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West Nile cases decline; science uncertain why

The use of DEET helps, but the complex factors that fuel some outbreaks are tough to decipher.

By Charles Piller / Los Angeles Times

West Nile virus cases have fallen dramatically from their peak two years ago, even as the mosquito-borne disease completes its migration across the United States to cover most of California.

The nation has logged 1,299 cases, down from 1,386 last year at this time and far below the 4,137 reported by mid-September 2003, the worst year so far, according to the federal Centers for Disease Control and Prevention.

In 2003, the disease had just barely touched the populous West Coast. The largest outbreaks were occurring in Nebraska, South Dakota and Colorado, which by itself reported nearly 3,000 cases that year.

Colorado has seen 48 cases this year.

California has so far reported the most cases in 2005 -- 593 with nine deaths, according to the state Department of Health Services. That's about the same as 2004, even though this is the first year that large numbers of birds and mosquitoes carrying the virus have been detected in the Central Valley and Bay Area.

Health authorities are uncertain why the total number of cases in the nation is dropping.

Scientists understand how birds and animals carry the virus. They know how it spreads to people from mosquitoes that have bitten infected birds.

But six years after the first West Nile virus case was identified in New York, the interplay between birds, mosquitoes and climate that fuels some outbreaks and stifles others has proved too complex to decipher.

"There probably never will be an ability to predict West Nile," said Theresa Smith, an epidemiologist who heads West Nile surveillance for the CDC.

The widespread message for people to use mosquito repellent, such as DEET, has also helped.

Colorado also had a wet spring followed by a hot summer, but mosquito populations fell dramatically from previous years, accounting for the sharp drop in West Nile cases, said John Pape, who heads West Nile surveillance for the Colorado Department of Public Health and Environment.

His program is mapping rainfall and temperature to West Nile's spread, looking for a precise sequence of conditions that may spark a major outbreak.

"Every once in a while every one of these factors that we are just beginning to understand are going to happen the right order, and we'll have a few thousand cases like we had in 2003," Pape said.

Most southeastern states, with steamy climates ideal for mosquito breeding, have seen few cases. So far this year, Alabama has reported five, Mississippi 23 and Florida 12, according to the CDC.

Hurricane Katrina has boosted mosquito populations in hard-hit states, but it also disturbed bird populations.

West Nile probably will not rise dramatically in states affected by the storm, according to mosquito-control experts.

Most people who are infected with the virus never develop a severe illness and don't realize they have acquired the virus. About 20 percent of those infected will develop symptoms similar to those of a three-day flu, and about one in every 150 will develop a serious illness such as meningitis.

A vaccine against the virus is at least several years away, so the only way to develop immunity is to be bitten by an infected mosquito.

But even in states where West Nile became established years ago, immunity levels remain trivial -- no higher than 3 percent where intense outbreaks have occurred, according to the CDC.