Northern cardinals act as “super suppressors” of West Nile virus, slowing transmission and reducing the incidence of human cases of the mosquito-borne pathogen, suggests a new study in Atlanta.
“Previous research has shown that the American robin acts like a ‘super spreader’ for West Nile virus in Chicago and some other cities,” says Rebecca Levine, who led the research as a PhD student in Emory University’s department of environmental sciences. “Now our study provides convincing data that northern cardinals and some other bird species may be ‘super suppressors’ of the virus in Atlanta.”
“The cardinals are absorbing the transmission of the virus and not usually passing it on.”
As reported in the American Journal of Tropical Medicine and Hygiene, the researchers also found that birds in Atlanta’s old-growth forests had much lower rates of West Nile virus infection compared to birds tested in the city’s secondary forests and other urban micro-habitats.
“This finding suggests that old growth forests may be an important part of an urban landscape,” Levine says, “not just because of the natural beauty of ancient trees, but because these habitats may also be a means of reducing transmission of some mosquito-borne diseases.”
Why is Georgia’s infection rate so low?
West Nile virus (WNV) is zoonotic, meaning that it is an infection of animals that can spill over to humans by a bridge vector, in this case Culex mosquitos. Since its introduction to the United States in 1999, WNV has become the most common zoonotic mosquito-borne pathogen in the country, infecting an estimated 780,000 people (including more than 1,700 fatal cases), in addition to birds and other mammals, such as horses.
The lab of Uriel Kitron, chair of Emory’s department of environmental sciences and senior author of the paper, wanted to find out why Georgia’s infection rate for WNV since 2001 is relatively low, at about 3.3 cases per 100,000 people, compared to some states in the north. A 2002 outbreak in Illinois, for instance, recorded about 7.1 cases per 100,000 people.
“When West Nile virus first arrived in the United States, we expected more transmission to humans in the South,” Kitron says, “because the South has a longer transmission season and the Culex mosquitos are common. But even though evidence shows high rates of the virus circulating in local bird populations, there is little West Nile virus in humans in Atlanta and the Southeast in general.”
Mosquitoes and the birds they bite
During the three-year study, the research team collected mosquitoes and birds from different sites across Atlanta, tested them for WNV, and ran a DNA analysis of the mosquitos’ blood meals to see which species of birds they had bitten.
“We found that the mosquitoes feed on American robins a lot from May to mid-July,” Levine says. “But for some unknown reason, in mid-July, during the critical time when the West Nile virus infection rate in mosquitos starts going up, they switch to feeding primarily on cardinals.”
American robins do a great job of amplifying the virus in their blood once they are infected. That trait means they can more efficiently pass the virus to other mosquitos that bite them, so robins are known as “super spreaders.” The virus does not efficiently reproduce, however, in the blood of northern cardinals.
“You can think of the cardinals like a ‘sink,’ and West Nile virus like water draining out of that sink,” Levine says. “The cardinals are absorbing the transmission of the virus and not usually passing it on.”
The study results showed that, to a somewhat lesser extent, birds in the mimid family—including mockingbirds, brown thrashers, and gray catbirds—also appear to be acting like sinks for WNV in Atlanta.
Levine has since graduated from Emory and now works as an epidemiologist and entomologist for the Centers for Disease Control and Prevention.
Coauthors of the paper are from the University of Georgia, Texas A&M University, and the Georgia Department of Transportation’s Office of Environmental Services.
Source: Emory University