the disease can be passed from horse to horse, horse to human or vice versa. Couetil and Thacker 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. Writer: Susan A. Steeves, (765) 496-7481, ssteeves@aes.purdue.edu Sources: Laurent Couetil, (765) 494-8548 Ralph Williams, (765) 494-4560 H. Leon Thacker, (765) 494-7460 Randy Crom, USDA, Animal and Plant Health Inspection Service, (800) 940-6524 Related Web site: Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; https://www.agriculture.purdue.edu/AgComm/public/agnews/ Purdue News Service: (765) 494-2096; purduenews@purdue.edu
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