When large groups of birds roost together the chances that an individual bird will get bitten by mosquitoes carrying the West Nile virus and subsequently contract the disease.
Although it would seem logical that large numbers of roosting birds would attract more mosquitoes that carry West Nile virus and contract the disease when bitten, recent research at the University of Illinois found the opposite to be true.
"Our study is the first field-based evidence to support what's called the 'encounter-dilution effect' acting in a vector-borne disease system with an experiment," researcher Bethany Krebs said in a statement. "There have been other laboratory and modeling studies that suggest that mosquitoes feed less per individual in a group than they do on a solitary bird but it's hard to get the information in natural settings."
The experiment was conducted over a period of three years.
For the study, researchers trapped mosquitoes inside and outside of roosts from 2010 to 2012 to determine whether roosts attracted more mosquitoes than non-roost sites. Then they sent the mosquitoes to a lab in Texas that ran analyses on them to determine if they carried the virus.
"Uninfected house sparrows were used as sentinel birds to assess host risk of West Nile exposure in 2012 -- the timing coincided with the historical period of peak West Nile virus transmission in the Chicago study areas known to be 'hot spots' for the disease," Krebs said.
The house sparrows were placed in flight cages -- 23 birds in cages near communal roosts and 25 in non-roost cages. Krebs explained that sentinel birds are used by public health departments as sort of a "canary-in-the-coal mine" early warning system to detect the presence of a vector-borne disease.
"Only three sparrows near roosts contracted West Nile virus whereas 11 birds in non-roost cages were infected," Krebs said. "So the risk of West Nile virus exposure for those sentinel birds caged within roosts was significantly lower than for birds caged in non-roost locations."
Jeff Brawn, U of I ecologist and department head of the Natural Resources and Environmental Sciences, said the study sheds light on the spread of vector-borne diseases, such as West Nile virus, compared to those transmitted via direct contact.
"If you're in a group, the probability of infection goes way up with direct contact diseases such as colds and flu," Brawn explained. "This study confirmed that the risk is spread out among the individuals in the herd; in the case of West Nile virus, which is a vector-borne disease, individual risk is minimized."
Brawn said that they don't understand why some birds roost and others of the same species do not. But this study shows that those who do choose to roost together benefit by the lower risk of exposure to West Nile virus infection.
Brawn said that although the study was on birds, it could provide an interesting implication with respect to human behavior and health risk.
"If you are in the woods alone, you may have a greater probability of getting bitten than if you are in a large group of people," he said.
The findings were detailed in the Proceedings of the Royal Society B: Biological Sciences and were supported by the National Science Foundation Ecology of Infectious Disease program.
