Nearly every day last summer, West Nile virus experts Jerry Kerce and Tom Burkot emptied 15 mosquito traps placed in rivers and creeks that catch the city's sewage overflows. The traps, designed by the U.S. Centers for Disease Control and Prevention, captured live mosquitoes that were later tested for the West Nile virus.
The results of the tests were startling. Last year, 63 of 1,553 pools of mosquitoes tested positive for the virus. All but five of the positive pools were found within a one-mile radius of sewage treatment facilities. Mosquitoes have a flight range of about one mile.
The findings suggest that the city's sewage treatment methods increase the chance of a West Nile outbreak.
It's groundbreaking research that Burkot, an entomologist with the CDC's infectious disease program, and Kerce, West Nile virus coordinator for the Fulton County Department of Health and Wellness, hope will be used to better keep mosquito populations under control.
"We know it's a huge problem," says Burkot, "and to my knowledge, no one else in the country is looking at this link between West Nile and sewage overflows."
Heavy rains cause sewage overflows that both kill predators that feed on mosquito larvae and provide a nutrient-rich food supply for the larvae. Burkot says the heavy rains and accompanying sewage create an ideal environment for rampant breeding of the Southern house mosquito, the main carrier of the West Nile virus.
Since it arrived in the United States in 1999, the West Nile virus has claimed more than 640 lives nationwide. In Georgia, 101 people have been infected and 11 have died. Two of the deaths and 23 of the infections occurred in Fulton County.
Burkot says there's a strong possibility that the number of West Nile cases will rise in Atlanta sooner or later, especially considering the number of positive mosquito pools Burkot found in and near city parks.
In Piedmont Park last year, three mosquito pools tested positive, and one was found in Grant Park. In Maddox Park, on the west side, two pools were positive, and a positive pool was found in Langford Memorial Park and Thomasville Park, both on the south side of the city. Ardmore Park, near Buckhead, also had a positive mosquito pool.
Each of those parks is within one mile of a sewage overflow station.
On a recent Friday morning, Burkot and Kerce hiked through Tanyard Creek Park in Buckhead to count mosquito eggs. In still pools along the creek bed, they found floating objects that looked like dark grains of rice. The objects were actually groups of mosquito eggs, between 100 and 250 eggs stuck together.
They found only a dozen or so such bunches, because a storm the previous day had washed away most of the larvae.
So far this year, Atlanta has dodged the West Nile virus bullet. Only one case of human West Nile virus infection, in Paulding County, has been confirmed.
Frequent storms, while creating more mosquito larvae, are also responsible for the lack of infection, says Kerce. "These rains have come at ideal times, washing the larvae away," he says. "We've been lucky."
But Burkot warns that it only takes a few rainless days to see mosquito populations explode.
"Under the right conditions, you get very rapid buildup of mosquito numbers, and they emerge into the surrounding area," he says. "It's been so bad that I've stood in the creek when there's quite a strong current, and I've seen mosquito larvae being washed past my feet. You never see that in the natural environment."
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