http://www.tigblog.org/group/health
As we are approaching winter (in the Northern hemisphere), we are about to hear about people catching the flu. And of course with influenza being discussed, we all remember the Avian Flu that has scared the whole world in 2005 and early 2006.
Let's first see what the word influenza means because it's interesting. The name comes from Italian and it means 'influence' because it was thought in the past that the disease was caused or "influenced" by stars. Anyway, the disease is caused by a virus and I guess everybody knows that. But why do you think you get influenza several times in your life and there's no such thing as permanent immunity against it? And why does the virus get monstrous sometimes and cause pandemics (appears in several countries simultaneously)?
The answer to that lies in the unique structure that this organism has which enables it to shift and drift. But what does that mean?
First let's see what "shifting" is but before that we need to know one thing or two about that virus.
Each virus is an entity by itself, so it has to do its life on its own. Have you ever wondered how a virus can "see" its host from the trillions of other cells it comes across. Each virus 'sees' around by what's called receptors, and in case of influenza, the 2 important types are called H and N (of course!). These receptors are the sensors of any cell in all God's creatures and they act like eyes and ears (in humans an estimated 100,000,000 receptors on average are to be found on each cell!!). A close-up to the virus would look a ball with plenty of protrusion coming out of it.
And the importance of presence of these receptor on a virus to the host, be it man, birds or any animal, is that they are what the immunity recognises and builds defenses against. (You can think of immunity's work as blinding the virus).
So, what does that all mean? It has been found that receptor type H isn't one shape (about 15) and N too isn't (about 9) and the combination of one type of H and another of N makes a 'strain' of virus. Again, so what? Well, the virus has a very special feature that makes this work in a nasty way. Imagine that there's a bird that has flu from type H5N3. Nearby, there's a man who has flu from type H2N1. Neither of both can transmit its virus to the other species. Here comes the role of pigs. Sometimes they call pigs "mixing vessels" as they can catch the flu from both humans and birds and then the magic starts. Influenza virus (as all viruses and even bacteria) has small amount of genetic material, so typically, this amount just form a single lump in the middle of a virus or bacteria. But the flu isn't so. It kinda mimics higher animals by having its genes divided into 8 separate segments (as if they're chromosomes). And the very important part is that the gene for making H lies in a separate segment from that making N and this makes a problem!
Let's get back to the pig thing we mentioned a while ago. So, a man has an "animal farm" where pigs are raised close to chicks. The man has H2N1 flu and the chick has H5N3. A pig catches the flu from both and viruses from both types infect the same cell (remember - mixing vessels). This all that should have happened to have H5N1 emerging as the rest of the process is completed by the normal life style of the virus.
We said that a cell is infected by 2 types of flu. As we know, viruses get their genome inside a cell and starts dividing (refer to earlier posts in the group if you wish to know more about the life cycle). And each virus has 8 pieces of DNA so a sort of mix-up occurs and scientists have coined a name for that (and boy they like to do that) they called it "genetic reassortment". So after a mix-up happens new viruses emerge. And from the example we said we could get H5N1 or H2N3 or whatever other combination.
This process is how pandemics occur in influenza. Every 20 or 30 years a new - completely new- strain comes up infecting thousands of people worldwide. And this is why it's called 'shifting' as if a virus is shifting to a whole new set of weapons. The last of these pandemics was the Avian Flu or H5N1 that emerged in Hong Kong in 1997.
And now let's shift to drifting (which mean slow and gradual moving). The virus's genetic code has a feature that makes the code changeable. In almost all living things (except for few viruses) there are 2 copies of their code in each cell which preserves the code unchanged, but with influenza virus the case is different. There's only one copy in each virus. In addition, influenza viruses are lousy copiers, that's to say, they often make mistakes in copying their code during division or multiplication. "So what?", you say. "Their code gets distorted each time a virus divide, so they get weaker by the time, that's awesome!" In fact, the absolute opposite happens as they kinda change their skin (or weapons) so that they evade your immunity. So the nice work of your immunity during your last influenza infection gets useless as your are now virtually facing a new enemy. And from here comes the name, drifting..the virus always drift from its original shape to another for the sake of it livelihood. This drifting still enable the cell to recognise their preys but immunity does not which means you'll have to suffer from the flu till your immunity tailors a new plan to face the danger.
Hope you enjoyed it!
