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West Nile Continues Westward Trek

Arizona is this year's hot spot so far

THURSDAY, July 15, 2004 (HealthDayNews) -- The West Nile virus continues its steady westward push, just five years after its mysterious arrival in the New York City area.

For reasons not entirely clear to scientists, the mosquito-borne illness is making its presence felt this year in the Southwest, especially Arizona, which has reported the most cases in the early weeks of the West Nile season. Cases have also popped up in California, New Mexico, Wyoming and Colorado.

But experts aren't recommending that people in the East let down their guard. While the virus seems to strike a state or region hard one year before moving on -- Colorado in 2003, Louisiana in 2002 -- it never vanishes entirely.

"Every state will have some West Nile cases every year on into the future," said Harry Savage, a research entomologist with the U.S. Centers for Disease Control and Prevention. "It's a virus we're going to have to learn to live with. It's not going to go away."

According to CDC figures as of July 13, the virus has infected at least 108 people nationwide and killed three. And while Arizona doesn't seem like a great place for mosquitoes to find swampy homes, it's currently the nation's hot spot with 66 reported cases and two deaths. California is in second place with 20 cases and no deaths. The sole death outside Arizona occurred in Iowa.

Since the West Nile season is still in its early stages, experts predict many more cases will pop up before the virus goes back into hibernation in the fall. And since doctors aren't required to report mild cases of the disease, there may be many more people infected than show up in the statistics.

The West Nile virus infects the salivary glands of female mosquitoes, which use chemicals in their saliva to thin the blood of the mammals they bite, Savage said. While the virus doesn't seem to make mosquitoes sick, it infects both birds and humans.

About 80 percent of people who get infected don't develop any symptoms for reasons that aren't known. "They may receive a smaller dose, or their immune system makes them better prepared," said Dr. Jennifer Brown, an epidemiologist with the CDC. "We're not clear on why those don't get sick while others do."

About 20 percent of those infected will develop a flu-like illness with headache, fever and muscle aches. And 1 percent -- typically the elderly or those with weakened immune systems -- will become severely ill as the virus attacks their central nervous system and threatens to kill them.

As scientists work to better understand the disease and develop a vaccine, they're also trying to figure out why it's on the move across the country from the Northeast to Southwest. Is it a natural migration or have humans unknowingly guided its progress?

As a "general rule," Savage said, the virus lands in a region one year and then spreads quickly among birds and humans the next year. Then, the virus seems to calm down.

This year, for instance, doctors in New York have reported just one case, even though the virus received national attention when it sickened dozens there in 1999.

How to explain the swift shifts in infection rates? One possibility: The birds that survive infection develop immunity and don't get sick again. Bird migration patterns could play a role, too. And humans might influence the ways of nature by developing mosquito-control programs and wearing more insect repellent.

Regardless of where you live, "the best way to avoid infection is to avoid being bitten in the first place," Brown said. "When you know you'll have exposure, consider wearing long sleeves and socks and applying a mosquito repellent containing (the chemical) DEET."

If you get bitten, don't hurry to the doctor unless you have symptoms of a fever-based illness, Brown said. Despite its spread across the continental United States, West Nile remains a rare disease.

In 2003, the CDC received reports of 9,862 cases of West Nile virus, with 264 deaths.

And remember that mosquito bites -- itchy and annoying as they can be -- are still many times more common than actual cases of illness caused by them.

More information

Has West Nile hit your state this year? Check with the CDC to find out.

SOURCES: Jennifer Brown, DVM, epidemiologist, and Harry Savage, Ph.D., entomologist, Division of Vector-borne Infectious Diseases, U.S. Centers for Disease Control and Prevention, Fort Collins, Colo.
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