Chicago lights up at night. Jim Young/Reuters

An ongoing study suggests light pollution’s effects on animals can help spread some viruses, like West Nile.

Light pollution is known to cause all kinds of problems. Studies show how it can throw sleep cycles out of whack and disrupt the natural ecosystem. Now biology researchers at the University of South Florida suggest yet another unintended consequence of artificial lights: They may help the spread of vector-borne diseases like the West Nile virus.

The West Nile virus is carried primarily by birds, and, in the right circumstances, can be passed on to humans via mosquitoes. Typically that infectious period in birds only lasts a few days as the immune system fights to lower the viral load. House sparrows, in particular, stay infectious for about two days.

But when Meredith Kernbach, a graduate student at USF’s Martin Lab, exposed a group of about two dozen infected sparrows to dim light at night—simulating what birds might experience in a city—she found that half of them continued to have significantly high viral concentrations for two days longer than the control group, which spent nights in darkness.

A man feeds a sparrow in Berlin. (Tobias Schwarz/Reuters)

“They did not die out at a greater proportion or a faster rate,” said Kernbach, who presented the initial result of her ongoing study earlier this month at the Society for Integrative and Comparative Biology meeting in San Francisco. “So these birds are staying alive just as long while remaining infectious to vectors.” That could result in as much as doubling the number of infected mosquitoes that can then go on to bite humans.

In 2017, the U.S. reported at least 2,000 cases of the West Nile virus, and it’s by no coincidence that, in states like California, Utah, and Texas, it tends to emerge in urban areas. For starters, house sparrows are urban-dwelling birds. Cities also make great breeding grounds for mosquitoes, and the density of humans allows a disease to quickly spread. And while the research is preliminary, light pollution may also be playing a significant role, Kernbach said, citing set of maps showing where light pollution in Texas overlaps with where West Nile virus cases were reported in 2016.

The areas that had the most reported cases of West Nile virus in Texas in 2016 (shaded in dark green on the map on the left) overlap with areas that have the most light pollution.  (CDC/Lightpollutionmap.info)

Past studies on birds suggest that artificial light can increase the production of a stress-regulating hormone known as corticosteroids, raising their stress levels and, according to previous research from the Martin Lab, increasing the concentration of the West Nile virus in infected individuals. Kernbach, though, didn’t see higher levels of corticosteroids in her sample, which led her to think that something else may be at play.

“We think that because there is light at night, the melatonin regulation might be a little wonky,” she said, referring to the hormone secreted at night to help us fall asleep. As with humans, blue light (with shorter wavelengths) suppresses the production of melatonin in birds, which can weaken their immune response to diseases—though Kernbach told CityLab they’d need to do more research to understand exactly how.

Though not a widely studied topic, researchers in the past have looked at the link between light pollution and vector-borne diseases. In 2010, two researchers in Brazil commented that, at the simplest level, artificial lights promote the spread of diseases by bringing vectors and humans together. More lights at night means people stay  outdoors longer, combined with the fact that insects are generally attracted to light. (It’s worth mentioning that scientists have been working on customizable LED bulbs that are less attractive to bugs.)

Take the deadly Chagas disease, spread through kissing bugs that carry the parasite Trypanosoma cruzi. Starting in the 1980s, Brazil led an aggressive insecticide spraying campaign against kissing bugs to control the disease, and for the most part it was successful. Then in 2005, the health department in the state of Santa Catarina reported an epidemic involving at least 12 confirmed cases despite the town never having reported such a case in the past. It turned out that a roadside sugarcane juice stand was to blame, according to the researchers:

The vector of Chagas disease does not live in sugarcane plantations, and there was no reason for it to be in stored sugarcane. The only positive indication was the high-intensity discharge lamp installed at the sugarcane juice kiosk. The bugs ... were attracted by the strong artificial light source in the sugarcane juice kiosk and were crushed together with the sugarcane when the juice was processed, thus transmitting the infection.

Kernbach’s study is limited in that she only focused on one species of bird and one particular virus—plus, her sample population is a captive one, so it’s hard to say how things play out in the field. “But it opens the door for what we should investigate,”and not just on birds, she said. “It's a good guess, for example, to make that rodents are also detrimentally affected by city lights specially because a lot of them are nocturnal.” That’s important because rodents also carry pathogens.

Next, Kernbach hopes to look at how light affects the insects that transmit diseases from one organism to another—particularly Culex mosquitoes, which carry the West Nile virus. But she said it will likely be applicable to other species that carry the Zika or malaria viruses. “The behavior is typically similar among them; they’re usually dawn and dusk feeders or nocturnal feeders,” she said. “We want to look at how [light pollution] affects their ability to develop the virus in their salivary glands.”

But for now, this is one more factor to keep in mind when making moves that would brighten a neighborhood, and one more reason to consider steps that would reduce light pollution generally.

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