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The Influenza Virus: Understanding Your Enemy

Posted Aug 24 2008 1:49pm
DAVID MARKS, MD: Hi, and welcome to our webcast. I'm Dr. David Marks. The influenza virus, or the flu, infects more than 90 million Americans each year. Most people get better within a week or two, but some can develop life-threatening complications. About 100,000 people land in the hospital each year, and more than 20,000 of them die. So what exactly is this virus, and where did it come from? And what are its potential health risks?

Joining me to answer these questions are two experts. First is Dr. Jason Kendler. He's assistant professor of medicine at Cornell Medical Center. Welcome.


DAVID MARKS, MD: His colleague at Cornell Medical Center is Dr. Adam Stracher, also an assistant professor of medicine there. Thanks.


DAVID MARKS, MD: What is a virus, to begin with?

ADAM STRACHER, MD: A virus is a very small living particle, that differs from bacteria because it's much smaller than bacteria, and it also differs from bacteria because it needs our cells, or the cells of another host to live within. It cannot live on its own.

DAVID MARKS, MD: Is it actually alive?

ADAM STRACHER, MD: It is alive. It is a living thing.

DAVID MARKS, MD: So the differences are, bacteria don't need us to live. But then can certainly live inside of us.


DAVID MARKS, MD: Now, influenza is a specific type of virus, correct?

JASON KENDLER, MD: That's correct.

DAVID MARKS, MD: Tell me a little bit about this type of virus.

JASON KENDLER, MD: The influenza virus is an RNA virus.

DAVID MARKS, MD: What does that mean?

JASON KENDLER, MD: It means that is the type of genetic material that it has. All viruses need to replicate themselves in order to cause infections and to spread infection from one host to another. The influenza virus in particular is spread by airborne spread, so that a patient who is infected with influenza will cough or sneeze, and can therefore transmit this to another host.

DAVID MARKS, MD: It comes up kind of cyclically during different seasons. Is it around all the time?

ADAM STRACHER, MD: Actually, here in our continent, and in this environment, it's around mostly in the wintertime. Actually between the late fall, around November, until March or April, basically in the winter. And that's, in the Southern climates, it's actually the opposite. It's there in their winter, which is opposite to ours. And in tropical climates, it is actually around all the time.

DAVID MARKS, MD: Why is it here in the winter?

ADAM STRACHER, MD: It's a little unclear. It probably has to do partly with crowding. People are all together often in the winter time, and so the virus is much more easily able to spread because of that. And it also probably lives better in cold, dry environments than it does in the summer time, in more humid environments.

DAVID MARKS, MD: We're talking about influenza virus as if it were one virus. But, really, there are different types within that category.

JASON KENDLER, MD: That's correct. There's influenza-A, influenza-B and influenza-C, all different types of virus. Most of the outbreaks, most of the disease we see in people are from influenza-A or B. Influenza-C is thought to cause a much milder disease.

DAVID MARKS, MD: I've never heard of influenza-C, frankly. We never hear about it in medical school.


DAVID MARKS, MD: Is it a problem at all?

JASON KENDLER, MD: It's generally not considered to be clinically significant.

ADAM STRACHER, MD: It's said to cause very mild cold sort of symptoms, almost. But the one that really, influenza-A and B are the ones that really cause human disease, and influenza-A is the one that can cause very severe outbreaks much more commonly than B.

DAVID MARKS, MD: How do we know which influenza type is going to be around each year?

JASON KENDLER, MD: The World Health Organization actually does active surveillance at different sites throughout the world. To try and anticipate, to try and prepare the flu vaccine for the coming season, they look at the most common serotypes of the influenza virus that they see in different cities in the world. Oftentimes the serotypes that we see at the tail-end of the season will be seen the next year. Oftentimes the serotypes that are seen in the southern hemispheres will be found in the northern hemisphere.

DAVID MARKS, MD: What do you mean by "serotype"?

JASON KENDLER, MD: The influenza virus has different proteins, different surface proteins, neuraminidase, hemagglutinins, and these proteins are on the surface of the virus. They help the virus infect cells. These serotypes, or these different proteins on the surface are recognized by the body and are the source of the immunity or immune response to fight off the virus.

ADAM STRACHER, MD: The virus is broken into influenza-A and B for the most part, and then those are also broken down into different types, so that based on these types of proteins, they'll be classified into a different type of influenza-A. And then they're also classified by where they were first discovered. So you may find influenza-A Hong Kong virus, and then another serotype designation. And that sort of tells us what type it is.

DAVID MARKS, MD: You answered my question about the different types. Basically, people from the World Health Organization or the CDC are trying to predict what's going to be coming down the road. Maybe that's why sometimes the vaccine is not as effective. At least it seems not to be as effective some years as opposed to others.

JASON KENDLER, MD: That is correct. The particular serotypes of the influenza virus that are used in the vaccine need to be, we need to come up with these six to nine months ahead of time, in order to properly develop the vaccine. So, yes, sometimes we guess wrong.

DAVID MARKS, MD: Does it start in humans, or does it start in animals?

ADAM STRACHER, MD: It starts in animals, and the circle of its development is a little bit unclear in some areas, but it is a circle between birds, pigs, and humans. It's one of the beliefs that many of these viruses start in Asia and in China, because in some of those environments, there is a very close proximity of those animals and humans together, and there does seem to be this development together of these viruses being passed from one to another.

DAVID MARKS, MD: How exactly does it pass from an animal to a human?

JASON KENDLER, MD: In general, it's through the airborne route. Influenza particles, they cause a respiratory infection through coughing, through sneezing. These particles get aerosolized and go from one to, either from an animal to a human, or from human to human.

DAVID MARKS, MD: So if you have a pet pig and you teach it to cover its snout with its hand, right? The flu has an interesting history that dates back decades and generations. Tell me some of the stories about some of the history of the flu, because it's very interesting.

ADAM STRACHER, MD: We decided I was the historian in this, so I'll take that question. It's a very old virus. There are descriptions back to outbreaks that seem to be influenza, dating back to 1100. One of the things about influenza is that it causes yearly disease, but then also it causes severe outbreaks every certain number of years, 10 or 20 or 30 years.

One of the most severe outbreaks ever, which are called pandemics, was in 1918, the so-called Spanish flu, which infected almost half the world's population, and killed about 20 million people, more people than were killed in World War I, actually. And then there have been several other pandemics throughout this century. Actually, throughout the 1900s, really. The virus itself was not discovered until the Thirties, but outbreaks are very, very classic. They can be found going back, as I said earlier, to 1100 and after that.

JASON KENDLER, MD: And the most recent pandemic was the Hong Kong flu of 1968, in which over 30,000 people in this country were killed.

DAVID MARKS, MD: Are we due for another one?

ADAM STRACHER, MD: We hope not, but that's always something that people are concerned about, and one of the reasons why the surveillance is done so closely every year, to try to predict if that's going to happen, and to try and get in front of it and have a vaccine available for it, to prevent it.

DAVID MARKS, MD: Now, let's say the vaccine is off. It's the wrong one. Are we out of luck, or are there things that we can do?

JASON KENDLER, MD: The vaccine is one means of prevention of the flu. Obviously there are other means. There are some medications, actually, that have been shown to be useful in prophylaxing people who have been exposed to the flu, or who are likely to be exposed to the flu. The first ones that came on the market were amantadine and ramantadine. More recently, there are two new products, neuraminidase inhibitors under the names of Tamiflu and Relenza.

ADAM STRACHER, MD: And also, it depends on how far off we are. If the vaccine is only a little bit off from one serotype, then it may help protect us somewhat. And obviously the common sense things like washing your hands and covering your mouth, whether you're a pig or a human, obviously that's helpful as well, to help prevent the spread.

DAVID MARKS, MD: How do these serotypes keep changing? Is it just that we're selecting out for them each year?

ADAM STRACHER, MD: Through mutations. And we select, basically. As people get immune to the different serotypes that are there, the virus mutates and changes. When it has small changes, it causes yearly outbreaks, but when it has very large changes, it causes these severe pandemics.

DAVID MARKS, MD: And hopefully this year will not be one of them. Hopefully not.


DAVID MARKS, MD: Thank you both for joining us. Thank you for watching our webcast. I'm Dr. David Marks. Goodbye.

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