Where has West Nile Virus been found in the U.S.?

Like other encephalitis-like viruses birds are an integral player in the life of the West Nile Virus. Mosquitoes that are infected with the virus feed on a bird, transferring the virus to the bird. Other mosquitoes that feed on the blood of that bird can then be infected, and so on. To date crows seem to be the most sensitive birds, and many hundreds of them have been killed by the disease. However, up to 20 other kinds of birds have tested positive for the virus, including other migrating birds such as mallards and robins. Several species of hawks as well as the Bald Eagle have tested positive as well.

It is possible that it is within these infected migrating birds that the virus spreads. While initially found in New York City in 1999, the known range of infected birds by October 2000 was as far south as North Carolina and as far north as Maryland and Washington DC. Wherever infected birds fly it is possible for the disease to be spread, and there even are dire predictions that the entire United States could become populated by infected birds.

How is West Nile Virus spread?

It is only by the bite of an infected mosquito that West Nile Virus is passed to people. An infected person cannot spread the disease to another person, and it cannot be passed directly from infected birds to people. It must be injected into the blood stream, and this is the mechanism mosquitoes use when they feed. Mosquitoes have the ability to ingest many other kinds of disease-causing pathogens, as they feed on human blood. However, almost all of these simply are "casual visitors" in the system of the mosquito, and they do not appear able then to pass that disease onto other people. Extensive studies find that they are ingested and passed through the digestive system and excreted. Encephalitis-type viruses, though, are intimately involved with the mosquito, and in fact MUST spend part of their life in a mosquito, where they propagate their numbers and then migrate to the salivary glands of the mosquito, to be passed into the next warm-blooded animal that mosquito feeds on.

Several species of mosquitoes appear capable of spreading West Nile Virus, including some that tend to fly in daylight hours as well as those that are primarily night fliers. These mosquitoes cannot survive the cold months as biting adults, but instead pass through the winter in the egg stage that has been placed in some protected environment by the adults the summer before. Obviously, then, the disease is only likely during those months when mosquito adults are active.

How dangerous is this disease?

West Nile Virus is considered to be a rather mild strain of encephalitis to humans. While many people have been killed by it, and that is a terrible thing, the vast majority of people who become infected with this virus may feel either no effect at all or very mild symptoms. They may experience a mild headache and fever and then recover completely. While it is possible that anyone might be seriously affected by the disease, those who are at the highest risk are the elderly, the very young, and those with weakened immune systems. There currently is no vaccine for this virus, and treatment of the disease for those severely affected is to treat the symptoms until the disease runs its course.

In 1999, in the New York City area, seven people were killed and 62 were seriously ill from West Nile Virus. In 2000 there was a single death and only 17 confirmed cases of seriously ill people. In other countries in Europe and the Middle East it has been responsible for dozens of deaths in several major outbreaks in the past 3 or 4 years, so the potential for major problems clearly exists.