Light pollution can prolong the risk of sparrows passing along West Nile virus

SAN FRANCISCO — Even moderate light pollution can roughly double the time a house sparrow remains a risk for passing along the worrisome West Nile virus.

House sparrows, about as widespread across the United States as artificial lighting itself, make a useful test species for a first-of-its-kind study of how night illumination might contribute to disease spread, said Meredith Kernbach, an eco-immunologist at the University of South Florida in Tampa. Passer domesticus brought into the lab and kept dimly illuminated at night were slower in fighting off West Nile infections than lab sparrows allowed full darkness, Kernbach reported January 7 at the annual meeting of the Society for Integrative and Comparative Biology.

Sparrows kept under a dim night light typically had enough virus in their bloodstreams for at least four days to turn biting mosquitoes into disease spreaders, she said. Sparrows housed in darkness had high virus concentrations for only about two days. Doubling the time a bird can pass along a big dose of virus could in theory increase the likelihood that a disease will spread.

The broader question of whether light pollution affects human health has been a concern for shift workers. Researchers have also looked at possible changes in reproduction and other behavior in wildlife (SN: 12/26/15, p. 29).

Kernbach’s project opens new territory by testing the effects of light on physiological factors that control how diseases that can infect humans might hopscotch among animals, says Jenny Ouyang, of the University of Nevada, Reno. As light pollution studies go, “I don’t know of anything like this,” says Ouyang, an integrative physiologist who also has studied light pollution and birds.

The tests intensify Ouyang’s curiosity about whether light might affect the spread of malaria among humans. There have been hints and speculation in the scientific literature, she says, that vector mosquitoes might be drawn to light sources in some circumstances, which could mean that excess illumination might compound urban disease risks.

Kernbach based much of her lab test on real-world conditions. The viral dose she gave the birds was strong enough to kill about 40 percent of them, and it was well within what a mosquito might pick up vampirizing birds or mammals.  She used white incandescent lighting, basically the last century’s universal light bulb, which is still common despite inroads by LED lighting.

The white incandescence in the experiment has plenty of warmer tones, but does include some of blue wavelengths from common cool white LEDs, or light-emitting diodes. The sparrows on average experienced about 8 lux of this white incandescence during their seven-hour nights. (A heavily overcast day, by comparison, ranks at about 100 lux.)

Other studies in birds are showing that artificial night lighting can affect concentrations of the hormone corticosterone, which helps orchestrate reactions of the immune system. But Kernbach said she found no signs in her experiment that corticosterone controlled the results she saw in house sparrows.

What lights do to the birds is only part of the story, points out Davide Dominoni, an eco-physiologist at the Netherlands Institute of Ecology in Wageningen. Researchers will also need to look for effects on the virus itself. And on the mosquitoes.