Mystery Behind Atlanta’s Low West Nile Numbers Solved

Michael Jones, a technician applicator with Mosquito Squad, prepares to spray a home with mosquito elimination product, Tuesday, July 23, 2013, in Suwanee, Ga. Scientists are predicting a large mosquito population in Georgia during the next few weeks. Elmer Gray, an entomologist with the University of Georgia’s Cooperative Extension Service, says the nuisance mosquitoes are expected to be particularly bad in coming weeks. However, Gray said the type of mosquitoes expected to attack Georgians in increasing numbers are not the primary carriers of West Nile virus. (AP Photo/David Goldman)

David Goldman / Associated Press

With the mosquito-borne illness Zika in the news, it may be hard to remember that just a few years ago, West Nile Virus was the big concern.

Well, it turns out West Nile never had major numbers in metro Atlanta – and now scientists know why.

Each year, only about three people out of 100,000 in the Atlanta area get infected with the virus.

That figure’s low  compared to other parts of the country, including Chicago.

For a long time the low infection rate puzzled scientists, since Atlanta’s long, hot mosquito season can last until November.

Enter a team of researchers led by epidemiologist and entomologist Dr. Rebecca Levine, who works at the Centers for Disease Control and Prevention. Back when she was a PhD student at Emory University, her team studied the patterns of the insects – and the birds they feed on.

What they learned: At mid-summer, precisely when mosquitoes are most likely to carry West Nile, they switch from feeding primarily on robins to feeding primarily on northern cardinals.

The difference there? Some bird species pass West Nile Virus on, while others do not. And when it comes to West Nile, the buck stops with the northern cardinal.

Or, as Levine puts it, “Whereas one mosquito biting a robin may be passing it onto future mosquitoes, a mosquito biting a cardinal, it’s not gonna pass it on.”

So why does all this matter, really? Levine says you can learn a lot about any virus – be it West Nile or Zika – imply by studying the ecosystem in which it reproduces. Her research may not specifically translate to the study of Zika, which, for starters, is carried by a different species of mosquitoes.  But, says Levine, “[T]here will always be some sort of ecological component to [a virus’s] transmission to humans, because transmission to humans necessarily also depends on the transmission within the animal.”