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West Nile virus, which surged in the Southwest this summer, has been silent or merely steady in Georgia and other parts of the country --- but recent hurricanes in the Southeast could provoke more activity this fall, health officials say.
The storms may have dampened the disease temporarily by flushing out mosquito pools. But that water, as it stagnates, could enable renewed insect growth in coming weeks.
"It's still too early to tell what could happen," said Katherine Bryant, an epidemiologist with the Georgia Division of Public Health, which has reported 14 cases of West Nile and no deaths this year, compared to 14 cases and one death at this time last year.
The deluges delivered by hurricanes Charley, Frances, Ivan and Jeanne diluted many of the dirty pools preferred by Georgia's main West Nile culprits, the Southern house mosquito (or Culex quinquefasciatus) and the Asian tiger mosquito (Aedes albopictus), said Rosmarie Kelly, a medical entomologist with the public health division.
But once the water sits a while, especially if the weather stays warm, the critters could return with a vengeance. Both mosquito species stay active through late October, and the Asian tiger mosquito in particular is not picky when prowling for a place to breed.
"These mosquitos will lay eggs in a bottle cap or on a magnolia leaf," if the surfaces contain water, Kelly said. "People must be very vigilant about dumping out containers."
Nationwide, 1,604 cases of West Nile and 48 deaths have been reported this year, compared with 4,666 cases and 88 deaths at this time last year, said Dr. Lyle Petersen of the Centers for Disease Control and Prevention's insect-borne disease lab in Colorado.
Many of those cases are mild West Nile fever, for which testing and reporting can vary greatly by state and year.
The CDC pays more attention to serious West Nile cases involving "neuroinvasive" disease, including brain swelling. In that category, 559 cases have been reported this year, compared to 1,338 cases at the same time last year.
Seven of Georgia's 14 cases this year are considered serious.
Half of the serious cases nationally this year have been in Arizona, California, Nevada and New Mexico --- a region that had little West Nile activity until this summer.
Last year's hot spot, Colorado, has been relatively quiet, with 32 serious cases and two deaths. Nebraska, also heavy last year, has had no serious cases. Several states in the Northeast --- West Nile was first recognized in this country in New York City five years ago --- have had no reports this year, even mild cases.
Health officials attribute the relative dormancy to a few factors: weather, education, immunity and the vagaries of nature.
It has been cool and wet this year in such places as Colorado, and West Nile generally thrives when it is hot and dry, Petersen said. People may be paying more attention to advice to clear standing water, wear protective clothing and apply mosquito repellent containing DEET.
Humans infected with West Nile become immune, and nearly half a million people are thought to get the virus each year without knowing it. More important, birds --- which carry the virus and which mosquitos feed on --- also develop immunity. So birds in much of the country may be less apt to harbor the virus now, Petersen said.
But scientists have been studying a cousin of West Nile, called St. Louis encephalitis, for decades, and they still can't predict or explain where, when and how strong it strikes.
Petersen expects West Nile to remain equally mysterious.
"A low year this year doesn't say anything about what's going to happen next year," he said. "I suspect we'll see big fluctuations every year."
Copyright 2004 The Atlanta Journal-Constitution