Cardinals 'Super Suppressors' of West Nile Virus

Authors of a new study took a closer look at a puzzling circumstance in Georgia: low rates of human infection with the West Nile virus (WNV), even as about a third of Atlanta's birds carry the disease.

The same situation -- many infected birds, not so many infected people -- applied to the broader American southeast, a team of researchers from Emory University, Texas A&M, the University of Georgia and Georgia's department of transportation noted.

Indeed, they said, Georgia's WNV infection rate, over the last 15 years, was only about 3 people per 100,000.

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To try to shed light on the discrepancy, the scientists spent three years testing birds and mosquitoes for WNV in Atlanta, analyzing the blood of the insects to determine on which birds they had fed.

The scientists were keeping a sharp eye for the number of American robin infections. They consider that species a "super spreader" of WNV, for its capacity to store enough of the virus in its blood to pass along to other mosquitoes.

While robins were certainly carriers, the researchers found a twist in the tale, one concerning a favorite of backyard birders everywhere: the cardinal.

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"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 the study's lead author, Rebecca Levine, 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," said Levine, now with the U.S. Centers for Disease Control and Prevention but at Emory University during the research. "That is why we called them 'super suppressors'."

Levine and her colleagues aren't sure yet why this sudden dietary change occurs in the insects, a shift that seems to be helping Atlanta's citizens.

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Humans become infected with WNV through mosquito bites, which the insects pick up when they bite infected birds. According to the World Health Organization most infected humans don't even show symptoms, but in rare cases the virus can be fatal.

Many things could determine how WNV is passed between birds and mosquitoes, and whether that puts humans at greater or lesser risk, the researchers said.

Changes in how the birds roost, or defensive behaviors are potential factors, they said.

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The team did see fewer instances of WNV in bird populations from old-growth forest areas, suggesting that the type of forest cover may be a factor that impacted transmission rates.

"We might find that keeping old-growth forests intact, even in urban areas, can provide more than just an interesting piece of history," Levine said.

Other parts of the country, such as the urban northeast and midwest seem to have the situation flipped. In Chicago, the researchers said, there are six time as many human WNV infections as in Atlanta, but from just 20 percent of infected birds.

But, her team's findings don't suggest cardinals are a magic red bullet against WNV, Levine said. The birds might not have the same roles elsewhere, depending on the local ecosystems in which they live.

Findings from the study have just been published in the American Journal of Tropical Medicine and Hygiene.