NATURE

Death Angels

Mosquitoes may be small, but they are deadly. As NATURE's BLOODY SUCKERS shows, these agile buzzers are probably responsible for more human deaths each year than any other animal. That's because they can pass along a host of diseases -- from malaria to dengue fever -- that claim millions of lives.

Today, most of the victims of mosquito-borne diseases live in the developing nations of the tropics, where a combination of climate and poor health care boosts the terrible toll. But in 1999, the citizens of wealthy nations got a fresh reminder of the threat posed by mosquitoes when West Nile Virus -- a potentially lethal mosquito-borne virus long known in Asia, Europe, and Africa -- appeared for the first time in North America. Since the virus claimed its first human victim in New York City, it has spread west across the continent, infecting at least 1,500 people and killing nearly 100.

Like much of the viral cargo carried by mosquitoes, West Nile Virus doesn't really target people. It's a virus that evolved to attack birds. Its human victims are innocent bystanders, and mosquitoes are the virus' unwitting carriers, ferrying the virus from infected birds to people (and back). The mosquitoes themselves aren't harmed by the virus, and even most people survive infection with little more than aches and pains.

Many birds, on the other hand, succumb: researchers estimate that West Nile Virus has killed at least 100,000 crows and blue jays in the United States. Indeed, the sudden presence of dead crows is often the first clue that West Nile has arrived. And bird experts worry that the virus could have a serious impact on other wild bird populations, since more than 100 species have become infected.

Although the virus can literally fly across the landscape, its quick and lethal spread across North America has surprised many scientists. They aren't sure how it got here, although they believe it came from the Middle East. But they do know that, as of late 2002, it had spread to 42 U.S. states and four of Canada's ten provinces. "It's really amazing to me how fast it's going," Laura Kramer, a virologist at the New York State Department of Health in Guilderland, recently told SCIENCE magazine. And the spread is likely to continue: researchers believe migrating birds could eventually carry West Nile to the northernmost plains of Canada and the southernmost reaches of South America. Infected mosquitoes could also be carried to new hunting grounds on planes, trains, and trucks.

Besides trying to understand how the virus is spread, researchers are also working on trying to figure out how to fight it. Many communities have stepped up spraying against mosquitoes, but it's not clear if that helps. One problem is that there are dozens, if not hundreds, of kinds of mosquitoes that might carry the virus, and they all have different habits. Some feed at night, for instance, while others snack during the day. Some go after people and birds, while others don't. And some are only vulnerable to pesticides when they fly; resting insects seem to survive just fine. Such complexity makes fighting West Nile Virus, and other diseases carried by mosquitoes, quite a chess game.

Other scientists hope to develop a vaccine that would help the most vulnerable people -- children and the elderly -- fight off any infection. But that may take years. In the meantime, public health experts recommend wearing bug repellent and protective clothing to ward off what one researcher calls "those buzzing death angels."


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