West Nile's Spread Linked to Robin's Migration
DEBBIE ELLIOTT, host:
From NPR News this is ALL THINGS CONSIDERED. I'm Debbie Elliott. The first official day of spring is still eight days away. But in many places the first sign of the new season has already arrived: flocks of red-breasted robins fresh from their wintering grounds. But a new study says the cheerful bird of spring holds a dark secret. Robins may be playing an important but unwitting role in spreading the dangerous West Nile virus. NPR's David Malakoff has more.
DAVID MALAKOFF reporting:
Just after dawn in a suburb of Washington, D.C. a robin serenades the neighborhood. Biologist Marm Kilpatrick says it's a pretty familiar song.
MERM KILPATRICK (Biologist): I think everyone knows American Robins. They have this kind of bright orange chest and like to hang out on lawns and eat worms. They are a fun and gregarious bird.
MALAKOFF: Kilpatrick works at the Consortium for the Conservation of Medicine in New York City. He studies how some diseases jump from wildlife to people. Recently he's been looking at the West Nile Virus. It arrived in the United States seven years ago and since then it has killed about eight hundred people. West Nile is mostly carried by birds. Mosquitoes move it to people. First they bite an infected bird and then a person. But it hasn't been exactly clear which kinds of birds are the big players in this disease triangle. To find out, a few summers ago Kilpatrick and his colleagues came here to Washington and started catching skeeters. They sucked up some with a customized bug vacuum. Others they lured to traps baited with a special recipe.
Mr. KILPATRICK: The water that we used to bait it with is actually called stinky water and it's made by taking rabbit chow or just grass, putting it in water and putting it in the sun for a week or two until it smells absolutely vile. The mosquitoes love that kind of water to lay their eggs in. So they come along to lay their eggs and we have a kind of fan apparatuses that sucks them up and catches them.
MALAKOFF: In four months Kilpatrick's team caught 30,000 mosquitoes. Then they looked at each and every one under a microscope. They were searching for a specific kind of mosquito. It's called Culex pipiens and is known to spread West Nile. More importantly, they were looking for Culex that were plump with fresh blood.
Mr. KILPATRICK: If they happened to have blood in their stomach then you yanked that one out and that goes in its own special tube. It's one of these little gems amongst the piles of mosquitoes.
MALAKOFF: That pile of gems was very small, about 150 mosquitoes, but the blood in those stomachs held scientific treasure. DNA that told the scientists exactly what kind of animal the mosquito had been feeding on.
Mr. KILPATRICK: What we found was actually quite shocking. Basically one species of bird was making up nearly half of all the blood meals and that's American robins.
MALAKOFF: Shocking because robins were nowhere near the most common bird in the study area. There were a lot more sparrows and starlings for instance. So a mosquito with a taste for robins' blood had to work really hard to find its pry. The bloody DNA also held another surprise. It showed that when robins stopped nesting in the late summer and fan out into the countryside the Culex mosquitoes go looking for another target.
Mr. KILPATRICK: They shift to their next favorite host which unfortunately appears to be us, humans.
MALAKOFF: And that shift happens just when the mosquitoes are most infected with the West Nile virus.
Mr. KILPATRICK: And so the thrust of this study is basically that mosquitoes appear to shift from feeding on birds, primarily American robins, to feeding on humans towards the end of the summer when West Nile Virus infections in mosquitoes are at their highest level.
MALAKOFF: The feeding shift, Kilpatrick says, could explain why human cases of West Nile spike in the late summer and early fall. The study is published in the latest issue of Public Library of Science Biology and Kilpatrick says it is important to realize that robins aren't the bad guy in this story.
Mr. KILPATRICK: Our study is not suggesting that where you find American robins you will have a lot of disease and therefore you should get rid of American robins. In fact I think the opposite. I think what this study suggests is if you were to get rid of the robins then what would happen is those mosquitoes might start feeding on you.
MALAKOFF: So keeping those robins in your back yard he says is a very good idea. David Malakoff, NPR News.