Light pollution makes sparrows sicker

Sparrows with West Nile Virus are infected for longer in the light - with implications for disease spread...
19 August 2020

Interview with 

Meredith Kernbach, University of South Florida


A house sparrow.


The coronavirus seems likely to have come to us from bats, possibly via a third party like a pangolin. If so, it would be far from the first time that an animal virus decided humans looked similar enough to make the jump - think of bird flu, mad cow disease, or even HIV. Zoologists from the University of South Florida have therefore been looking into the ways animals live closely alongside humans, and in particular testing out how light pollution affects sparrows that are carrying a common but dangerous disease called West Nile Virus. Phil Sansom heard from Meredith Kernbach...

Meredith - House sparrows, when they're exposed to light at night, they're not able to cope with infection with West Nile virus as well. They maintain the disease for longer and may actually die more easily from it.

Phil - So the light seemed to screw with their health in some way? And they couldn't deal with the virus as well?

Meredith - Right. Correct. So individuals are actually staying sick for longer, which may increase their opportunity to cause a West Nile virus outbreak in their area. West Nile virus is one of those things that spills over to people in the United States, and in Europe, and in Africa. And basically, we're broadly interested in why these diseases spill over. And so we chose light at night because we also know that light at night is rapidly expanding across all continents, except for Antarctica of course; but we know that it has a lot of other negative effects.

Phil - What was your version then, of a sparrow in a suburb? Presumably you didn't actually release sparrows with viruses into the suburbs of a nice city.

Meredith - Right. So we performed experimental infections in captivity, which has its caveats, because it's removed from nature. And these sparrows are housed basically on one side of the room, and we rig up a small light bulb, like a light bulb you'd find in your house, on the other side of the room, at a dim enough level that you might find in nature.

Phil - And then you give them a bit of the virus?

Meredith - Right. So we expose them to a very low amount of virus.

Phil - What does that look like - a sparrow with West Nile virus?

Meredith - It depends on the sparrow. There's a lot of different variation. Some birds will fluff up or look lethargic, and then other birds you'll never even know that they were sick.

Phil - If it's hard to tell whether these birds are actually that sick sometimes, what are you measuring to figure out how bad the light pollution is hitting them?

Meredith - We'll measure an individual's body mass, which indicates how well they're coping with disease. You might assume that a bird that's losing a lot of body mass is a very sick bird. And then we'll also take a blood sample to measure how much virus is in circulation, or how sick they are by how much virus they have in their blood sample.

Phil - And what do you find? How big a deal is light pollution for that?

Meredith - Light pollution actually does end up being a pretty big deal for these sparrows. The sparrows that are exposed to light at night actually have much higher amounts of virus in their blood. The hormone melatonin might be involved. It's a hormone that's secreted at night, but is suppressed in birds that are exposed to light at night. And melatonin is a regulator of the immune system, which means, without melatonin, your immune system might be all out of whack. Another mechanism is that the immune system will fluctuate throughout the day. So certain immune cells may be at higher concentrations, or there may be more of one immune cell during, say, the daytime. And so when they don't have the difference between light and dark anymore, their immune system may just get all scrambled up with not knowing what time of day it is.

Phil - Now, we're on Naked Genetics here. Do you have any clues for me from the genes of these sparrows?

Meredith - Yeah, absolutely. So, in another study we performed a transcriptomic analysis, which basically tells us which genes the sparrows are upregulating in order to cope with West Nile virus infection. And the birds who are exposed to light at night, upregulate these genes that are associated with say, pathogen resistance; which is counter-intuitive because you would expect that the birds with more virus would have worse resistance, but these birds that have more virus are actually upregulating these pathways earlier. And we also found from some of these other genetic signatures that individuals that are exposed to light at night are incurring more pathogen-induced damage, which means suffering a lot more from infection.

Phil - Huh. So it seems like whatever light is doing to these birds, the genes are desperately scrambling to keep up.

Meredith - Exactly.


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