Tuesday, March 28, 2006

Evolutionary no man’s land — pandemic flu and the underreported meme

The deadly influenza pandemic of 1918 seems to have first broken out in the trenches of World War I. We now know it was a bird flu that jumped species. What made it so lethal? Will it happen again? What about today’s outbreak of H5N1 bird flu? And how do wild birds fit into the picture?

The NYT stories this week about the bird flu threat, like most stories in the mainstream media, are long on questions and short on answers. One reason is that they overlook evolution. Neither the NYT story about the 1918 flu pandemic nor their story about the current bird flu outbreak mentions evolutionary biology or natural selection. However an expert quoted in the latter article does attempt to read a chicken’s mind.
"If you're a chicken," Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention, said at a recent conference on avian flu, "this is a pandemic. We have to be aware that other species are thinking about this differently."
It’s not clear that other species are thinking about this so much as suffering from it, but that’s a quibble. The real issue here is the underreported meme. Wendy Orent, the author of "Plague,” filled in some of the missing pieces in the LA Times last fall.
Somehow, somewhere, the mysterious gene collection that made up the 1918 killer influenza acquired its adaptive and lethal abilities in people. Most influenza viruses are respiratory and require mobile human hosts, who become viral distribution machines. You might be miserable with the flu, but you're still able to walk around, shake hands, sneeze on your keyboard and talk to colleagues with a halo of virus around you.

But the 1918 pandemic strain was different. According to evolutionary biologist Paul W. Ewald of the University of Louisville, its lethality evolved in the trenches, the trucks, the trains and the hospitals of World War I. Infected soldiers were packed shoulder to shoulder with the healthy, and even the deadliest virus can jump from one host to another. The Western Front was a disease factory, and it manufactured the 1918 flu. The packed chicken farms of Asia are a close parallel. H5N1 evolved the same way as the 1918 flu did in the trenches.

We don't know what will happen to H5N1 as it moves through Europe. It is certain, though, that the longer it lives in wild birds, the more likely it will become mild, at least for its wild-bird hosts. This is what happened to the 1918 flu after soldiers abandoned the Western Front. In just over a year, the virus lost its virulence and wandered the planet as an ordinary flu.

The lesson here is that the flu virus, like all of life, is subject to evolution. Lethal diseases don't fall out of the sky. They evolve in the context of a host and that host's conditions of life. There is no sign, so far, that H5N1 is turning into a human disease — effectively spreading from person to person. Even if it does, it needs a Western Front to become more than ordinary.
The entire article is worth reading. It also sparked a debate that John Hawks covered in his science blog. He’s an anthropologist at the University of Wisconsin-Madison, and while this is not his area of research, he does work with population genetics and evolutionary models, and his conclusions are interesting.
I don't have a dog in this hunt, but I notice several points:

1. Almost no mainstream press accounts of the bird flu threat discuss anything about the evolution of influenza. This is probably the most important public impact of evolutionary theory today, but we hear almost nothing of the evolutionary modeling of how the virus may change.

2. Ewald is very well known for studying the evolutionary dynamics of disease. He is making an argument that is sound, as far as the dynamics of selection are concerned. Thus, there are good reasons to think that the worst will not happen, and this is a perspective that has been underplayed.

3. So far, the theory has only been tested by a relatively small number of instances -- there just haven't been so many pandemics that we can infer accurately from past events what the future will be like. It could certainly happen that some new influenza strain could violate the model in some unexpected way, and for this reason governments should play it safe rather than assume that no high-virulence pandemic will emerge.

4. A lot of public health scientists are going to be well-employed for as long as the bird flu remains in the public perception. This doesn't mean that they are wrong to convey alarm, but it does mean that they don't benefit by playing down the threat. It's sort of like NASA and the asteroid impact threat --- partly they are more concerned because they know more about the threat and its terrible effects, partly because it's their job to be concerned.

5. There are a lot of biologists who don't use or understand selection.

No comments: