More Info on Viruses

[Avalux]

I wrote this and posted it in the Science forum way back in 2005 when there was discussion of an avian flu outbreak (luckily, it didn't happen). My intention was to provide basic information on viruses - influenza A viruses and in particular bird influenza A. The virus causing this current "swine flu" outbreak is an influenza A virus that infects pigs, so the the information in my post is relevant. I'm so glad DU has a searchable Archives (thanks Skinner, et al).

There are a couple of other very informative posts in this forum so consider this an additional resource. I think it's important to understand the basic science so news stories are better put in context.

Here’s an unsolicited lesson in viruses and how they survive – particularly the influenza A virus causing the current outbreaks of avian flu. I’ll do my best to write an explanation without being too technical. Let me add a disclaimer here – no one can predict when a pandemic will occur but I do believe it is inevitable; either in a few years or as many as 50. As advanced as we humans are, we can’t stop mother nature - it's that pesky "E" word (evolution).

A virus is a parasite – it is comprised of genetic material enclosed in a protective coat. The genetic material may be DNA or RNA depending on the type of virus. The protective coat is called a capsid. Not only does it protect the genetic material inside, but the capsid also helps the virus infect host cells. Some viruses even have an additional outer envelope of proteins, sugars and lipids stolen from the host cell in which it has been made. The complete virus "particle" – genetic material, capsid and envelope (if it has one) - is called a virion.

We aren’t sure of the origins of viruses, but most virologists agree that each virus got started by copying a few useful genes from their host cells. Viruses are ignorant of any patent or copyright laws. They are brilliant at making copies out of whatever they find useful in the host’s genetic code and move on from there. The ‘useful’ gene that’s stolen is intimately involved in the host cell’s reproduction, food gathering, cell communication or other essential function. Viruses mutate the stolen genes and when a particularly useful mutation comes along, the virus uses it for it’s own survival, usually to the detriment of its host. Over many generations and years, viruses have the ability to switch over the other species, find new genes to copy and then continue evolving in their own selfish way.

The avian flu virus is an orthomyxovirus – it’s genetic material is RNA. When a virus with RNA replicates, the copies tend to have more errors than when a virus with DNA replicates. These extra errors provide mutations upon which natural selection may act. That means RNA viruses have a high mutation rate and can evolve quickly - faster than a DNA virus or any DNA cell. Over time these mutations accumulate and eventually the virus evolves into a new strain. This progressive accumulation of individual mutations is called antigenic drift. The shape of the viral protein (antigen) slowly drifts into a different shape with each generation of virus. Eventually they drift so much that the original antibody can no longer bind to it. That means a host can become infected with this newly evolved virus. All viruses show antigenic drift, but RNA viruses mutate faster so they drift faster. Antigenic drift is responsible for many of the localized outbreaks of different strains of influenza, especially influenza A and B. In practical terms, antigenic drift is the reason we need to develop a new flu vaccine every year for human influenza and why it’s never 100% effective.

Here’s where it get pretty amazing – influenza A. This virus has the ability to undergo a kind of gene swapping or genetic reassortment that other viruses do not have. If a host cell is simultaneously infected by two different strains of influenza A, the copies of the virion may contain mixtures of each parents' genes. This makes it very easy for influenza A to quickly evolve into new combinations of genes. This is called antigenic shift; a newly created virus strain with mixed genetic material that’s different from it’s parents. This phenomenon is different from antigenic drift which occurs slowly and without change in the gene associations. The new combinations are such a unique strain of virus that the immune system has to start all over to make new antibodies to combat it.

Influenza A can infect mammals other than humans including birds (avian flu). It's very unusual for a virus to have such a wide host range, but influenza A somehow manages this trick. It probably has to do with the fact that the virus gains entry to a host cell by using receptors common to many species. That means a strain of influenza A may infect one species for decades and then suddenly jump to a new species. This sudden jump, due to antigenic shift, can produce a very serious epidemic. For example, several years ago many seals washed up on the east coast of the US dying from a strain of influenza A that, until then, had only been found in birds. Horse and swine influenza A have turned up in humans. Influenza A is the nightmare of science fiction - a virus that normally causes only a slight illness, undergoes genetic recombination with other species and comes back as a very deadly virus.

We know that influenza A has been conducting random, unlicensed recombinant genetics "experiments" for centuries and will continue to do so regardless of what any of us try to do about it. We watch and wait. Avian influenza A virus has a very high mortality rate (about 75%) and that’s why the present scenario is so scary. If avian influenza A were to recombine with human influenza A within a host cell, it may give the virus the ability to pass from one human to another quite rapidly. This would cause a worldwide pandemic. We may be getting dangerously close to this reality as there were two documented cases of human to human transmission last year in Asia. Another significant change we’ve seen in bird to human transmission is the large amount of viral particles in respiratory secretions of birds where in the past the virus was only found in bird feces.

And when it comes to vaccines, there are obstacles to the rapid development of a vaccine for a pandemic outbreak. Flu vaccines are normally grown in chicken eggs, but this will not be useful because avian influenza A is deadly to the chicken embryo. The only other option will be to use ‘reverse genetics’, which involves merging selected genetic material from the natural virus with a laboratory virus, with the resulting virus stimulating an immune response, but no disease when injected into humans. This of course, is easier said than done.

I happen to respect viruses a great deal - they are amazing creatures. I ask myself frequently how a one-celled organism dependent on others for survival can be so smart – they always seem to be one step ahead of us.

My advice if an avian flu pandemic happens - stock up on supplies and stay home.

http://www.democraticunderground.com/discuss/duboard.php?az=show_topic&forum=228&topic_id=5742#6140

Please follow the link to the original post - good info in the thread too. :hi: