Saturday, October 08, 2005

Links to avian influenza sites

These last months the press has been bombarding us with news concerning a bird flu that can sometimes infect humans. Lately the tone has become more alarmist. Is there a pandemic looming? What will cause it?

This bird flu virus is currently spreading among bird populations. Sometimes it can infect humans, and when it does the mortality rate is of 55%. That is pretty bad if your job involves some kind of contact with chickens because currently you can only catch it from them. However, if the virus becomes able to travel from human to human, that is all you are going to see in the news for a long time to come.

There are two ways that this situation can come about. First, a human virus could acquire the deadly characteristics of the avian virus. This requires both viruses to be in the same host at the same time. In the second scenario, you have the virus mutating. I quote the following part from this sobering article:

...Unfortunately, scientists have never had the opportunity to use modern methods to study the emergence of a pandemic strain. Our understanding of how the human influenza virus incorporates avian elements is, as a consequence, very limited. Predictions that the current behaviour of the H5N1 avian virus is indicative of a growing pandemic threat are really just speculation.

There are really two quite distinct scenarios being promoted. Davis, along with many scientists, makes no real effort to distinguish between them. The first is the antigenic drift scenario described above. It is plausible, if poorly understood.

The second is the mutation scenario. This is the stuff of science fiction. It predicts the direct conversion of the H5N1 avian virus into, in effect, a human virus. The emergence of a brand-new virulent and highly infectious human virus is theoretically possible, but fortunately, as history teaches us, vanishingly rare. We have to look back to the sixth-century plague of Justinian, if not earlier, for a precedent. Predictions that we are now on the brink of such a rare biological event speak more to hubris than to good science.

It is of course very reassuring after one is done reading the article: it is not pessimistic. Even if such an epidemic takes place, people in many countries are better prepared than in previous pandemics (although I have many doubts about general practitioners in the Netherlands. I will leave that to another post). Yet his assessment of the second scenario has rebuked by some recent work.

As unbelievable as it sounds, scientists have managed to recreate the virus from the 1918 Spanish influenza epidemic using tissue from a frozen body exhumed in Alaska. They showed that this strain is particularly virulent:
In the Science article, Terrence M. Tumpey and colleagues report that they generated a flu virus bearing all eight gene segments of the 1918 virus in order to study what made it so virulent. They exposed groups of mice to that virus and to other viruses in which some of the 1918 virus's genes were replaced by genes from recent flu viruses.

The 1918 virus turned out to be extremely virulent. Mice infected with it died in as little as 3 days, and mice that survived as long as 4 days had 39,000 times as many virus particles in their lungs as did mice infected with a control flu virus, a Texas strain of H1N1 from 1991. All the mice infected with the 1918 virus died, while those exposed to the Texas strain survived. Further, the 1918 virus was at least 100 times as lethal as an engineered virus that contained five 1918 genes and three genes from the Texas H1N1 strain.

They did a related experiment on human lung tissue obtaining similar trends. Now, the characteristic that relates this 1918 virus with today's situation is:

In the other study, researchers at the Armed Forces Institute of Pathology (AFIP) report that the close resemblance of the 1918 virus to avian flu viruses suggests that the 1918 virus was an avian strain that managed to adapt to humans without first acquiring any genes from existing human flu viruses. Further, the researchers found that several of the same mutations that differentiated the 1918 virus from avian flu viruses are found in the H5N1 virus, which has killed more than 60 people in Asia. The report appears in Nature

This seems to support the possibility of scenario two.

I am trying to read a bit more to learn about it. If you want to do the same the Flu Wiki is a good place to start.


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