Atlanta should have a higher rate of West Nile virus, the mosquito-borne pathogen that doesn’t cause symptoms in most infected people, but which can kill those who are elderly or have compromised immune systems. After all, about one-third of the area’s birds—which serve as reservoirs for the pathogen—shows signs of being exposed. That’s a large percentage, and rates of infection of humans typically go up when more birds carry the disease.
In Chicago, for example, only 20 percent of birds have been exposed to the virus. But human infection rates there are six times higher than in Atlanta.
What could explain the low rates of infection in Atlanta and elsewhere in the Southeast?
New research suggests that cardinals may be a major reason for the disparity. The birds, which are plentiful in the area, are particularly prone to being fed upon by mosquitoes. But they also carry low quantities of the virus in their blood, and aren’t effective at passing it on to the blood-sucking insects.
Robins, on the other hand, easily transmit the virus to mosquitoes. But a new study published in the American Society of Tropical Medicine and Hygiene found that the insects shift from feeding on robins to cardinals and their relatives in the summer, just when conditions are perfect for the virus to be transmitted to humans. For this reason, scientists have dubbed cardinals “supersuppressors”—winged saviors of the Southeast.
"What we found is that, for some unknown reason, around the middle of July, mosquitoes in Atlanta seem to decide that they have had their fill of robins and they switch to feeding on cardinals," said Rebecca Levine, Ph.D., study lead author who conducted the research while at Emory University, in a statement. "But cardinals, even though they can be infected with West Nile virus, are much less likely to have enough virus circulating in their blood to transmit the disease back to feeding mosquitoes.”
The scientists also found lower rates of transmissions surroundings forests, especially patches of older-growth, where there may be more cardinals. That’s yet another reason for preserving woodlands, says Levine, who is now with the Centers for Disease Control and Prevention.