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 New Threat for Birds, Horses, Humans

Pet Column for the week of October 8, 2001

Related information:

Services - Public Health

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

In 1999, veterinarians at the Bronx Zoo Wildlife Conservation Park in New York were perplexed by the sudden death of 24 birds from some kind of encephalitis (inflammation of the brain). A few cases of human encephalitis occurred in New York around the same time and were attributed at first to the St. Louis encephalitis virus, which is native to the United States, but does not usually cause disease in birds.

The real culprit was eventually shown to be the West Nile virus, which had never before been identified in the Western Hemisphere. Since that time, the incidence of the disease has increased dramatically and cases have been reported in New York, Michigan, Connecticut, Ohio, Ontario, Indiana. In August, several birds testing positive for the disease were found in Illinois. This indicates that the disease is moving across the country carried by infected birds.
Dr. John Andrews, director of the Veterinary Diagnostic Laboratory at the University of Illinois College of Veterinary Medicine, says, "The most significant reason to be concerned about this virus is that it is zoonotic, which means that it can be transmitted to humans."

Because the disease is caused by a virus, there is no effective treatment once infection has occurred. The virus infects domestic and wild birds, horses, and humans, but no disease in cats and dogs has been reported. It causes inflammation of the brain, which is often accompanied by neurological problems, including difficulty walking, tremors, abnormal head posture, circling, and convulsions. Although there have been a few human deaths from the virus, people tend to get flu-like symptoms and eventually recover, whereas the disease is usually fatal in certain birds, such as crows and blue jays. Death in horses has also been reported.

The disease is spread by mosquitoes, which are a secondary host to the virus. When the infected mosquito feeds on an animal, the virus enters the animal's bloodstream through the saliva of the mosquito. It is also thought that occasionally the disease may also be spread via feces and urine, and by birds of prey that may eat other birds already infected by the virus.

Dr. Andrews says, "It is likely that the disease may have been brought to this country by people traveling from abroad since the virus closely resembles the West Nile virus identified in an earlier outbreak in Israel." Since 1999, when the virus was first identified in the United States, efforts to monitor and control its spread have been put in place. All states have now instituted a surveillance program, and efforts to control the mosquito population have become a high priority. Ironically, there have been more cases of human infection in cities than in rural areas. Dr. Andrews says, "This is partly because the human population is much more dense in urban areas, and partly because urban areas may accumulate more standing water in which mosquitoes can breed."

The best thing that can be done to avoid contact with this virus is to make sure to empty containers of standing water in your yard and avoid being bitten by mosquitoes, for example, by using insect repellant.

If you have questions regarding the West Nile virus, contact your local veterinarian.