West Nile Virus Declines Sharply Despite Its Coast-to-Coast Spread

By Charles Piller

Sept. 16, 2005 12 AM PT

Times Staff Writer

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

The nation has logged 1,299 cases, down from 1,386 last year at this time and a far cry from 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 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 highest number of 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 number of cases nationwide 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 Dr. Theresa Smith, an epidemiologist who heads West Nile surveillance for the CDC. Public health measures have undoubtedly helped contain the disease to some degree.

When West Nile became a major problem in Southern California last year, the state added $12 million to local mosquito abatement programs, boosting overall spending to more than $90 million.

“Without those funds, without a doubt we would be seeing many more West Nile cases,” said Vicki Kramer, an entomologist who heads California’s West Nile monitoring program.

The widespread message for people to use mosquito repellent containing DEET has also helped.

Analyzing the weather has proved to be little help in predicting outbreaks of the disease.

Southern California and the Central Valley both experienced wet springs and unusually hot summers, which favors the mosquito species Culex tarsalis and Culex pipiens, notorious among entomologists for spreading the virus.

“It was the perfect mix of ingredients for West Nile,” Kramer said.

The number of cases did shoot up in the Central Valley, but Southland numbers have plummeted.

Colorado also had a wet spring followed by a hot summer, but Culex populations fell dramatically compared to prior years, accounting for the sharp drop in West Nile cases, said John Pape, an epidemiologist 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 in 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 only 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 will probably 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 even realize they have acquired the virus. About 20% of those 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 low -- no higher than 3% where intense outbreaks have occurred, according to the CDC.

Even in places such as Israel, where immunity levels have been measured at up to 73% of the population, outbreaks still occur.

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West Nile’s cycles

West Nile virus cases in the U.S. have decreased in the last two years. The disease, which is most commonly transmitted from birds to humans by mosquitoes, tends to peak in the summer months.

Human cases in U.S.

1999: 61

2000: 21

2001: 66

2002: 3,727

2003: 9,862

2004: 2,539

2005: 1,299 (1)

(1) Through mid-September

U.S. cases by month

2004

9/1 - 9/28: 731