Stressed-out birds more inviting to mosquitoes

Birds exposed to stress such as road noise, pesticides and light pollution were twice as likely to be bitten by mosquitoes that transmit the West Nile virus, finds a study. West Nile fever is a mosquito-borne infection by the virus with the same name.

The findings showed that an elevated concentration of corticosterone – stress hormone raises the level of host attractiveness, potentially affecting the transmission of mosquito-borne diseases in a number of ways.

“Mosquitoes seem to be able to ‘sniff out’ the stress hormone and key in on individual birds,” said lead researcher Lynn Martin, Associate Professor at University of South Florida (USF) in the US.

“For vector-borne diseases such as West Nile virus, the presence of corticosterone could influence pathogen spread through effects on contact rates with the mosquitoes that transmit it,” added Stephanie Gervasi, post-doctoral student at Monell Chemical Senses Center – a non-profit independent scientific institute in Philadelphia, US.

These stress hormones were also found to have other negative effects, including immunosuppression and increased susceptibility to infections in animals. Further, birds who underwent corticosterone treatment showed an increase in tail flicks, head shakes and other defensive behaviours much more than the birds without the hormone treatment. However, “the mosquitoes managed to breach those defenses and feed more on stress hormone-treated birds,” Martin said.

Mosquitoes use a variety of cues to locate a target, including carbon dioxide output, body size and temperature. These signals coming from a bird could convey information about stress hormones making the birds more appealing targets for the insects, the researchers pointed out.

For the study, the team experimentally manipulated songbird stress hormones levels. Then they examined mosquito feeding preferences, feeding success and productivity as well as the defensive behaviours of birds trying to avoid being bitten.

The study also found that the mosquitoes that fed on birds with higher levels of stress hormones tended to lay different sized clutches of eggs at different rates than the mosquitoes which fed on control birds. 

“Stress hormones altered the relationship between the timing of laying and clutch size in mosquitoes,” explained Thomas Unnasch, Professor at USF, US. 

These effects of bird stress on mosquito reproduction suggest that mosquito-feeding choice might also affect disease cycles in nature by changing the number of newborn mosquitoes that could be infected later by stressed birds, the authors noted. The study also has implications for the transmission of other viruses such as Eastern Equine Encephalitis, and perhaps even Zika.