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West Nile Virus a Threat to Horses
Dr. R. Dean Scoggins, Retired
September 20, 2001
The West Nile encephalitis virus first arrived in the United States in 1999, possibly brought by infected birds or humans. The virus has since spread westward from New York via migratory birds. Crows and blue jays are especially susceptible. The virus was first identified in Illinois in the past month, found in dead crows in the Chicago area.
In addition to affecting birds and humans, West Nile virus seriously affects horses. The disease agent is transmitted among the hosts--from birds to humans or horses--by mosquitoes. This means that areas where West Nile has been identified, including Illinois, are at risk until a hard freeze or several frosts kill off the mosquito populations.
Signs, including depression, unsteadiness, head pressing, and other neurologic signs, appear within 7 to 10 days of infection. According to one report, as many as 60 percent of horses infected with the virus showed neurologic signs of disease. Of those showing signs of disease, 36 percent either died or were euthanized due to the severity of the disease signs. Horses can get several neurological diseases. Any sudden behavioral change in a horse requires an immediate call to a veterinarian.
Because this is a viral disease, there is no specific effective treatment. One company (Fort Dodge Laboratories) has developed a killed-virus vaccine against West Nile virus for horses. The vaccine has been proven safe, with no adverse reactions more serious than an occasional case of hives and temporary soreness at the injection site. Its level of protection is still being studied; on the basis of the effectiveness of other vaccines for similar diseases, this vaccine is expected to provide good protection.
The vaccine has been given a conditional license by the U.S. Department of Agriculture. Owners or their agent are required to give permission for the vaccination of their horses. The vaccination entails two injections, 4 weeks apart. Significant protection will be present 10 to 14 days after the second injection. Single boosters of the vaccine should be given the following spring to stimulate a strong response and give good protection against the virus as the mosquito season returns.
This vaccine is the only protection against West Nile virus currently available. Supplies of the vaccine have not kept up with demand. Horse owners should discuss vaccination with their veterinarians.
Mosquito control is also an important preventive measure. However, a warm, wet fall will increase mosquito populations and increase the risk of viral encephalitis from West Nile as well as from Eastern and Western encephalomyelitis.
EDITOR'S NOTE: This article was written by Dr. R.D. Scoggins, who passed away in December 2006.
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