TAMPA — A hungry mosquito is more likely to bite the bird with ruffled feathers, according to new research from the University of South Florida, another step in understanding the spread of mosquito-borne viruses.
The new study, published this month, found that birds with higher levels of stress hormones are twice as likely to get bitten by mosquitoes. During the year-and-a-half-long study, disease-carrying mosquitoes proved to be attracted to the hormone corticosterone, which is nearly identical to the stress hormone cortisol produced by humans.
That attraction can spark a big ripple effect, particularly in urban areas, the researchers said. If more birds are exposed to stresses such as light pollution or road noise from urbanization, they'll attract more mosquitoes. If those birds happen to be carrying diseases like West Nile Virus or Eastern Equine Encephalitis, there is a greater chance the disease could be spread and humans infected. (Scientists do not believe birds carry the Zika virus.)
"We were asking whether mosquitoes could, in a sense, smell stress and, lo and behold, not only do they do it, they seem incredibly adept at doing it," said the study's principal investigator Dr. Lynn Martin, associate professor in the USF Department of Integrative Biology.
It's still unclear if elevated stress hormones in humans also attract mosquitoes, but the birds' hormones are similar enough that it's worth studying, he said.
The team began by putting three zebra finches in a cage with mosquitoes. One bird was injected with high levels of corticosterone, one with medium levels of the hormone and one with normal levels. The mosquitoes were left to feed on the birds overnight and, in the morning, underwent DNA testing that showed which blood in their bodies came from which bird.
Mosquitoes key in on their hosts using a multitude of signals, from physical movement to body temperature to detecting carbon dioxide. Yet the birds with the higher hormone levels were bitten twice as much as the others, said co-investigator Aaron Schrey, an assistant professor at Armstrong State University in Georgia.
"When I did the analysis I intentionally didn't know which insects were which, but the results I saw were really clear. They would definitely bite on the ones with the higher hormones," Schrey said.
In a second study, each zebra finch was kept in a cage by itself to see how the different levels of stress hormones helped them defend against the mosquitoes. The stress hormones made the birds more defensive, increasing actions like shaking their head, hopping or preening their feathers, but surprisingly the mosquitoes were still able and willing to bite the birds with elevated hormone levels, Martin said. The researchers also found that those birds didn't necessarily make the most nourishing meal, or result in the mosquitoes laying more eggs after feeding.
"I think that the signals coming from the birds are so incredibly compelling that the mosquitoes out in the wild, if they get any signal at all that some tasty host is around, they're going to go for that, even if it's not the best meal," Martin said. "They bite the first one they find and, often times, that's going to be the stressed one."
The next step for the researchers is to see if the same results are found out of the lab and in the wild. The researchers will also continue to "scale up" their thinking, and raise more questions about how the hormonal changes of one animal can affect a population, said Stephanie Gervasi, postdoctoral fellow on the project who is now at the Monell Chemical Senses Center in Philadelphia.
"The cool thing is it really led to a lot of questions about the birds, the mosquitoes and what it is they liked so much about those birds, something they smell that changes what the mosquitoes use to find them and bite them," Gervasi said.
An earlier version of this story suggested that birds carry Zika, which has not been proven.
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