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