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West Nile Virus: A New Challenge

West Nile Virus is now in our headlines and our vocabularies. When any new challenge enters public health, education is the key to reasonable reactions and good choices. Who catches the disease and how? What are experts doing about it? What steps can you and your familes take to protect yourseves and avoid infection? Explore what we know and what we're learning about West Nile Virus. Then make an informed opinion with the activities and questions at the end of this page.

Mosquitoes: Sometimes More than Just Pesky
Virtually every mosquito bite we get may make us itchy but doesn t cause any other problems. In the tropics some mosquitoes carry dangerous illnesses like malaria and yellow fever, but these diseases don t live in the US or Canada. But even up here, if a mosquito bites a person or animal who is sick, it swallows virus or bacteria that is in the blood. Then when it bites another person or animal, it can put that virus or bacteria in that person or animal s blood. Fortunately, most of the diseases that North American mosquitoes carry don t hurt us, and the itch from the bite is not from any disease.

But in 1999, a disease arrived in North America that had never been here before: West Nile Virus. The disease most likely was brought here in a horse or a human. It's not a new disease. People have known about it since 1937, and it may have been around centuries before that. West Nile Virus used to exist only in Africa, West and Central Asia, and the Middle East. Over time, most people and animals there have developed resistance to the disease. When West Nile Virus came to America in 1999, the animals and people here had not been exposed to it before. Fortunately, people are pretty strong and don t usually even get sick from it.

West Nile Virus lives in blood. Fortunately, when an infected mosquito bites a person, usually the person can fight off the disease, and many people build up antibodies that will kill any new West Nile Virus that mosquitoes may bring them in the future. But some people do get sick from West Nile Virus. A few get so sick that they can die. In 2002, an estimated half-million people were bitten by mosquitoes with West Nile Virus; 3667 people got sick from it, and 212 died of the disease. Many of the people bitten by infected mosquitoes now are protected by antibodies.


Targets for West Nile Virus
Mosquitoes bite far many more animals than people. West Nile Virus is far more dangerous for some mammals and many birds than it is for humans. When horses are bit by an infected mosquito, they get the disease easily, and a high percentage of them die. The first known case of the disease in America was a horse in New York in 1999. Since then many more horses have died from it than people. Also, some birds are in real trouble if they get bit by infected mosquitoes. Almost every crow that has been exposed to the virus has died. Blue Jays, in the same family as crows, also get sick and die easily if they get bit by infected mosquitoes. So far, scientists have found that more than 140 species of birds can die from West Nile Virus.


Using Pesticides to Stop the Disease
In hopes of stopping the spread of West Nile Virus, many cities and states have started spraying insecticides in the air to kill mosquitoes. There are two different kinds of insecticide sprays for mosquitoes, and both affect more species than the targeted mosquitoes.
  • Adulticides kill adult mosquitoes. But adult mosquitoes move around so much that this kind of spray program doesn't work well. Mosquitoes can fly over 11 miles in a single day, and many move over 20 miles between their breeding grounds and their feeding areas. So just a day after spraying, a lot of new adult mosquitoes may fly in from other areas. Insecticides that kill adult insects also harm other living things. They kill dragonflies and other insects that eat mosquitoes. The insecticides also kill frogs and fish, which both eat mosquito larvae and are beneficial in many other ways. And these pesticides harm many birds, too. In 2000, researchers in New York collected 80,000 dead birds and performed necropsies on over 4,000 of them. Fully 31% tested positive for West Nile Virus. But pesticide poisoning directly or indirectly caused the deaths of 48%. Some died in accidents or from trauma while sickened or dying from pesticide exposure. Others directly from the poisoning. And even when insecticides were sprayed in the Chicago area in 2002, they still had more cases of West Nile Virus than places in the state where no spraying was done.
  • Larvicides kill mosquito larvae, which live in water. The kind most often used is actually a bacteria called Bacillus thuringiensis. The strain used to kill mosquitoes kills fly larvae but not other orders of insects. So it's safe for dragonflies, butterflies, and other insects. Another kind of larvicide used is a hormone that prevents mosquito larvae from developing into adults. This may be harmful to other insects as well, and may cause other ecological problems. Larvicides are not normally used over backyards, where a great many mosquito larvae grow up.

Insecticides are not the only ways to control mosquitoes. Healthy populations of birds, bats, dragonflies and other "mosquito-eating machines" will get rid of huge numbers of mosquitoes.


How to Protect Against West Nile Virus
Mosquito control is first. The best way to keep mosquitoes from developing in most neighborhoods is to make sure there is NO standing water anywhere. Gutters, sandbox toys, unused flower pots, old tires, and open garbage cans are a few places where mosquitoes lay their eggs. Check them often, and empty out any water. Mosquitoes may lay eggs in birdbaths, too. If you have one in your yard, make sure you change the birdbath water every two or three days.

Prevent mosquito bites. Even if no mosquito larvae live in your area, adult mosquitoes fly in from many other places. So if you live in an area where West Nile Virus exists, the most important thing is to protect yourself and the people you know from mosquito bites. Wear long sleeves and long pants when mosquitoes are around. Insect repellents with DEET work well, but read labels and use only 10% DEET or less. (Spray only outdoors, and wash off repellent when you get home. Don't spray repellent on your hands or face, and don't breathe repellent.)

When you find dead birds, make sure you report them. Call your state or province's department of natural resources to find out what they'd like you to do with the dead bird. Some cities and states are no longer testing dead birds for West Nile Virus if they already know that the disease is in an area; the tests are very expensive. Where dead birds are actually tested to discover the cause of death, many times it turns out to be other things, especially pesticides.


The Outlook: Minds at Work
A vaccination to prevent West Nile Virus in horses already exists. Rehabilitators and people working with captive endangered birds are also using a bird vaccination. According to several experts, a vaccination for people probably won't be made because so few people get sick from West Nile Virus; a vaccination may cause more harm than good.

In the long run, most kinds of animals will develop immunity to West Nile Virus. But until then, there may be a few years when the disease kills a lot more birds and other animals. Figuring out how to solve a tricky problem like this is something we humans are masters at. Information is our best defense.


Try This! Making an Informed Opinion
Print Journey North's lesson : Exposing All Sides: Guide to Making Informed Opinions. Use it as a class to help students generate their own informed opinions about one issue in the West Nile Virus challenge:

To Spray or Not to Spray?


Try This! Discussion or Journaling Questions:

1. How does West Nile Virus affect YOU? Is West Nile Virus in your state or province? What numbers have been reported? To see exactly how many cases of West Nile Virus have been recorded in the U.S this year, go to:


West Nile Virus hasn't spread as quickly in Canada. Could the reason be because it is farther north? One person died of West Nile Virus in Ontario in 2002. The Canadian government is tracking dead birds to keep tabs on the threat. See:

2. Brainstorm a list of people who have direct connections with West Nile Virus. Tell what their connections are, and how they might feel about the issue of spraying or not spraying to kill mosquitoes or their larvae. (Examples: doctor, veterinarian, wild bird rehabilitator, someone whose family member has West Nile Virus, ornithologist, hoser rancher, someone wh works at a place that makes insecticides, someone whose job is applying insecticides, someone who spends a lot of time outdoors in swampy areas....)

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