Light Pollution Affects West Nile Virus Spread

A new study from researchers at the University of South Florida (USF) highlights the correlation between exposure to light and the spread of West Nile virus (WNV) through birds.

To measure how light transmission could translate into more avian cases of WNV, nearly 50 house sparrows were intentionally infected with the virus. All were monitored regularly for disease signs as well as body weight, viremia, and stress hormones. A dim light was kept on during the night in 50% of the study birds’ cages; the other half of the cages remained in complete darkness.

Interestingly, exposure to light did not impact how many birds displayed clinical signs or how long they stayed sick—sparrows from both groups got sick within 2 days and almost half died. However, the researchers identified that the group exposed to light at night remained infectious for 2 additional days, doubling the length of time before a bird’s immune system fought back to bring virus levels low enough so that biting mosquitoes could no longer be infected.

Although WNV primarily affects birds, animals and humans are also susceptible when bitten by mosquitoes that have previously fed on infected birds. This study’s findings suggest that the prolonged presence of light could translate into more cases of WNV both in birds and people.

Since the first case of WNV was identified in Queens, New York in 1999, the virus has spread throughout the country and news coverage of reported cases has become commonplace. But the familiarity of the virus doesn’t negate its seriousness. Nearly 44,000 WNV cases have been reported in the United States since 1999 and more than 1900 people have died. This is according to the Centers for Disease Control and Prevention, which also predicts that many other cases of illness have gone unreported. By the end of 2017, a total of 47 states and Washington, DC had reported West Nile virus infections in people, birds, or mosquitoes for the year.

About 20% of infected people are symptomatic, with fever, body aches, joint pains, vomiting, or rash. Most people recover from the virus, but about 1 in 150 infected people develop a serious, sometimes fatal, illness affecting the central nervous system.

Additional Studies

USF has conducted related research on how light exposure can increase stress hormone levels in birds, how those stress hormones affect the likelihood that a bird could be bitten by an infected mosquito, and how the stress hormones might impact a bird’s resistance to the virus.

A day before presenting their findings on light pollution, researchers from the university (some of whom worked on both studies) spoke about how glucocorticoid hormones affect birds’ ability to cope with WNV infection. Although previous research on rodents indicated that the short-term, elevated presence of the stress hormone corticosterone helps hosts resist or cope with infection, the same results were not duplicated in the sampled birds. Instead, birds infected with WNV tested positive for levels of the virus in their blood and died as a result. Both studies were presented this month at the Society for Integrative and Comparative Biology’s 2018 annual meeting in San Francisco, California.

In 2016, the USF College of Public Health reported on a study that determined that birds with elevated levels of the stress hormone corticosterone are more likely to be bitten by mosquitoes that can carry WNV. “Mosquitoes seem to be able to ‘sniff out’ the stress hormone and key in on individual birds,” said the study’s principal investigator Lynn Martin, PhD, associate professor in the USF Department of Integrative Biology.

The researchers concluded that the corticosterone levels in their test birds were within the range of normal for birds in the wild when exposed to stressors in natural their environments, such as road noise, pesticides, and light pollution.