Light at night lengthens how long birds can spread West Nile virus | Science News for Students

Light at night lengthens how long birds can spread West Nile virus

Under dim light, house sparrows take longer to fight off their infection
Apr 16, 2018 — 6:45 am EST
house sparrows
Another reason for caution in lighting up the night: Study finds it makes it harder for house sparrows to fight off West Nile virus. Mosquitoes can then bite infected birds and later spread the virus to people.

SAN FRANCISCO, Calif. — In many places, a moonless night sky is no longer inky black. Artificial lighting can give the night a persistent glow. This so-called light pollution can affect animals. And new data now suggest those effects might trickle through ecosystems.

Even moderate light pollution, a new study finds, can roughly double how long a house sparrow infected with West Nile virus remains at high risk of spreading disease. If bitten by a mosquito, that virus can now spread to other animals, including people.

In the United States, house sparrows are about as widespread as is artificial lighting. So they made a useful test species in a new first-of-its-kind study, says Meredith Kernbach. Her team used these birds to test whether light at night might affect the spread of West Nile disease. Kernbach’s work combines ecology with the study of immune systems. She works at the University of South Florida in Tampa.

For the new study, she brought sparrows into the lab. Some spent the night in an area that was dimly lit. These birds were slower in fighting off West Nile infections than were lab sparrows that spent the night in full darkness. Kernbach reported her findings here, January 7, at the annual meeting of the Society for Integrative and Comparative Biology.

The West Nile virus needs a mosquito to spread from bird to bird, or from bird to human. If a mosquito doesn’t pick up enough of the virus from the blood of an infected animal, its new victim might be able to avoid getting sick. That’s why it matters how much virus an infected bird had coursing through its veins.

Sparrows kept under a dim night light typically had enough virus in their blood to be a potent source of virus for at least four days, Kernbach reports. Sparrows housed in full darkness had enough virus to only spread the disease for two days.

This doubling of how long a sparrow could pass along a big dose of virus might up the chance that mosquitoes will be able to spread the bird's disease to others.

Scientists have been studying the effects of light pollution for years. Some have looked into whether it even affects human health. This has been a particular concern for people who work at night. Researchers also have looked at possible impacts on wildlife behaviors and reproduction.

Jenny Ouyang works at the University of Nevada, Reno. She, too, has studied light pollution and birds. Kernbach’s project opens new territory, Ouyang says. As light-pollution studies go, “I don’t know of anything like this.”

The findings heighten her curiosity about whether light at night might have a similar effect in spreading malaria. It, too, is a mosquito-borne disease. There have been hints in other studies, Ouyang says, that mosquitoes may sometimes be drawn to light sources. If true, night lights may boost risks of certain infectious diseases in urban areas.

Kernbach based much of her lab test on real-world conditions. The dose of virus that she gave the birds was enough to kill about 40 percent of them. That was well within what a mosquito might pick up as a vampire of birds or mammals.  She used white incandescent bulbs. Used widely throughout the last century, these bulbs are still common, despite big inroads by fluorescent and LED lighting. The sparrows encountered about 8 lux of light during their seven-hour nights. (A heavily overcast day, by comparison, ranks at about 100 lux.)

What lights do to the birds is only part of the story, points out Davide Dominoni. He’s a physiologist at the Netherlands Institute of Ecology in Wageningen. Researchers will also need to look for effects on the virus itself, he says. And  of course  in the mosquitoes.

Power Words

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annual     Adjective for something that happens every year. (in botany) A plant that lives only one year, so it usually has a showy flower and produces many seeds.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

biology     The study of living things. The scientists who study them are known as biologists.

ecology      A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.

fluorescent lighting     A technology that uses electricity to create glowing tubes of mercury vapor. It has been in use since the late 1890s.

immune system     The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.

incandescent lighting     The old-style lighting technology that relied on a glass bulb. Electricity passing through the bulb heated a thread-like tungsten filament, making it glow white hot. Thomas Edison commercialized this technology in 1879. By that time, the technology was already about 50 years old. Incandescent lights have been used to illuminate everything from tiny flashlights to whole rooms. Many governments have moved to ban these bulbs because they waste so much of their energy as heat.

infection     A disease that can spread from one organism to another. It’s usually caused by some type of germ.

insect     A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

LED     (short for light emitting diode) Electronic components that, as their name suggests, emit light when electricity flows through them. LEDs are very energy-efficient and often can be very bright. They have lately been replacing conventional lights for home and commercial lamps.

light pollution     The intrusion of unwanted light into areas that would naturally remain dark. Light pollution interferes with our ability to view the night sky. It also alters the circadian rhythms of plants, animals and people.

lux     A measurement unit for the level of light.

malaria     A disease caused by a parasite that invades the red blood cells. The parasite is transmitted by mosquitoes, largely in tropical and subtropical regions.

mammal     A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.

physiologist     A scientist who studies the branch of biology that deals with how the bodies of healthy organisms function under normal circumstances.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

urban     Of or related to cities, especially densely populated ones or regions where lots of traffic and industrial activity occurs. The development or buildup of urban areas is a phenomenon known as urbanization.

virus     Tiny infectious particles consisting of RNA or DNA surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact no virus is truly alive. It doesn’t eat like animals do, or make its own food the way plants do. It must hijack the cellular machinery of a living cell in order to survive.

West Nile     A disease caused by a virus that is transmitted by mosquitoes. Most people develop no symptoms. But about one in five infected people will get a headache, body aches, joint pains, vomiting, diarrhea, fever or some rash. A very small share of infected people will also develop coma, seizures or paralysis.


Meeting: M. Kernbach et al. Light pollution increases host competence to West Nile Virus in a reservoir species. Society for Integrative and Comparative Biology 2018 meeting, San Francisco, January 7, 2018. 

Journal: J.Q. Ouyang et al. Restless roosts: Light pollution affects behavior, sleep and physiology in a free-living songbird. Global Change Biology. Vol. 23, November 2017, p. 4887. doi: 10.1111/gcb.13756. 

Journal: J.Q. Ouyang et al. Stressful colours: corticosterone concentrations in a free-living songbird vary with the spectral composition of experimental illumination. Biology Letters. Vol. 11, August 2015, p. 13. doi: 10.1098/rsbl.2015.0517