Has the West Nile Virus Come And Gone? Hardly, Experts Warn.

The region's fixation on the West Nile virus began with the sighting of a dead crow in Baltimore in 1999, and it mushroomed three years later, when the virus killed 11 people in the District, Maryland and Virginia.

But although nearly 100 people have died of West Nile in the country this year, the Washington region is poised to end 2007 with just 13 reported cases -- 10 in Maryland, three in Virginia, none in the District -- and no deaths.

What happened to a virus that just a few years ago was considered one of the region's greatest public health threats? Unusually dry weather hampered the breeding of some mosquitoes that carry the virus, and some health officials credit a concerted public education campaign.

Despite the decline in reported West Nile cases, many health experts remain focused on the virus, now considered endemic to the region. West Nile could reemerge for a variety of reasons, some of which might not be understood now, experts said.

"There's no shortage of mysteries with West Nile virus," said David Gaines, public health entomologist for the Virginia Department of Health. "There's still a lot that needs to be learned."

West Nile is spread to people by mosquitoes that get the virus by biting infected birds. The spread has followed a predictable pattern since it was first detected in the New York area in 1999.

Health officials carefully tracked its progress in the Washington region: first in crows, then mosquitoes and eventually horses and humans. In 2001, two years after the initial infected crow was found in Baltimore, the first human case of West Nile was reported in the region, and the public was informed of subsequent cases in the following years.

"People are a lot more aware of West Nile virus," said Kimberly Mitchell, Maryland's chief of rabies and vector-borne diseases. "They understand what it is and what the steps are they can take to protect against it."

LaSh¿n Seastrunk, a D.C. Health Department spokeswoman, said the lower numbers are due to "extensive outreach education programs" that have taught people about precautions, as well as increased applications of substances to kill larvae in catch basins and sewage drains where mosquitoes breed.

"It's consistent, and I think that's what's led to it being so successful," she said. "We have a very targeted response."

The number of West Nile-related deaths peaked as the virus spread from cities to rural areas. In 2002, 11 people died in the region, and the toll dropped to nine the next year. By 2004, the number fell to one, followed by none in 2005 and one in 2006.

Meanwhile, the virus has continued to spread west across the country, with a high of 284 deaths in the nation in 2002, according to the federal Centers for Disease Control and Prevention. This year, the virus has been concentrated in the western United States, with Colorado reporting 555 of the country's 3,404 human cases, according to the latest figures available from the CDC. Ninety-eight deaths have been reported nationally, according to the CDC.

But experts said the numbers can be deceiving because most West Nile cases are not reported. Many who contract the virus are unaware of it, experiencing mild flulike discomfort, and reported cases and deaths typically involve people who develop meningitis or encephalitis.

Gaines estimated that "not even 1 percent of the population" in Virginia has been exposed to West Nile, meaning that few people have developed resistance to the virus and could be susceptible to infection.

"I've heard people say West Nile virus came through, it affected people on the East Coast and it's gone," he said.

"There's an awful lot of people on the East Coast who have never been exposed to it. I think we might get a confluence of just the right water conditions and just the right mosquito species."

The perfect conditions for a West Nile resurgence, Gaines said, would include a relatively large bird population that had not built up immunity from previous exposure to the virus. There would have to be enough rainfall to provide breeding conditions for some mosquitoes but not so much that mosquitoes that favor underground storm sewers are flooded out.

"That's essentially what would lead to another bad West Nile virus year," Gaines said.

Experts will ask residents to continue to take precautions as researchers try to learn more about the virus, he said.

"Even if you know it all, you can't predict what's going to happen from year to year, because it's too complex a system," Gaines said.