- House sparrows exposed to light at night had higher levels of West Nile virus in their blood for two days longer than sparrows that were exposed to darkness, according to a new study.
- The research sought to mimic the effects of light pollution common to urban environments on virus levels in a known reservoir of West Nile virus, which can cause a flu-like fever in humans.
- The team’s research suggests that an outbreak could be 41 percent more likely to happen as a result of the persistence of the virus in this host.
Light pollution could lead to more infections with West Nile virus by increasing the amount of time that small songbirds hold on to the virus, according to a new study.
“The findings may be the first indication that light pollution can affect the spread of zoonotic diseases,” Meredith Kernbach, a doctoral student in global health at the University of South Florida and lead author of the study, said in a statement. Kernbach and her colleagues published their findings in the journal Proceedings of the Royal Society B: Biological Sciences on July 24.
Scientists already know that exposure to artificial light can affect animal biology, including our own, interfering with immune system functioning, metabolism and behavior.
“Many hosts and vectors use light cues to coordinate daily and seasonal rhythms,” Kernbach said. “[D]isruption of these rhythms by light exposure at night could affect immune responses, generating the effects we see here.”
She and her colleagues wondered whether artificial light might influence the way the birds’ bodies react to the virus that causes West Nile fever. Symptoms, when they do appear, are typically similar to those of the flu in humans, and in rare cases can be fatal.
Research has shown that songbirds like house sparrows (Passer domesticus) carry West Nile virus, along with other diseases. They’re also frequent visitors to towns and cities, where light pollution abounds and where there are dense human populations to which they can hand off the virus through successive bites by the same mosquito.
To test their hypothesis, the team kept two groups of wild house sparrows under different lighting conditions. The control group experienced 12 hours of light and 12 hours of darkness each day for up to three weeks. The second group of birds was kept in an area with 12 hours of light as well, but then the researchers exposed them to 12 hours of dim light meant to mimic the nighttime street and building lights of an urban environment. In the midst of the light exposure experiments, Kernbach and her colleagues inoculated the birds with West Nile virus.
Beginning two days after exposure to the virus, the team measured the amount of the virus in the blood of each bird. They all had comparable levels of the virus after four days, but six days in, the birds being exposed to the nighttime lights had significantly higher levels of West Nile virus in their blood than the control group.
The researchers also created a statistical model demonstrating that the lingering viral load in light-pollution-exposed sparrows could increase the chances of an outbreak of West Nile fever by 41 percent.
Earlier research had shown that higher levels of the stress hormone corticosterone made another species of birds more enticing to hungry mosquitos, and the scientists did notice a slight bump in this compound in the birds exposed to the dim night lights. But that alone didn’t explain the persistence of West Nile virus in the animals’ blood samples, pointing to the need for more research. The stress that light induces could have other effects, for example, on the secretion of the hormone melatonin, that could affect bird behavior, the authors write.
In the meantime, the team suggests that motion-sensitive lights might diminish exposure to light pollution and that lights could be turned off at night when the transmission of West Nile virus is particularly high.
Banner image of a house sparrow courtesy of the University of South Florida.
Kernbach, M. E., Newhouse, D. J., Miller, J. M., Hall, R. J., Gibbons, J., Oberstaller, J., … Martin, L. B. (2019). Light pollution increases West Nile virus competence of a ubiquitous passerine reservoir species. Proceedings of the Royal Society B: Biological Sciences, 286(1907), 20191051. doi:10.1098/rspb.2019.1051
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