COMMENTARY: West Nile: The Role of Weather and Climate

Dr. A. Marm Kilpatrick is a senior scientist at the Consortium for Conservation Medicine, based at the Wildlife Trust in New York City. He was part of a team, working with the New York State Department of Health and the Smithsonian Migratory Bird Center producing findings last May that American robins are the primary carrier of the West Nile virus in the northeastern US.

Dr. A. Marm Kilpatrick in the field, checking an owl for signs of infection.

Taking censuses of different species, the research team determined that mosquitoes feed on robins 43 percent of the time, even though robins make up only four percent of the birds at those sites in Washington, D.C. and Maryland. Robins carry a moderately high level of the virus in their blood when they’re infected, which makes it more likely that they"ll infect the mosquitoes that bite them.

"This is a dramatic result," Dr. Kilpatrick says. "Our data clearly shows that American robins are the focus of West Nile virus transmission at our sites, even though they are only a small part of the bird community. Even more importantly, our research shows that sites with the most intense feeding on American robins have earlier West Nile virus epidemics—data that can be used to help manage outbreaks." E talked to Dr. Kilpatrick in an effort to understand how environmental factors might spread this scourge:

E Magazine: I understand that the West Nile virus, which was first discovered in New York City in 1999, has infected 26,000 people in North America and killed 962. And it has become a very virulent scourge. Why do you think we are seeing an outbreak of West Nile now, and what is its potential in human hosts?

Dr. Kirkpatrick: The introduction of West Nile virus in North America appears to be a consequence of increased globalization and trade in goods—basically airplane flights and goods being moved all around the world. The available evidence suggests that West Nile got here via trade from the Middle East. It was either a bird or a mosquito on an airplane probably traveling from somewhere in the Middle East. Some of the genetic similarities between the viruses here and there make that link. That process, of course, is also spreading other diseases, including bird flu or avian influenza.

The mosquito that spreads West Nile, Culex pipiens, is that native to the U.S.?
There are a number of species that spread West Nile virus around the US. Culex pipiens is probably the most important West Nile virus vector for about a quarter of the country, including the Northeast, north central U.S. and central Canada. Culex pipiens is actually an introduced mosquito from Europe and the old world.

If that mosquito had not come here, would the virus have simply found another vector?
Good question. There are a few other species that transmit it in the area where it was introduced in New York. One species is Culex restuans, which is somewhat similar but a little bit different as well. It’s an earlier-season mosquito, and so it peaks a month or two earlier and then fades as Culex pipiens is really taking off towards the middle and end of the summer. So it’s a pretty interesting question to ask whether or not the viruses would have taken off and done as well as it has if Culex pipiens had not been introduced in North America.

One way I’ve heard that mosquitoes come here is in tires that gather water on docks. For example, Japanese tires are imported into the U.S. for recapping, and as they sit on docks full of water they’re a breeding ground for mosquitoes.
That has shown to be one of the more important entry ways for a number of different mosquitoes, including one called Aedes albopictus or the Asian tiger mosquito, that definitely bothers a lot of people in the middle Atlantic states. It hasn’t reached New York yet, but in D.C. and further south it’s quite prevalent and a nuisance mosquito. Those mosquitoes love to lay their eggs in tires, so it has been spread all around the globe. Some research has linked the volume of shipping with the spread of that mosquito.

So the timing is crucial. Culex pipiens prefers to feed on robins, but when the robins leave the mosquitoes go to their second-best choice, which is mammals. So the weather probably plays a part in it, and perhaps global warming.
It’s interesting, What the data suggests is that Culex pipiens, also known as the Northern House Mosquito, appears to prefer the robin of the huge range of its hosts. So when the robins leave it appears to prefer humans next—not just any mammals. They don’t switch to squirrels or cats, for instance. And so far, data suggest that in this case weather isn’t driving that feeding preference. But weather is important in driving other aspects of the mosquitoes, especially reproduction and survival. And so as you get warmer temperatures, mosquitoes can lay eggs and those eggs can develop faster. And in addition, the virus replicates faster in mosquitoes when it’s warmer.

Culex pipiens is probably the most important West Nile virus "vector" for about a quarter of the country.© U.S. Geological Survey

Why did we see such a large outbreak of West Nile virus in New York City in 1999? I’ve seen speculation that climate change was a factor in that as well, because it dried up water sources for birds, causing them to leave and making it more likely that the mosquitoes would bite humans.
There is some speculation about West Nile virus and climate change, specifically about how droughts might affect it. A professor at Harvard, Dr. Paul Epstein, has tried to put this idea out there, and so far there really isn’t any good evidence supporting that. There’s no research that actually links years that are drier on average or warmer on average with years that are especially intense for transmission. We’re just starting to look at the past eight years that West Nile has been around and saying, what are the factors that drive epidemics of this disease?

The summer of 2006, how bad was that for West Nile?
I think it ranked as the third-worst year. The worst year was 2003, when we had about 10,000 human cases across the US. Last year, 2006, was about 4,000, as was 2002. And the last two years have each been around 3,000. These numbers, of course, are reported cases, and that has a lot do to with how much energy and money is being spent by public health officials who test patients when they come in with flu-like symptoms. A number of other pathogens can cause those symptoms. The research suggests there have been about 1.3 million humans infected with West Nile virus in North America so far. And we only have about 26,000 reported cases. So there’s a disconnect between infections, the number of people who get sick, and the number of people that get sick, get tested and are then found to be positive.

Most people survive it according to the data. There have been 770 fatalities since 1999, or is that number higher now?
It’s actually higher now, almost a thousand, but your generality is completely correct. People survive it, and in fact most people don’t even show any symptoms of it. On the order of four of every five are asymptomatic—they show no illness at all. Only one in five or so seem to get symptoms, and then an even smaller fraction gets serious meningitis or encephalitis in the brain, and then an even smaller fraction actually die. So while this is a serious disease causing sometimes life-debilitating illnesses, it kills a relatively small fraction of people that get infected—about one in a thousand.

Has West Nile now spread to all parts of the U.S.? The last place

I heard where there was no West Nile was Hawaii. Is that still true?
It’s still not in Hawaii, and it has not been found in Alaska, either. The lower 48 have got it, but the other two states still remain free. Most of Canada has it.

That raises the interesting question: Do climates get too cold for West Nile? Is Alaska simply too chilly?
That’s what it looks like, yes. It’s found in Maine but mostly in the southern areas, and only for a short period of the year. And so it does look like climate is limiting the northern distribution of the disease. At least as current conditions are now, I would guess it would never get to Alaska. But if it gets warmer there

The Consortium for Conservation Medicine and the Wildlife Trust look at the intersection of animals, environment and the spread of disease. Would you say that both animals and the environment are a factor in spreading this disease?
There are a couple of interesting questions. One, how did it get from the Middle East to the U.S. and North America? And once it got here, how did it spread? There are a number of different hypotheses and ideas that people have about how this might have occurred. One is that airplanes spread it around, and another is that birds disperse it. After birds fledge, they try and find a new territory away from their parents. There are some other ideas, including that it is moving around simply by mosquito flights—they’ve been known to fly a few kilometers. People have also suggested that maybe mosquitoes are getting on airplanes or trucks and being moved around by commercial transportation. I think the real answer is it’s really a combination all of these things. The data so far suggests that it got to the U.S. and North America by human modes, either a bird or a mosquito on an airplane, but then from there it seems they have spread primarily by dispersing and migrating birds.

Do you think that West Nile will stay at this relatively mild level, or will we see a much larger outbreak of it sometime soon, and if so, what would be the likely cause?
That’s a good question. We have had only since 1999 to study this disease, and it appears there is a lot of year-to-year fluctuation, with small or regional epidemics followed by a few years of not as much activity and then sometimes another more-intense year. My guess is that we will continue to see a few thousand cases a year, then occasionally there will be a year where the conditions will be just right for the disease. So we"ll see maybe a few thousand more cases. Some things might happen to change that prediction. For instance, the virus itself is evolving. There are some recent studies showing that the virus that’s circulating now is a little bit more efficient at replicating in North American birds and North American mosquitoes than the original virus that was introduced. That will allow it to spread faster and create more of an epidemic. So there’s certainly cause for concern. And then, of course, climate change may have an effect as well. It’s a complex interaction.

Research assistance by Jennifer Greco.

JIM MOTAVALLI is editor of E.

CONTACT: Consortium for Conservation Medicine; Wildlife Trust; E cover story on conservation medicine