One more reason to love the northern cardinal: In addition to being beautiful to look at, in Atlanta these birds appear to help shield humans from West Nile virus. (Photo by Stephen Wolfe.)
By Carol Clark
Northern cardinals act as “super suppressors” of West Nile virus in Atlanta, slowing transmission and reducing the incidence of human cases of the mosquito-borne pathogen, suggests a new study published in the American Journal of Tropical Medicine and Hygiene.
“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 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.”
Levine has since graduated from Emory's Laney Graduate School and now works as an epidemiologist and entomologist for the Centers for Disease Control and Prevention.
Uriel Kitron, chair of Emory’s Department of Environmental Sciences and an expert in mosquito-borne pathogens, is senior author of the paper.
Rebecca Levine in the field with one of the cardinals that was tested. The birds in the study were captured with mist nets and released unharmed after blood samples were drawn. (Photo courtesy of Rebecca Levine.)
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 Kitron lab 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.”
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.
The researchers found significantly fewer avian WNV infections in the old growth forest sites sampled in Atlanta – including Fernbank Forest and Wesley Woods Preserve – compared to secondary forests such as Grant Park and the Atlanta Botanical Garden. The rate of infections in mosquitos, however, was similar for both types of forests.
“These are really complex ecosystems, so we cannot single out the specific reasons for these findings,” Levine says. “They suggest that there is something unique about these old growth forests and how they affect avian systems in Atlanta.”
Atlanta, nicknamed “City in the Forest,” is one of only seven U.S. cities with a high population density to have urban tree cover of 40 percent or more. In contrast, Chicago retains only 11 percent tree cover.
“As new mosquito-borne diseases enter and spread in America, we need to better understand all aspects of pathogen transmission cycles, said Stephen Higgs, present of the American Society of Tropical Medicine and Hygiene. “By shedding light on the reasons behind a curious discrepancy in West Nile virus human infection rates in different regions of the United States, this study has the potential to better protect Americans’ health while continuing to demonstrate the link between animal and human health.”
Co-authors of the paper also include researchers from the University of Georgia, Texas A&M and the Georgia Department of Transportation’s Office of Environmental Services.
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