Understanding the Threat: A Focus on the Science: Two Infectious Disease Specialists Describe and Discuss What the Scientific Community Knows about the Avian Flu Virus H5N1 and How Pandemic Influenza Might Emerge
Osterholm, Michael, Nieman Reports
Michael Osterholm, Director, Center for Infectious Disease Research and Policy and member of the National Science Advisory Board on Biosecurity
Understanding influenza: From the virus to the pandemic.
It's not enough for Americans to be worried about influenza in the United States. It's critical that Americans worry about influenza in every country, because we're so dependent on the rest of the world. All that crosses our borders each day in the global just-in-time economy is growing exponentially. And today one out of every nine people who's ever lived is on the face of the earth. If similar to previous pandemics, the numbers will obviously be very large, relative to that world population.
Influenza A viruses are extremely diverse and evolve rapidly. [But] I know less about influenza today than I did 10 years ago, and the more information we get, in many instances, the more questions we have. Influenza pandemics emerge sporadically with variable characteristics. There is no such thing as the influenza pandemic; pandemics differ and are varied, but with one commonality: They're caused by the influenza virus. Influenza causes disease in domestic animals like poultry, horses and swine, and new strains continually emerge. The host range is extensive in birds and mammals. The last count I saw there were 121 different species of birds or mammals that have been infected with H5N1, so just that one strain can be very extensive.
Dealing with other infectious diseases, whether it's mumps, measles, rubella, so many other agents, they are basically glaciers in genetic change compared to the influenza A virus, which is a hurricane in its truest sense. And one of the problems we have today when we talk about H5N1 is there isn't such a thing as an H5N1 virus. There are multitudes of H5N1 viruses, and they're doing different things. They're causing different problems in different areas. Some may actually be more likely to be the next pandemic strain if, in fact, it's going to be at all. There may be specificities for animals vs. humans. Their interrelationship with humans and genetic aspects of humans is all there. This is the part I must tell you I understand less about, and I think many of my colleagues understand less about today, than we did a few years ago. And we're learning a lot, but each week it's almost a new learning experience.
Now to understand pandemic influenza, it occurs when a novel influence of strain emerges from the avian population that has the following features. One, it can be readily transmitted between humans. It's about the birds now, but it's not about the birds once it becomes a human-to-human transmitted agent. That's when we worry about us transmitting to and by ourselves. It will be genetically unique, and clearly we lack immunity to this. Studies have demonstrated over and over again a lack of H5N1 infection in recent history in humans. It is critical for us to understand that we look at influenza through a microscope when we need a telescope. We only have 70-some years of virologic data to really be able to talk about influenza when we know influenza goes back to antiquity.
Today we have this incredible genetic roulette table out there, with billions and billions of poultry to feed the billions and billions of people that we didn't have before. There are more domestic poultry today than we probably have had collectively in the past 1,000 years and so therefore we have this new reservoir. Maybe in a past life this virus would not have been effective enough to basically develop genetic changes, given a limited population. Today it has this unlimited roulette table to play and play and play again, and that is what we're concerned about.
Pandemic influenza is different from avian influenza. If we learn nothing at this meeting, then understand we're not talking about avian influenza as the problem. We're talking about it as the intermediate problem. …