Mosquito Season Turns Deadly

Few things appear more threatening than new diseases, especially ones that are potentially fatal. The West Nile virus, which can cause a deadly encephalitis (or brain swelling), has already killed seven patients this year and infected at least 128 more--and the mosquitoes that transmit the disease are still biting. Equally alarming, the virus is spreading rapidly. Since its first known U.S. appearance in New York in 1999, it has moved as far west as Texas. It's unlikely to stop short of California. "I'd bet my life savings on it," says infectious-disease expert Dr. Lyle Petersen of the Centers for Disease Control and Prevention.

Does that mean that the West Nile virus is slated to become a major killer? Probably not. It is moving quickly now, in part because every new area represents virgin territory, where neither people nor animals have developed immunity. (Mosquitoes pick up the virus by feeding on the blood of infected birds, such as crows.) But ultimately, Petersen speculates, the virus will likely settle into a pattern resembling that of its close relative, St. Louis encephalitis. When SLE first appeared in Missouri in 1933, it infected at least 1,000 people and killed 200. Since then, it has generally caused no more than a few dozen cases a year--with occasional spikes into the hundreds and, in 1975, to almost 2,000.

If this analogy is correct, then West Nile virus calls for vigilance, but not panic. The vast majority of mosquitoes do not carry either West Nile or SLE, and less than 1 percent of people who are bitten will develop serious complications-- the majority of them elderly.

Still, it's best to take precautions. Avoid the outdoors between dusk and dawn, when mosquitoes are biting. Cover your arms and legs, and use insect repellent. And eliminate sources of standing water. "I've seen mosquitoes breeding in containers as small as a bottle cap," says Joseph Conlon of the American Mosquito Control Association. "They've been around for 400 million years, and it's not because they're stupid." They've been carrying diseases for eons, too, and smart humans will do what they can to stay out of their way.