Light pollution could make West Nile virus in birds more infectious, study suggests

Light pollution may be increasing West Nile virus spillover from wild birds
House sparrow used in study that concludes birds exposed to artificial light at night are infectious for 2 days longer than those who do not. Credit: University of South Florida

Light pollution can extend by 41% how long sparrows are infectious with West Nile virus, which could make it more likely that they transmit the lethal disease to mosquitoes and on to people, a new study from researchers in Florida has found.

Meredith Kernbach, a University of South Florida doctoral candidate and lead researcher, recently published her findings in Proceedings of the Royal Society B.

"This is really the first study that's kind of investigating the effects of light pollution on infectious disease," Kernbach said in an interview. "We have to admit this is a captive (laboratory) study, but if we see something similar in urban areas near people, it may increase the opportunity for spillover to humans."

Kernbach chose house sparrows because they often live in urban areas and because they're a good example of what's known as a reservoir species, which doesn't often die from West Nile but can pass it on to mosquitoes. The study exposed some birds to light at night, with the control group in darkness then, she said. The birds exposed to artificial light stayed sick with West Nile an average of two days longer than the control group.

"We also know light pollution has a lot of negative consequences for animals and people. It's a real threat," Kernbach said. "So we wanted to examine the direct role of light in fighting infection."

Birds use light cues as part of daily and seasonal rhythms, informing what time they begin chirping as well as when to migrate or produce young, said Doug Stotz, a bird expert at the Field Museum.

Kernbach's study, however, may be the first indication that light pollution can affect the spread of zoonotic diseases, ones that can pass from animals to people.

Patrick Irwin, assistant director of the Northwest Mosquito Abatement District, read the study and has had many conversations with other scientists about its implications.

"This is something that has a lot of people in my field excited, and I'll admit, it's not something I would've thought of," Irwin said. "But it's awesome that somebody did and now we have at least some idea that this could play an important role in the transmission of West Nile virus."

Irwin said that because nature is so much more complex than a lab, light might not have as profound an effect in the wild. That's particularly true because the control group was kept entirely in the dark at night, conditions not easily found anywhere in the Chicago area. Still, he hopes additional studies, ideally outside, are performed.

"I could envision somebody going into the city and looking at what you find with sparrows who are living in a parking garage, for example, and those who are still in the city, but maybe at a forest preserve where there is less light pollution," he said.

Across Chicago, LED lights are being installed to replace more than a quarter of a million high-pressure sodium light fixtures in use since the 1970s—a move that could exacerbate the dangers of West Nile virus infection by making light pollution worse, experts said. The city says light pollution will be reduced.

The change from high-pressure sodium to LED lights will boost energy efficiency by about 50% and drop the electricity expenditure for streetlights from $18 million in 2017 to about $10 million annually once the entire conversion is complete, said Chicago Department of Transportation spokesman Michael Claffey. The city also can expect a total rebate of roughly $35 million from ComEd, which offers an incentive for each light converted to LED.

CDOT also said light pollution will decrease with the new lights.

"The LED fixtures we are using are designed to focus light downwards on streets and sidewalks, limiting light trespass into people's homes and the night sky," Claffey wrote in an email. "As a result, we are confident that the conversion is reducing light pollution in Chicago."

Some experts on birds and the night sky, including Kernbach, disagree with that assessment.

When the city began considering new streetlights, LED lights only came with bright white bulbs that gave off a cool-toned blue light. That blue-toned light has since been making headlines as the sleep-depriving culprit in phone and computer screens, Kernbach said.

Less stark LED lights will be used in Chicago, but critics say that because they're still cool-toned they'll emit blue light. The city's current high-pressure sodium lights give off an orange glow that is considered less harmful, even if some people have always hated the color.

Andrew Johnston, the Adler Planetarium's vice president of astronomy and collections, agreed there have been strides in LED lights in the time since the city began talking about the move and the current installation. Adler works to preserve Chicagoans' ability to enjoy the night sky, so Johnston said he was grateful that the earliest proposed lights weren't used, as they registered on the lighting color scale about 4,000 Kelvin, what's known as "pure white."

Claffey said the city is installing "soft white" lights instead, with a color temperature of 3,000 Kelvin or less, "which is what the American Medical Association guidelines and the International Dark-Sky Association call for."

Johnston said that's a significant stride for stargazing, although even soft white light is cool-toned compared with sodium streetlights.

"But we also understand the new lights will have shielding that concentrates the light downward, so we remain optimistic," he said.

Adler also will undertake its own study by continuing to send high-altitude balloons into the stratosphere to map the light pollution a city emits, eventually creating a display of the findings between old and new streetlights.

Beyond the color hues, Stotz from the Field Museum and Annette Prince of the Chicago Audubon Society said bright lights generally are bad for birds. Birds use stars to navigate during migration, and it's believed that's why they find bright lights hard to resist, Stotz said.

"The analogy I use is if you turn the light on at the back of your house, all the moths come into that light," Stotz said. "Birds are doing much the same thing when they are migrating and see a city lit up."

Stotz said he has studied McCormick Place and noted that when it is dark, nearly 90% fewer birds will hit it and die than when it is lit up. Separately, Prince said a lone building in Chicago has killed 1,000 birds in a single event before. Both brought up "Lights Out Chicago," a voluntary program that asks buildings downtown to turn off outdoor ornamental lighting between 11 p.m. and daybreak in spring and fall when birds are migrating.

Prince said the new streetlights will create a more dangerous environment for birds and people. One hundred years ago, most of the country was totally dark at night, and widespread outdoor lighting has only increased in popularity, she said. In that time the brighter environment has affected myriad animals and the food web overall, from insects up the chain to humans. Birds have adapted over millions of years to instinctively know the best time to migrate or hatch young based on when certain plants will be in bloom or when insects can be found along migratory routes, she said.

"The new LED lights are going to save energy, but they are bright white and will absolutely attract birds and increase the amount of light pollution in the city. Overall our whole urban environment will be a lot brighter," she said.

Like Prince, Irwin, the mosquito expert, said Kernbach's findings are an important step in learning more about the ways light not only throws off light and dark cycles in animals and humans but how nocturnal light affects a body's ability to fight infection.

"This one paper is spurring a lot of scientific conversation, and that's a good thing," Irwin said.

More information: Meredith E. Kernbach et al, Light pollution increases West Nile virus competence of a ubiquitous passerine reservoir species, Proceedings of the Royal Society B: Biological Sciences (2019). DOI: 10.1098/rspb.2019.1051

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