Birds exposed to city lights are better able to spread disease. 

Sandesh Kadur/Minden Pictures

Light pollution may promote the spread of West Nile virus

SAN FRANCISCO, CALIFORNIA—After the West Nile virus appeared on the U.S. East Coast in 1999, it spread across the entire country in just a few years, sickening thousands of people and striking down whole flocks of robins, crows, and other birds. Now, a new study suggests the mosquito-borne virus may have had an unexpected helper: light pollution. Birds infected with West Nile can spread the virus twice as long when they are exposed to night light, according to a study presented here over the weekend at the annual meeting of the Society for Integrative and Comparative Biology.

The work “shows that light pollution is not only bad for our [daily rhythms], but also can affect disease prevalence and transmission,” says Jenny Ouyang, an integrative physiologist at the University of Nevada in Reno. “Perhaps infection in humans and other animals is also affected by light,” adds Yale University epidemiologist Durland Fish. (Neither Ouyang nor Fish were involved in the study.)

West Nile is primarily a bird virus, but people occasionally get infected because some bird-biting mosquitoes dine on human blood as well. Human infections can cause fevers, body aches, rashes, diarrhea, long-term fatigue, and, in some cases, inflammation of the brain and its membranes. In the United States, almost 2000 people have died from West Nile since its arrival.

Because birds play such a key role in West Nile’s epidemiology, bird health is an important topic, says Meredith Kernbach, an ecoimmunologist at the University of South Florida in Tampa. Extra light at night is known to increase birds’ levels of corticosteroids and other stress hormones, which in turn can affect the animals’ health. So Kernbach decided to see whether city life makes birds more susceptible to disease.

She and her colleagues infected about 50 house sparrows with the West Nile virus; they left a dim light on at night in half the birds’ cages, whereas the other half spent the night in complete darkness. The team watched both groups for disease symptoms and monitored their body weight and blood levels of virus and stress hormones.

In both groups, most birds got sick and became infectious within 2 days, and almost half died. Those exposed to light didn’t get sicker, or stay sick longer. But they did remain infectious for a longer period of time, the team reported at the meeting.

Previous research had shown that after an average of 2 days, a bird’s immune system beats back a West Nile virus infection to levels low enough that biting mosquitoes no longer get infected with the virus. That’s what happened with the “dark night” group in this study. But in the birds exposed to light, viral concentrations remained high for 2 more days, Kernbach reported. “This doubles the infectious period, which may double the number of infected mosquitoes,” she says. As a result, adds Fish, “we expect to have greater [disease] transmission in urban areas.” More transmission could translate into more cases of West Nile disease both in humans and in birds.

Kernbach and her colleagues had thought that the light would stress out the birds and that this might lower their resistance to West Nile. But the blood tests indicated that the birds were not more stressed; their stress response systems remained intact. Kernbach suspects that instead, the light may have altered levels of another hormone, melatonin, which may also have an influence on immune responses.

Light pollution occurs in the country as well, Kernbach warns, so it’s not just city dwellers who may be at increased risk of West Nile. Humans “may be influencing infectious disease dynamics to a level far greater than we thought.”

Clinton Francis, an ecologist at the California Polytechnic State University in San Luis Obispo, agrees. “The ways that we influence sensory environments”—whether it’s with sound or light”—can have all sorts of unintended consequences,” he says.