West Nile Weather

West Nile Weather Photo Credit: Clipart.com

In 2004, nearly twenty-five hundred human cases of West Nile virus were reported to the Centers For Disease Control. But in 2003, there were about four times as many. Now, scientists have found a way to predict how severe a particular year's outbreak might be.


A barometer for West Nile virus. I'm Bob Hirshon and this is Science Update.

Hot summers mean a higher risk for West Nile virus epidemics. That's according to Ken Kunkel, an atmospheric scientist at the Illinois State Water Survey, and a team of entomologists and ecologists.

In the northern United States, West Nile is spread to humans by a mosquito known as Culex pipiens. Its population depends on the number of spring and summer days above 81 degrees Fahrenheit.


When that number of days is greater than what we normally experience, we observe that Culex pipiens becomes dominant earlier in the season.

And the sooner that happens, the greater the threat to the public health. Dr. Kunkel says other aspects of the weather, like rainfall patterns, may also help predict the risk. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

West Nile virus is a fairly new problem for Americans. It is common in Africa, Asia, and the Middle East, but probably first appeared in the Northeastern United States around the summer of 1999. By 2002, it had spread virtually nationwide. About 80 percent of people infected with West Nile virus have no symptoms. For some, however, the virus can cause symptoms ranging from fever and headache to paralysis and vision loss.

The number of cases of West Nile virus in America has varied wildly from year to year. For example, in the summer of 2003, there were nearly 10,000 reported cases; in the summers of 2002 and 2004, on the other hand, there were about one-quarter as many. Predicting how severe the disease might be could help public health officials decide how best to prevent it and where to focus their energies.

The life of the West Nile virus is tied to the life cycle of two kinds of mosquitoes: Culex pipiens, the northern house mosquito, which you heard about in the report, and Culex restuans, the white-spotted mosquito. C. restuans infects primarily birds, but not people, and is dominant in the spring and early summer; C. pipiens infects people and its population explodes later in the summer, usually peaking in early August.

C. pipiens mosquitoes hang around until the weather turns cold, so the earlier their population peaks, the more time they have to infect people. Kunkel's team figured out that 81 degrees Fahrenheit is a "magic number" for predicting the trade-off in mosquito populations. If the daily high temperature in the spring and early summer is warmer than 81 degrees more often than normal, C. pipiens peaks earlier, so West Nile cases should go up. If the temperature goes above 81 degrees less frequently than normal, C. pipiens will peak later, and the incidence of West Nile should be lower.

With global warming on the rise, the future does not bode well for heat-related diseases like West Nile virus. Most of the hottest summers on record have occurred within the last 20 years. Scientists predict that global temperatures will continue to climb for at least the next century. So, although the West Nile virus may be worse in some years than others, it doesn’t appear to be going away anytime soon.

Now try and answer these questions:

  1. What two mosquitoes carry West Nile virus? How are their life cycles related?
  2. What's so special about 81 degrees Fahrenheit?
  3. Why do you think rainfall might also affect the prevalence of West Nile virus?
  4. Can you think of other diseases that might be affected by global warming?

For Educators

The Centers for Disease Control's West Nile Virus site has a wealth of information about the virus, how it spreads, and how to prevent it. You also can find statistics on the disease.

Rutgers University's Mosquito Biology page features information about the mosquito's biology and life cycle.

In the lesson Viral Invaders, found on the New York Times Learning Network, students explore scientific information about the West Nile virus in order to analyze the accuracy of their own background knowledge on the topic.

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