Burness
A bird species that does a poor job spreading West Nile virus
(WNV) but is particularly likely to get mosquito bites may explain why human
infections with the disease are relatively uncommon in Atlanta, Georgia --
despite evidence of high rates of virus circulating in the local bird
population, according to a new study published online in the American
Journal of Tropical Medicine and Hygiene.
Researchers at Emory University, University of Georgia, Texas
A&M and the Georgia Department of Transportation set out to find clues
explaining why Georgia's infection rate for WNV since 2001 is quite low (about
3.3 per 100,000 people), even though evidence showed that about one-third of
birds in the Atlanta area have been exposed to the disease.
They noted that a similar pattern is seen throughout the
American southeast, where a large amount of WNV in birds and mosquitoes has not
translated into a large number of human cases.
In urban areas in the northeast
and midwest, the situation is frequently the opposite -- infection rates in
Chicago, for example, are six times higher than Atlanta's, even though less
than 20 percent of Chicago birds show evidence of WNV infection.
Disease experts often refer to human cases of WNV as "spillover" infections, because they happen when disease circulating between birds and mosquitoes spills over into nearby human populations via mosquito bites.
While most human infections cause few, if any symptoms, some
cases can result in headaches, joint pain, and fatigue that linger for weeks.
In rare instances WNV produces potentially fatal brain inflammation. Since the
disease arrived in the continental United States in 1999, there have been over
780,000 human infections and 1,700 fatalities documented.
The Emory University researchers were particularly interested in
the role of American robins, considered "super-spreaders" of the
disease because they efficiently pass the virus along to mosquitoes that bite
them.
Those mosquitoes in turn can feed on humans, potentially
infecting them. Over three years they collected mosquitoes and birds across
Atlanta, tested them for WNV, and ran a DNA analysis of the mosquitoes' blood
meals to see which birds they had bitten.
"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, PhD, the lead author of the study who conducted the research while at
Emory University.
"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. That is why we called
them 'supersuppressors'."
The shift occurs just as mosquito populations are rising, which
typically should mean more WNV circulating in the area and hence a greater risk
of human infections, said Levine, now an epidemiologist and entomologist at the
U.S. Centers for Disease Control and Prevention (CDC).
She said it's not clear what prompts the shift in feeding
behavior in Atlanta but that this shift may be what is protecting Atlantans
from WNV spillover.
Also, throughout the city, researchers found a high rate of
exposure to WNV among mockingbirds, brown thrashers and gray catbirds. There is
evidence from previous studies that gray catbirds and brown thrashers also act
as suppressors slowing the transmission of WNV.
Levine said there could be many factors that determine how WNV
is spread between birds and mosquitoes and how that in turn affects human risk.
Changes in roosting behavior of birds are one possibility,
defensive behavior is another, and there are other possibilities. For example,
they found less evidence of WNV among bird populations in small patches of old
growth forest. Thus, original forest cover may reduce transmission.
"So, we might find that keeping old growth forests intact,
even in urban areas, can provide more than just an interesting piece of
history," she said.
"Ultimately, if we can identify certain features of the
local ecosystem that help prevent disease transmission, then we can consider
preserving and enhancing them."
Overall, Levine said the findings point to the complex role of
many factors in the local environment interacting to determine the risk of
human infections.
For example, she said the study should not prompt officials in
Chicago to start releasing flocks of cardinals across the city because the same
species may perform different roles in different ecosystems.
"As new mosquito-borne diseases enter and spread in
America, we need to better understand all aspects of pathogen transmission
cycles," said Stephen Higgs, PhD, president 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."