Cardinals may protect Atlanta residents from West Nile virus

In Atlanta, Georgia, West Nile virus (WNV) is rampant — but only in birds. Humans in the area are relatively free of the mosquito-borne illness, and they may have cardinals to thank, according to new research. Unlike American robins (Turdus migratorius), northern cardinals (Cardinalis cardinalis) rarely transmit WNV, even after being bitten by an infected mosquito. In a recent study in the American Journal of Tropical Medicine and Hygiene, researchers report that the cardinals may serve as a “sink” for the virus, bearing the bulk of mosquito bites just as West Nile season is ramping up.

“In mid-July, the mosquito infection rate really skyrockets — which is the same time that the mosquitos, instead of feeding on robins, start to feed on cardinals,” said Rebecca Levine, an epidemiologist who conducted the research while she was a graduate student at Emory University. “The cardinals are absorbing these infectious bites.”

In the eastern United States, WNV normally travels between mosquitos and certain species of songbirds such as robins. These birds serve as effective viral incubators, allowing the virus to multiply until there is enough of it to infect a mosquito. When other species such as humans are bitten by an infected mosquito, they may become sick, but they won’t produce enough virus to transmit the illness. Past research has shown that cardinals are right on the edge of being competent hosts for WNV, sometimes becoming infectious, but more often serving as dead ends from the virus’s point of view.

In Atlanta, only 330 humans are known to have contracted West Nile since 2001, and human cases are similarly rare in other parts of the Southeast, according to the researchers. Human cases are much more common in the Northeast and the Midwest. For example, the researchers write that people in Illinois catch WNV around five times as often as those in Atlanta, even though the virus is no more common in Illinois’ birds.

To unravel this mystery, the researchers collected more than 45,000 mosquitos, several hundred of which still had blood in their stomachs. By analyzing DNA from the mosquitos’ last blood meals, the researchers found that robins and cardinals together accounted for more than 40 percent of the mosquitos’ diets. The mosquitos focused on robins during the spring and early summer, then, for unknown reasons, switched to the disease-suppressing cardinals.

The researchers also netted 630 birds from 41 species in various habitat types across Atlanta, including parks, residential areas, and patches of old-growth forest. When the researchers tested the birds’ blood for West Nile antibodies, they found that birds in old-growth forest were significantly less likely to have been exposed to the virus, suggesting that forest habitats are less conducive to transmission.

The low infection rates in forests and the mosquitos’ taste for cardinals both emphasize the value of studying complex ecosystem relationships, says Levine.

“This kind of research is important,” she said. “When we can find something in a complex ecosystem that seems like, ‘hey, this is a free service that the ecosystem is providing to us,’ then it behooves us to pay attention.”

Header Image: Researchers handle a wild cardinal in order to test it for exposure to West Nile virus. Cardinals don’t transmit the disease easily, and they may help protect Atlanta’s human residents by absorbing infectious mosquito bites. ©Will Gravlee