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The bird virus is called H1N5. Does anyone know what the numbers and letters mean? Why does a virus attack only certain species, like this one only birds? Are our cells not very similar in function? Thanks
Question Date: 2005-10-20
Answer 1:

One of my students last year was really interested in the "bird flu" ("avian influenza", H1N5), and wrote his senior thesis on it, so I know something about it. H1N5 is a strain of the influenza virus (or "flu" for short). As you know, the influenza virus can infect humans, but there are MANY strains of this virus. There are multiple strains that can infect humans, but there are also strains that infect birds (chickens, ducks, turkeys), pigs, etc. The "flu shot", which changes every year, is meant to be a vaccine for the three influenza strains that scientists think will be the most common or dangerous that year. It is a guess based on data from doctors and scientists that monitor influenza infections around the world.

There are two types of influenza that can cause epidemics in humans: Influenza A and Influenza B.

Influenza A is further divided into subtypes based on two antigens (molecules that stick up on the surface of the virus that your body's immune system uses to recognize and destroy the virus particles). One antigen is called hemagglutinin (the H part of H1N5) and one is called neuraminidase (the N part of H1N5). To date, there are 16 different H types and 9 different N types! Influenza B is not divided into subtypes.

Scientists that study viruses ("virologists") believe that, in general, viruses tend to infect only one species of organism. That is, viruses are usually species-specific. Influenza is an exception. Almost every organism is known to be vulnerable to viruses (there are exceptions), so that means there are viruses that only infect certain species of bacteria, viruses that only infect certain species of plants, etc. This makes them very diverse. We, as humans, have our own unique set of viruses. Occasionally, a virus will mutate and be able to jump from its usual host to another host (e.g. from chickens to humans). This is what happened with HIV. What started out as SIV (Simian Immunodeficiency Virus) jumped from monkeys to humans, and has mutated into two different HIV (or Human Immunodeficiency Virus) strains, HIV-1 and HIV-2 that are transmissible by human-to-human contact. Virologists can tell this by comparing the genetic code of the SIV and HIV viruses.

The ability of a virus to jump from animals to humans is affected by the similarity of the animal's immune system to humans (monkeys and humans are pretty similar) and their degree of contact with humans (humans can get more viruses from domesticated animals than wild animals, e.g.). The same thing that happened with SIV is currently happening with H1N5. This virus attacks several species of birds, but can also "jump" to humans. So far, the virus hasn't mutated enough for it to be transmissible from human to human, which means you can only get it by handling sick or dead birds, or by eating their meat. If it does mutate enough so that it becomes a new virus specific to humans and transmissible between humans, then it will be very dangerous.

To date, those that have contracted H1N5 are people working with sick chickens (poultry farmers) or people that eat sick chickens. Human cases have largely been confined to Asia (9 different countries) and, most recently, Europe (Turkey - no pun intended - and Romania). What makes H1N5 special, and what has everyone worried, is that when H1N5 does infect humans, it has a high mortality rate. Also, it is known to infect wildfowl (ducks, e.g.) without any symptoms, so a duck could visit a poultry farm, pick up the virus, then fly to other poultry farms and spread the virus to uninfected chickens. Ducks are migratory, so could be a real problem.

Scientists are scrambling to control the virus in countries where outbreaks are common, in the hope that if we keep the number of infected people down, the chance of the virus mutating and becoming a new, deadly influenza strain in humans will be small. Some scientists are comparing this new strain of influenza to the strain that killed so many people in 1916. That particular strain was very lethal for young, healthy people with good immune systems (the strain basically used the person's own immune system to them off). Young people in their 20's were literally dropping dead in the streets. We knew much less about medicine back then, so hopefully this new influenza strain won't get that bad if it does manage to become transmissible from human-to-human contact. Cross your fingers.

Check out what the US Centers for Disease Control and Prevention (CDC) has to say about flu influenza and avian influenza.

Answer 2:

The bird flu virus is known as an avian influenza type A virus, which means that it is a flu virus that infects birds. The designation of Type A indicates that it is the kind of virus that can cause a pandemic. The problem is that there may be many different kinds of avian Type A flu viruses that infect birds! To solve this problem, when a virus is discovered it is usually named according to particular unique characteristics of the strain. Thus H1N5 refers to some particular characteristics of the proteins that are in this virus: H5 stands for hemagglutinin type 5, which is a protein that helps the virus stick to the cell it wants to infect, and N1 stands for neurominidase type 1, which a protein involved in helping the virus break open an infected cell to spread more viruses.

So, if a scientist comes across an avian flu virus, he can do some biochemical tests to determine what type of hemagglutinin and neurominidase proteins it has to determine if it is the H1N5 flu.

>In order for a virus to infect a cell, it must be able to attach itself to the outside of the cell. In order to do this it has some little sticky proteins on its surface. However, these proteins don't just stick to anything. They have to match up with another kind of protein on the surface of the cell that they can then stick to. If the cell doesn't have those matching proteins, the virus cant stick to the cell and infect it. After the virus has managed to stick to the cell, it also has to inject its own RNA or DNA into the cell, and then it has to take over the cell. If the cell isn't the right kind of cell, the proteins of the virus won't be able to take over the cell. Even though birds and humans have similar cells, it may be that human cells just don't have the right sticky proteins and other parts for the virus.

You can think of it like a jigsaw puzzle: when you look at all the pieces of the puzzle in the box, they all look similar and work the same. However, if the parts of two pieces don't match exactly right, they won't fit together! If the virus parts and the human cell parts aren't exactly right, the virus cant infect the cell.

Did you know that your own body has a similar system in it? Your immune system works in much the same way. There are special cells traveling around inside your body that look for invaders like bacteria and viruses. If the cells find an invader, they will attack and destroy it! But how does your immune system recognize a bad invader from a normal cell that belongs there? It looks for strange, unfamiliar proteins on the cells! If it encounters a cell with some weird proteins it hasn't seen before, then chances are that is one that is an invader that doesn't belong there!

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