Don't fear the (bird) reaper

November 4, 2005 6:28 AM

Evolutionary biologist Paul Ewald, author of The evolution of infectious disease and an expert on the development of pathogen virulence (see this, this and this for a good intro), responds to this editorial in Scientific American and pours cold water on fears of pandemic influenza.
posted by docgonzo (23 comments total)

Interesting argument. It makes a lot of sense, but his arguments rest quite a bit on hindsight (particularly with regards to 1918), which really weakens them.

It's easy to come up with a theory that explains the past, but more difficult to come up with a theory that explains the future.
posted by delmoi at 6:45 AM on November 4, 2005

I'm not sure if hindsight is the proper term in this case, as it's a term of art rather than of science. Ewald certainly doesn't seem to be speculating, but actually to be building scientific theories about how viruses evolve. What I find interesting is that many of the components of his theory are pretty widely accepted, yet they seem not to be being applied in the case of flu. For instance, every time there is an ebola outbreak the reporting seems to stress the link between virulence and transmissability, or at least, I've read more than one previous report suggesting that the risk of a huge ebola outbreak is mitigated by precisely the virulence that makes it so terrifying (people die quickly in the place where they contracted the virus). This is exactly what Ewald points out,

Evolution of increased transmissibility of H5N1 from human to human is bound to go hand in hand with drastic evolutionary reductions in virulence. The history of influenza provides strong evidence for this conclusion. Except for the 1918 pandemic, all the trustworthy evidence from all the years of well documented influenza epidemics and pandemics indicate that influenza viruses maintain themselves evolutionarily at low to moderate virulence when transmission depends on host mobility.

but he goes even further and explains that the fears presented in the first editorial (that influenza's transmission pre-symptom) actually misses the evolutionary link between virulence and transmission. Revere (and implicitly the SciAm guy, who seems to want to disagree with Ewald based more on desire than science) bring up the low transmission rate of ebola (and several other nasties), but just assume that this is a happy coincidence, as if Nature had dealt us a good hand in those cases. Ewald points out that this is rhetorical slight of hand, and that transmission and virulence are results of the same evolutionary process. Good post.
posted by OmieWise at 6:55 AM on November 4, 2005

Aw, you're no fun anymore. I'd rather get like all hysterical and fearful and stuff. Give me the sensationalist mass media any day.
posted by ZenMasterThis at 7:00 AM on November 4, 2005

More correctly: It is *required* to come up with a theory that explains the past, or the theory is rejected out of hand. Any theory about the potential for a H5N1 pandemic has to take into account the 1918 H1N1 epidemic, or it is just pseudoscientific bullshit.

The test isn't how it jibes with the past, it's how it jibes with the future, and Ewald's theory -- that the 1918 H1N1 pandemic was caused by a very constrained set of circumstances -- is a good workable theory that both takes into account the past and makes proveable (at least, falsifiable) assertions of the future.

The idea that viruses tend to be either very transmissible (the common cold) or very virulent (Ebola) isn't new, and as a general rule, has held up very well. SARS was rather virulent, but not that easy to catch. H1N1 was unusual when it first appeared, because it was as transmissible as any other influzena virus (very) but, for reasons we still don't understand, was much more virulent (kicker: it was apparently even more so in the young and healthy, which is the exact opposite you would expect.) Dr. Ewald's theory is that this was a fluke of circumstances -- a reasonable conclusion, given the data we have, but by no means a certain one.

I, of course, hope that Dr. Ewald is correct -- simply because another pandemic like 1918 would be horrible. However, Hope, as they say, Is Not A Plan, and there are reasons to be wary of H5N1. It's clearly lethal to birds, and it has been documented to have jumped to swine. There's some good (but not great) evidence of transmission to humans, and there's hints of Human to Human transmission, and it seems that H5N1 in humans is more virulent than the typical influenza virus. All that data needs more study, of course, but being wary and watchful of this particular incarnation of influzena is a very, very good idea.
posted by eriko at 7:00 AM on November 4, 2005

First, ebola and denge and all those hemorrhagic fevers were supposed to wipe us all out, a la Hot Zone. Then, it was West Nile virus. Now, this wacky bird flu.

And yet - here I am, not coughing or bleeding uncontrollably! Disease-riddled corpses are not stacked like cordwood in the streets! Where's my pandemic, dammit! Don't tell me this has all just been hysterical hype . . . that would be so much worry and stress for nothing.
posted by ToasT at 7:38 AM on November 4, 2005

eriko right, obviously a theory needs to be consistent with the past.

The problem is that there are, actually, an infinite number of theories that can perfectly describe any set of data. If you can't do experiments, then you're not going to be able to cull theories that fit the known data, but have no predictive power.
posted by delmoi at 7:38 AM on November 4, 2005

But ... but ... Bush wants to limit travel dammit! A populace that walks is so much easier to control!
posted by WolfDaddy at 7:53 AM on November 4, 2005

A good point from the author:

Also, might it be a mistake to focus too exclusively on mortality as a measure of how devastating a flu pandemic could be? Even an outbreak with mortality well within the range of what we've seen post-1918 could be devastating to economies--and the measures needed to contain it would be the same.

I'm not building a bunker, but I'd rather over-prepare and be wrong than vice versa...
posted by jalexei at 8:01 AM on November 4, 2005

To be fair, ToasT, West Nile and Marburg/Ebola-whatever were never considered threats by actual scientists, just the hysterical media.

Bird flu is a tad different.
posted by mek at 8:06 AM on November 4, 2005

It's easy to come up with a theory that explains the past, but more difficult to come up with a theory that explains the future.

Maybe because the future hasn't yet happened?

Perhaps you mean *predict* the future. Well, isn't that the test of a theory? That it explains past events, and, if valid, should predict future events?
posted by Ayn Marx at 8:26 AM on November 4, 2005

It's great to read sensible articles like this, but they'll never get into the mass circulation media, which seems to have made up its mind. The people hunched over their computer models predicting armageddon, getting research funds and selling newspapers as a result of their dismal speculations are the same bunch that brought you the media panic on SARS, Foot and Mouth and of course BSE. In all these cases it seems the massive over-reaction is the real thing to fear, particularly the economic impact.
posted by grahamwell at 8:53 AM on November 4, 2005

I believe that the reason that Ebola is not such a virulent virus is the immunity present in Europeans from the Black Death. These two authors convincingly describe how the Black Death could not be caused by a Bubonic plague due to the quick death and virulence exhibited in populations during the Black Death.

Also it seems that a disease has to have a very specific mode of entry and execution to be both virulent and widespread -- something that influenza by design seems incapable of.
posted by geoff. at 8:56 AM on November 4, 2005

SARS was rather virulent, but not that easy to catch

Unless, of course, you were in Toronto. At least, that's what the media told me.
posted by antifuse at 9:01 AM on November 4, 2005

I believe that the reason that Ebola is not such a virulent virus is the immunity present in Europeans from the Black Death.

I think you might have your pathogens mixed up. There is a theory that the reason for the existence of the CCR5-delta32 mutation -- which provides partial or near complete protection from infection by HIV-1 and is common in many northern European populations -- was due to the Black Death although subsequent models have indicated the selection pressure was more likely from episodic smallpox outbreaks in Europe.

I don't think any protective factors -- physiologic, genetic or molecular -- have been identified for ebola. In fact, non-Africans have been sickened and died from both Ebola and Marburg -- a related filovirus -- most often in lab accidents. (The reason Marburg is called Marburg is because the first outbreak was in that German town, sparked by a bunch of imported lab monkeys.)

Also it seems that a disease has to have a very specific mode of entry and execution to be both virulent and widespread -- something that influenza by design seems incapable of.

I'm not sure what you mean here. The 1918 H1N1 virus was both transmissible and highly pathogenic -- although, as Ewald argues, this was the result of a fluke of evolutionary pressures.

And all viruses have very specific modes of entering target cells, for example HIV-1, which needs both the CD4 cellular receptor and either the CCR5 or CXCR4 receptors as co-receptors, in order to attach to target cells and inject its genomic material into the cell. In fact, getting into target cells is one of the largest barriers to infection and, therefore, is under intense evolutionary pressure. Viruses which cannot enter host cells effectively are not highly transmissible and do not last long.
posted by docgonzo at 9:11 AM on November 4, 2005

There's no way it will be both highly contagious and highly lethal. That simply doesn't happen.

Except, well, the last time it happened.

Does this actually comfort anyone?

I do not understand some of those in the medical field who want to bury their heads in the sand on this issue.

Is it likely? No, not at all. Is it possible? Absolutely. Do we as individuals, companies, cities, states, nations prepare for unlikely outcomes? Daily.

I do not see how this particular argument is persuasive. Ewald argues that the virulence of the 1918 flu was due to the immobility of the original infected populace. Hopefully without misinterpreting him, he seems to be describing an incubatory circumstance.

Does he truly believe there are no insular populations that could provide incubatory conditions in today's world?
posted by Ynoxas at 10:00 AM on November 4, 2005

Docgonzo nailed me again. I'm going to start just making stuff up now to get him to comment on a thread. PS Your blog was down forever, what was the deal dude?
posted by geoff. at 10:45 AM on November 4, 2005

Do we prepare for unlikely outcomes? Yes, sometimes, but we know that such preparation commits finite resources and may be unwise. It's a trade off, do we divert money and talent from real problems to counter imagined ones? Tough call.

While we panic about a possibility, Tuberculosis and Malaria are killing millions, now. Our finite healthcare resources might be better employed dealing with the devils we know.
posted by grahamwell at 10:45 AM on November 4, 2005

grahamwell: calling this possible pandemic "imagined" is not accurate.

There is a world of difference, or perhaps better stated, several million possible lives of difference, between "unlikely" and "imagined".

However, to accept some of your point, I would agree that ALL worldwide health concerns should be treated seriously, not just the ones that have media cachet.

Again, people pointing out that this is never going to happen when it has actually occurred in the modern era is being disingenuous, or at the very least, understating the possible risks.
posted by Ynoxas at 10:58 AM on November 4, 2005

...Bush wants to limit travel dammit! A populace that walks is so much easier to control!

Ok. I just pictured Bush's face atop a giant quasi-immortal man-worm and am now going to cower in a hole for the next week or so.
posted by Dormant Gorilla at 11:13 AM on November 4, 2005

Ewald is not arguing that it is never going to happen. He is arguing that we do not now have the necessary preconditions to produce a pandemic like that of 1918 and, therefore, the current panic is unjustified. As he writes, "With regard to the future I am predicting that such a highly lethal pandemic (i.e., 1 death per 50 infections) will not occur, not from H5N1 and not from any other influenza virus that will arise unless regional conditions allow transmission from immobile hosts, as they did on the Western Front in 1918. "
The argument is over whether it is just unlikely or extremely unlikely.
If the imagined (i.e. not imminent) flu pandemic is extremely unlikely, then the current panic and allocation of resources is hugely exaggerated, as was the panic over SARS.
posted by Zetetics at 11:45 AM on November 4, 2005

Apologies, the word should have been unlikely. As Zetetics says, Ewald's point was not that it couldn't happen but that the circumstances which created pandemic of 1918 don't exist now. I agree with you however, I don't quite understand why not. Possible reasons would be better medical knowledge, drugs, improved sanitation and the like. A worrying counter-argument would be to point to the modern development of super-slums.

It's amazing to think that the death toll from Malaria and Tuberculosis, every year, is roughly the population of Birmingham or the Washington DC Metro area. These are diseases that we know how to fight - a treated mosquito net cuts malaria by 90%, permanently, at a cost of pennies. History will judge us harshly I think.
posted by grahamwell at 12:02 PM on November 4, 2005

History will judge us harshly I think.

Surely, unless we begin to peel back the layers and start to get at the real issues. Why is it that some "immanent dangers" (WMD?) are hugely over-inflated or invented, while others (Climate Change?) are hugely underestimated or ignored? I hear the strains of "the giant wurlitzer" in this, a coordinated message in a scary minor key. To what ends we can only guess; certainly social control mechanisms are high on the list of proposed "solutions".
posted by dinsdale at 1:29 PM on November 4, 2005

The Daily Show (which we can get in Britain now, wonderful) had a superb montage of Bush - trying to put the fear of God into us.

Why?

"No more Katrinas" is part of the message .. the rest was Sales, Sales of Tamiflu®, of HoffmanLaRocheGlaxoSmithKlein stock, sales for all of the bright-eyed opportunity seekers who can see fun and profit in all of this. The Wurlizer you hear is powered by Seven Billion Dollars of borrowed money.

Which should be harmless and could be life-saving. Fine. The problem is that this media story has a life of its own. Pictures of fluffy birds and doom-laden voice overs make good Television. No-one can control what happens next. A scare in Indonesia, fights cancelled, travel advisories ...

Fancy a Chinese?

The SARS panic killed - hundreds? It is estimated to have cost 15 Billion Dollars, but we can't really know. That's mighty expensive birdshit.
posted by grahamwell at 2:07 PM on November 4, 2005