A few months ago, the Alameda County Mosquito Abatement District discovered four dead birds and seven groups of mosquitos in Castro Valley and Union City. All tested positive for West Nile Virus. As of early November, the numbers had grown to 19 dead birds and 15 groups of mosquitoes and had spread north to Oakland and south to Milpitas.
Two of the birds that died from West Nile Virus infection were house sparrows. One was found in a Castro Valley neighborhood in late May and the other in a busy Hayward neighborhood in early October. It’s not clear how long the sparrows were infected with West Nile before they died. But according to abatement district lab director Eric Haas-Stapleton, house sparrows are especially susceptible to contracting the disease and spreading it.
“In house sparrows, the virus can generate a chronic infection, which can be transmitted to mosquitoes that bite birds,” Haas-Stapleton explained. “Those sparrows can live for a very long time with West Nile Virus,” thereby increasing the risk that the infected birds will be bitten by mosquitoes, which, in turn, will then spread the disease further.
House sparrows are among the few avian species left in urban areas due to habitat loss from urbanization. Because they are also considered a reservoir species that naturally breeds infectious pathogens for West Nile Virus, they were the go-to species for a study earlier this year that is raising new concerns about the impacts of urbanization — specifically, light pollution — on the spread of disease.
The study, first published February in bioRXiv, a repository for biological sciences papers, and followed up July in Oxford’s Integrative & Comparative Biology journal, was led by University of South Florida global health Ph.D. student Meredith Kernbach. The findings suggest that light pollution from artificial lighting can lengthen the infectiousness period of West Nile Virus in birds, and thus could allow the disease to spread more easily. The study revealed that light pollution could ultimately increase West Nile Virus outbreaks in urban areas by 41 percent.
West Nile Virus landed in North America nearly two decades ago, from where it was first identified in the West Nile District of Uganda. While most people do not display symptoms when infected with West Nile Virus, statistics from the Center for Disease Control show that 1 in 150 people develop serious illnesses — including high fever, convulsions, muscle weakness, vision loss, paralysis, and coma — that affect the central nervous system. Within this group, 1 in 10 people die from the illnesses. There is no cure or specific treatment for West Nile Virus infections.
The disease has killed millions of birds, decimating populations, including those already endangered and threatened. Horses have also been known diagnosed with West Nile Virus and once infected, the virus can damage the spinal cord and brain.
The lack of previous research on light pollution and its potential link to the spread of West Nile Virus motivated Kernbach to pursue the subject. “There’s been a lot of work done that’s found that light pollution has a lot of negative consequences on individual fitness and health,” Kernbach said in an interview. “It causes them to change behaviors, but nobody has really looked at infectious disease dynamics.”
Just how does light pollution play a role in infectious diseases in birds — and perhaps in humans?
It has to do with artificial lighting — specifically, blue light. A shorter wavelength light, blue light is commonly used in both fluorescent lighting and energy-efficient LEDs in many urban areas. Blue light also emits from other artificial sources that accompany our lives: computer display and TV screens, tablets, and perhaps most importantly, our smartphones.
Blue light has its benefits; it can help our bodies produce vitamin D, boosts alertness and mood, and supports memory and cognition. But blue light also contributes significantly to light pollution — the glow or glare of dim light at night from artificial lighting, emitted by lighted buildings, streetlights, vehicle headlights, and lit billboards.
Kernbach’s study subjected house sparrows to two environments: one that mimicked dim light at night (dLAN), a common form of light pollution, and the other that mimicked the natural light at night emitted by the moon and stars. Both groups of sparrows were exposed to the same environment for up to three weeks, both before and after they were injected with West Nile.
Using a strain known as West Nile-NY99 to inject the experimental group, the researchers subjected the house sparrows to 12 hours of natural light followed by 12 hours of dim light to mimic the light pollution that is now widespread in urban places.
On the sixth day following infection, Kernbach’s team found that the sparrows exposed to dLAN had higher levels of West Nile Virus in their bodies than the control group for longer periods. Though it largely depends on the species, infected birds will typically carry West Nile Virus for about two days, according to Kernbach. The study found that in house sparrows exposed to light pollution, the infectious period lengthened by two days.
“The chance that a mosquito has West Nile Virus or contracts it from a [bird in a] light-polluted habitat and then transmits it to a human might be increased,” Kernbach added.