Robins are considered "super-spreaders" of West Nile virus. They’re especially good at passing the virus to mosquitoes, and mosquitoes, of course, can then pass it to us.
It turns out a different bird species – cardinals – might be shielding people from getting the virus in some parts of the country.
Rebecca Levine is the lead author of a new study in the American Journal of Tropical Medicine and Hygiene. She's an epidemiologist and entomologist at the Centers for Disease Control and Prevention.
“We set out to investigate if there might be any feature in the ecology of our system here in Atlanta that might explain the very low human infection rates that we see," she says.
She says because birds are the reservoir for West Nile virus, they took samples from bird species in the area and captured lots of mosquitoes and tested them for the virus.
"The other interesting thing we did is that when we caught a mosquito that had blood inside of its abdomen still, we were able to use molecular techniques to actually determine what species the mosquito fed on," Levine explains.
Before about mid-July, she says the mosquitoes were feeding mainly on American robins.
"But right about mid-July, which is the same time the West Nile virus infection rate in mosquitoes started to rise, for some unknown reason, the mosquitoes almost completely stopped feeding on robins and instead they started feeding on cardinals," she says.
Shielding people from the virus
Cardinals are not as good at transmitting West Nile virus to mosquitoes.
“We can tell they’re getting a lot of mosquito bites, we actually found the blood of cardinals inside of mosquitoes, so we know that they’re getting bitten a lot, but what we suspect is that these bites that the mosquitoes are giving the cardinals are actually doing less in terms of spilling over into human populations because the cardinals are not as good at amplifying the virus as robins are," says Levine.
There’s a discrepancy in West Nile virus infection rates in people in different regions of the U.S. In Chicago – for example – it’s about six times higher than in Atlanta. Levine says it's possible these different bird species could be part of the reason why.
"We know that infection rates in cardinals in Chicago are also quite high, so for whatever reason, perhaps the mosquitoes in Chicago don’t shift onto feeding on a less good host, and therefore during the critical time when the virus is really growing in the mosquito populations, there is more spillover to humans," she says.
In other words, she says, it's possible the mosquitoes in Chicago don't switch over to feeding on more cardinals.
"It’s just one theory. Ecological systems are very complex, but it’s one of the fascinating things about the way nature works together," she says.
Levine says it's still a mystery why mosquitoes in Atlanta switched from robins to cardinals for their meals. She says roosting behavior and migration patterns could play a role, and some birds might be better at warding off mosquitoes than others.