Risk of West Nile virus remains low despite mosquito boom in flood areas

COLUMBIA, Mo. – Mosquito populations along the Missouri River have skyrocketed due to recent flooding.

The good news is that mosquito species linked to transmission of West Nile virus (WNV) aren’t part of this population explosion, said a University of Missouri Extension entomologist.

“The threat of WNV transmission is not necessarily elevated because the types of mosquitoes associated with flooding are not the same as those associated with transmitting WNV,” said Richard Houseman. “The abundance of these different species also varies hugely, with the floodwater species being many times more abundant than the vector species, even during years when there is average flooding.”

In surveys conducted in recent summers, traps at 12 central Missouri sites caught more than 65,000 mosquitoes belonging to the floodwater species Aedes vexans. During the same collection period those sites caught only about 750 mosquitoes from the three species associated with WNV, called “vector” species.

“This represents a ratio of 99-to-1 floodwater to vector mosquitoes collected over an entire summer in an average year,” Houseman said. “In a flood year, when mosquito trap collections are 3-5 times higher than normal, the ratio of floodwater to vector mosquitoes could easily increase to more than 400-to-1.”

Laboratory screenings of more than 30,000 mosquitoes from St. Louis County found only six mosquitoes that tested positive for West Nile virus, he said.

The floodwater species Aedes vexans gets its name because it’s an aggressive, persistent and “vexing” biter. It feeds on mammals, including humans, and is most active from early evening until midnight. It lays eggs on the soil just above standing water, and these eggs can withstand drought, cold and rain for up to four years. They don’t hatch until they are flooded and floating in water with reduced oxygen levels (caused by decaying organic matter). While not linked to WNV, Aedes vexans can transmit other diseases to humans and animals, including heartworm in dogs.

“The vector species Aedes albopictus, Culex tarsalis and Culex pipiens feed on birds as well as mammals, which enables them to acquire WNV from birds and transmit it to humans or horses,” Houseman said. These species lay eggs in standing water, which they can find around homes and other buildings in such places as old cans, buckets, birdbaths, potted plants and clogged gutters.

“While it is good news that the risk of WNV is remains low despite increased mosquito populations, do not become complacent,” Houseman said.

Try to eliminate mosquito breeding habitats by emptying cans, buckets and other containers. In places where you can’t eliminate or drain the standing water, products called mosquito “dunks” or “donuts” will kill the larvae. “These products contained a bacterium called Bt and are nontoxic to humans,” he said.

Reduce your risk of mosquito bites by keeping windows and doors closed and staying inside during dusk and dawn. When outside, wear long sleeves and long pants and use repellents.

Many insect repellents are usually effective against mosquitoes for less than an hour. “Products containing DEET usually provide protection for up to four hours,” Houseman said. DEET is available in various concentrations, but concentrations higher than 35 percent aren’t much more effective and don’t last any longer. “Use lower DEET concentrations on children.”

Houseman notes that horses have a much higher incidence of death than humans following infection from WNV and should be protected even during an average year. There is a vaccine for horses to protect against WNV. See a licensed veterinarian for more information.

For more information about mosquitoes, see the MU Extension publication “Mosquitoes” (G7400), available for free download at http://extension.missouri.edu/p/G7400.