After the floods, West Nile?

Mosquito breeding grounds breed disease fears, others argue region may be safer

By | September 22, 2005

As the waters from Hurricane Katrina begin to recede, Gulf coast residents may have something else to worry about: mosquito-borne disease. Some scientists are concerned that the large, stagnant bodies of water that Katrina left behind will be ideal breeding grounds for vector mosquitoes, perhaps amplifying the low level of West Nile virus present in the area this season. However, experts also suggest that the hurricane may have wiped out most of the mosquitoes and birds that bring the disease to humans, making the region safer than before.

"It's going to be difficult to predict whether or not [the hurricane] is going to have an impact on West Nile," said Anthony Fauci, director of the National Institute of Allergy and Infectious Disease in Bethesda, Md.

Stagnant waters certainly favor the proliferation of mosquitoes, but most of the birds that serve as reservoirs for West Nile and other arboviruses have likely been killed or cleared out by the hurricane, Fauci told The Scientist. "Luckily, those two factors will neutralize each other."

Birds seemed to have disappeared entirely in southern Louisiana after the storm, according to Colonel Dean L. Winslow of the Delaware Air National Guard, who served as the U.S. military coordinator for public health and force protection in 13 Louisiana parishes from September 5-15. "One of the things that I was struck by was just this eerie silence," said Winslow. "The birds, which are the main reservoir, were basically blown away."

Mosquitoes, considered the most effective vectors of West Nile virus in the Gulf Coast, are also facing some obstacles to their ability to spread the disease. Two species that are particularly good West Nile vectors, Culex pipiens and Culex quinquefasciatus, live mainly in low, enclosed areas, so many were probably killed by the floods that followed the hurricane, said David Gaines, state public health entomologist with the Virginia Department of Health in Richmond. Furthermore, the species most likely to have survived the storm and flooding are those that live high in the hollows of trees, or "floodwater" species whose eggs are triggered to hatch when water floods over them, and those species are not as likely to spread West Nile efficiently, Gaines added.

But even if the hurricane's violent winds and rains have flushed out many mosquitoes, they've started to make a comeback, Winslow said. Some parishes in Louisiana have reported mosquito populations 800 percent above normal, said Kyle Moppert, a medical entomologist with the Louisiana Department of Health and Hospitals (DHH) in Baton Rouge. Furthermore, Culex quinquefasciatus, considered the best vector for West Nile in the Gulf states, thrives in septic water and therefore will probably reproduce prolifically in the region's pools of backed-up sewage, Moppert added.

An immediate concern is that people who live in areas affected by the hurricane now have increased exposure to infectious mosquitoes, said Roger Nasci, an entomologist with the Centers for Disease Control and Prevention (CDC) Division of Vector-Borne Infectious Diseases in Fort Collins, Colo.

It hasn't been a big year for West Nile virus in the Gulf states, but Louisiana, Mississippi, and Alabama have all reported some human cases this year, so infected mosquitoes must have been around before the storm, Nasci said. Katrina has increased "human exposure dramatically by displacing people, by compromising houses, by putting power out, by bringing a lot of responders into the area that are spending a lot of time out of doors," he noted.

Human West Nile virus cases are generally seen in Louisiana all year, but only sporadically after September. Consequently, it's not clear whether new mosquitoes will acquire the virus in time to infect humans, Nasci said. As a precaution, officials decided to spray several Louisiana parishes with the organophosphate pesticide naled, said Moppert, which is routinely used by mosquito control programs in Louisiana. National Guard soldiers are also monitoring standing water to make sure that mosquito larvae aren't forming, Winslow said.

The mosquito-borne diseases Eastern equine encephalitis and St. Louis encephalitis—both endemic in the southeastern U.S.—are also a mild concern, according to Nasci. "There's been a lot of speculation about malaria and dengue," Nasci said, but "the likelihood is so low as to be negligible."

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