Naked Science Forum
Life Sciences => Cells, Microbes & Viruses => Topic started by: j9sully on 20/04/2004 18:21:19
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I know that all viruses act in different ways when they enter the body but if asked the above question what would be a general answer to it??
[:I]
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Hi J9sully, in my ever decreasing withering knowledge base, I understand that a virus is a tiny itsy bitsy organism or entity that infects or infests the cells of other animals or plants and replicates itself much to the hosts cells utter annoyance !!! which is how they cause a disease !!...I think !!...err...is that 'general' enough ?.....cos, it's about as complicated as I can get !!!(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.world-of-smilies.de%2Fhtml%2Fimages%2Fsmilies%2Fkrank%2Fkrank01.gif&hash=09b79fe96b30f6dc524607b997c73953)
Don't you worry though, all the lovely nice scientific/medical etc etc bods here will give you the correct answer soon enough.[:)]
'Men are the same as women...just inside out !'
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I don't know much, and I hope Chris will correct me if I'm wrong, but I'm going to stab at this one.
A virus enters a cell and hijacks the RNA to make copies of the virus (instead of the normal cell proteins and such). After a while, the cell is full of new virus particles and literally bursts, thus sending thousands more out to infect other cells. I don't know the details.
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John - The Eternal Pessimist.
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A good place to start would be to have a listen to the interview we did with virologist Dr. Stacey Efstathiou from Cambridge University on the subject of "How do viruses cause disease" :
Here's the link : http://www.thenakedscientists.com/html/shows/2002.05.26.htm
I'll write more about this later.
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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Thanks all for your quick replies, you've been fantastic help, although i am only part way through the assignment so i may be back for more!!!!
And thanks chris for the link, just about to have a listen now!!
J9.xx
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There are all different types of viruses and they each affect the body in all different ways. A general summary though is that they get into a cell and -when they actually cause illness or disease -subvert the normal functioning of the cell. They can cause the cell to die; to divide too much and potentially become cancerous; to interfere with the specialised function of the cell (eg cause heart cells to contract irregularly during a myocardial infection); or to do something it isn't meant to do. Or they may not bother the cells at all.
Unique genes code for different proteins, and these proteins are made (translated) using information from genes. Viruses don't have their own machinery for translating proteins. By a variety of different methods, when viruses enter cells, they hijack the cells translation machinery and force it to translate the viruses own genes. This allows for more viral particles to be made which can go on to infect other cells in the same human, other humans or other animals. At times, viruses are content to sit within a cell without taking it over. This is called 'latency'. Some viruses end up being latent throughout their lifespans. Others such as HIV and Ebstein-Barr Virus (EBV -which causes mononucleosis among other illnesses) alternate between latency and a 'lytic phase' when viral particles are actively being made.
Viruses of course do not only infect mammals. They can potentially infect any organism that has machinery to translate viral proteins from viral genes, including plants and bacteria.
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quote:
Originally posted by dalya
There are all different types of viruses and they each affect the body in all different ways. A general summary though is that they get into a cell and -when they actually cause illness or disease -subvert the normal functioning of the cell. They can cause the cell to die; to divide too much and potentially become cancerous; to interfere with the specialised function of the cell (eg cause heart cells to contract irregularly during a myocardial infection); or to do something it isn't meant to do. Or they may not bother the cells at all.
I have a thing on my mind. In case of heart cells, don`t you think that it is an inflamatory reaction that causes trouble, at least in some cases? I don`t realy know, i should study about it in a few months.
One more thing. Are you maybe a doctor or medical student ?
Thanks in advance.
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It could very well be the inflammation response. There's a kind of viral meningitis where the virus gets into the meningal cells but doesn't do very much harm, just sits in the cell, occasionally cranking out a copy or two...until the immune system kicks in and causes inflammation response and Tk cells start hosing down infected cells with cytotoxins. Then you develop memingitis and quite likely die unless you get a dose of anti-inflammatories quickly.
Most viruses cause illness almost exclusively through outright cellular damage and the resulting inflammation response. The preferred portal of entry often dictates what types of cells get affected and what symptoms are developed. For example, influenza...the aches are from damaged muscle and joint tissue, the coughing from damaged lung cells, and the runny nose/congestion from response to damaged muscous membrane cells. There are of course exceptions and slower-acting viruses.
It may be interesting to note that some pathogenic bacteria don't produce toxins unless they are infected with a prophage. (a lysogenic/latent virus hiding in its genome) Botulism, scarlet fever, and diphtheria are all examples of this.
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I completly agree!
Very thoroughly explanation, thanks !
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You mention viral meningitis. I just wanted to clarify this a bit. Viral meningitis is common. There are many viral infections capable of initiating meningitis, but they are virtually never fatal.
The infection is usually self-limiting and responds well to symptomatic treatments and rest. It tends to be noticed because the symptoms can mimick those of the far more serious bacterial meningitis.
Bacterial meningitis is more frequently fatal, and the condition for which we must be vigilant. Gratifyingly, the incidence is now much lower owing to the effectiveness of the vaccination programme introduced in the last few years targeting meningococcal strain C (which accounted for the bulk of meningitis cases amongst young people).
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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I'm surprised nobody has taken it into their concern to mention lysis and lyonization. Viewed generally, viruses mainly destroy a cell. Plain and simple. Diseases occur from the lack of activity or contribution for which that particular cell is responsible. It infects in two ways...
Lysis is the most basic way a virus mercilessly kills a cell. The virus attaches to a particular cell specific to special cell receptors and injects its viral genome into the cell. The gene then (for lack of a better word) merges with the present DNA and encodes instructions on creating the viral coats and DNA. The viruses assemble in the cell and eventually break the cell, releasing more viruses into the environment. The lysogenic pathway is a dervative of the lytic cycle. When the viral genome is injected into the hapless cell then it recombines (oh my god there's that better word) with the already present gene and remains dormant (for lack of a better word :/ ). The cell undergoes mitosis but still retains the viral DNA and information. Eventually the cell returns to the lytic pathway and creates its viral coats and stuff and realising itself back into the environment.
Viruses inhibit normal cell production. That's how people get sick.
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Do all viruses cause disease?
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quote:
Originally posted by alex
Do all viruses cause disease?
No.
As an extreme case, trere are many people HIV positive, but not having symptoms of AIDS.
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Rokit Man,
Do you think these people are going to get the disease eventually, or they'll just stay HIV positive to death of other causes? I've heard about these people, but how long have any of them been HIV positive yet asymptomatic of AIDS?
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The individuals you refer to, Bezoar, are so-called 'chronic non-progressors'. These individuals test positive for HIV yet do not seems to succumb to AIDS. Interestingly precisely the same phenomenon is seen in Monkeys infected with the simian version of HIV (called SIV). They seem to live with the virus yet never become unwell.
HIV is a recent human phenomenon. The virus jumped into people from Chimpanzees (which harbour the closest genetic relative of HIV) at some point over the last 100 years, but its primate ancestor, SIV, is found throughout the monkey kingdom, arguing that it has been there a long time and that these monkeys are evolutionarily well-adapted to its presence. It is tempting to suggest that, over time, a population of monkeys akin to human chronic non-progressors has been selected and hence they do not become unwell. Left to its own devices, HIV would probably do the same to human population. Indeed, the Black Death (for which traditionally the bacterium Yersinia pestis has been blamed) we now believe was the result of a vicious virus. But it didn't kill everyone. Some people carried a genetic mutation in their chemokine receptor gene (CCR5-delta-32) which seemed to protect them. By a strange twist of fate, precisely the same mutation is now protecting the modern-day descendents of those Black Death survivors from HIV.
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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so when you get a cold, is the tickly itching feeling you get in your throat and lungs basically the feeling of your cells gradually beign destroyed, one by one?
also, as far as i can see, no-one has mentioned how the virus gets into cells. I think the bacteriophage is supposed to 'inject' its DNA (or is it RNA?) into the cell, but also that others don't do this. there must be some kind of recognition, so does the protein coat bind to extracellular protein receptors or something?
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When you are infected with a bad cold, or the flu, even though the infection is confined to the respiratory tract, it is not unusual to develop systemic symptoms (aches, pains, lethargy, headache, fever, loss of appetite) as well as local symptoms (sore throat, blocked nose, copious nasal secretions, cough).
The systemic symptoms are a side effect of the immune system gearing up to eliminate the virus responsible for causing the problem. By making you feel unwell, the immune system essentially forces you to take things easy to help speed up recovery.
The local symptoms are due to viral replication, cell death and inflammation in the (usually upper) respiratory tract. When cold-causing viruses (like paraflu, rhinoviruses, coronaviruses or RSV) get into these cells they turn them into viral factories which churn out millions of new viral particles which you then cough, blow or sneeze out into the air where they lie in wait for the next susceptible victim. Cells hijacked in this way inevitably die or are destroyed by the immune system. When they die, the virus-laden shrapnel left behind is a potent stimulus for the immune system which rushes in and produces more pro-inflammatory factors. These produce nasal stuffiness, sore throat (by aggravating local nerves), swelling and discomfort within the affected area.
If you are really unlucky, the damage done by the virus (and the immune system's efforts to remove it) to the protective mucosal surfaces can make you susceptible to opportunistic bacterial 'superinfections'. This is a common consequence of flu infection which can precipitate otitis media (middle ear infections) in children, and even bacterial pneumonia (often caused by Staphylococcus aureus).
Superinfections like these may require a course of antibiotics. But as an advertising campaign currently running in Australia says "Common Colds need Common Sense...not antibiotics" so wrap up warm, take regular paracetamol and try not to share your virus with anyone else - i.e. don't feel obliged to visit the doctor just for some reassurance and, in the process, give the infection to everyone else at the surgery (including the doctor) !
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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OK, some people are infected with a virus but don't succumb to illness, so their viruses are presumably dormant, or possibly even destroyed by the immune system, but are there any viruses that are actually a positive thing? like symbionts, good bacteria in the gut etc?
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Absolutely - bacteriophages - viruses that prey specifically upon bacteria - are currently being (re-)investigated as potential alternatives to our present (dwindling) stocks of antibiotics.
Here are some useful phage-therapy references :
Scientists use phages to protect a mouse model of infection :
http://www.thenakedscientists.com/html/shows/2002.01.20.htm#2
Scientists 'reawaken' dormant phages from with the genome of superbug MRSA for therapeutic use :
http://www.thenakedscientists.com/html/shows/2004.01.11.htm#1
And a previous thread in which we discussed phage therapy in some detail :
http://www.thenakedscientists.com/forum/topic.asp?topic_id=706
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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Much disease due to infection is caused by inflammatory responses induced by the immune response.
Peter C. Sayles
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quote:
Originally posted by Rokitansky
I have a thing on my mind. In case of heart cells, don`t you think that it is an inflamatory reaction that causes trouble, at least in some cases? I don`t realy know, i should study about it in a few months.
One more thing. Are you maybe a doctor or medical student ?
Thanks in advance.
My own feeling is that there are very many cases of disease, particularly viral disease, where the infectious agent is totally harmless, but it is only the host response that is damaging.
This would make some evolutionary sense. From the point of view of a virus, it wants to be able to use the host to reproduce, and a dead host is not going to reproduce viruses. On the other hand, a host is trying to maintain the purity of its DNA, and does not want to be reproducing alien DNA; so it is logical for a host to try and destroy infected cells as a means of quarantining the alien intruder. In the extreme case, it might even be valid for an entire multicellular organism to commit suicide in order to protect its children and siblings from infection.
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Virus take up the control over protein formation machinery of human body cells once inside and these protein machinery now instead of forming human proteins produce viral proteins. Thus viral proteins formed unite to form a new viral particle. The human dead cell now busts and releases new viral particles.
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How viruses cause disease is an interesting and complex topic
Viral virulence depends on many factors and you you cannot readily compare virulence of different viruses as symptoms can vary so much. For example you cannot say that rabies is more virulent than, Ebola or vice versa because the way you measure virulence in the two is different, attempts to say which is the most virulent virus are very
problematic.
Some viruses, enter at mucosal surfaces and replicate in the epithelium.
Some viruses like HPV stay there and cause disease there, but others like spread
somewhere else like the CNS or the kidney where they cause disease.
One expression of viral virulence is tissue damage. Some viruses are cytopathic, they kill cells, they take over the host cell molecular processes like nucleic acid synthesis and protein synthesis that probably contributes to killing cells. Damaged cells may undergo apoptosis or may be detected by the immune system that sends killer cells to take them out.
Some viruses lead to syncytium formation, that is multiple cell fusions resulting in
large multi-nucleated cells.
In many non-cytolytic virus infections, there is not a lot of virus induced tissue damage.
Rhinoviruses which cause around half of all the common colds, there is not much direct viral tissue damage observed, the virus really doesn't seem to kill the cells.
So the pathology of the infection is due to something else, that is your immune response to virus infection.
This is known as immunopathology, you need an immune response to clear the infection, but you pay a price in clinical symptoms, fever, tissue damage, aches, pains, nausea.
The virus itself is not damaging you so you feel pain, it's your immune response, your cytokines that cause your fever and the nausea.
One of the reasons suspected for the seriousness of avian flu is that birds have a naturally higher body temperature so the virus comes pre adapted to cope with the temperatures of our fever response, when the immune system continues to detect virus more cytokines are produced. This cytokine cascade also triggers rapidly proliferating and highly activated T-cells or natural killer (NK) cells that kill infected cells. If uncontrolled the damage produced by the immune response may be fatal.
http://www.ncbi.nlm.nih.gov/pubmed/1883518?dopt=Abstract&holding=npg
http://www.ncbi.nlm.nih.gov/pubmed/2364437?dopt=Abstract&holding=npg
http://www.ncbi.nlm.nih.gov/pubmed/12480361?dopt=Abstract&holding=npg
http://chemport.cas.org/cgi-bin/sdcgi?APP=ftslink&action=reflink&origin=npg&version=1.0&coi=1:STN:280:DC%2BD2Mnotlygtg%3D%3D&pissn=0818-9641&pyear=2007&md5=e1805470d7fe1b4e173c03b197df9e5d
There is a great deal more to this and it would probably take several volumes to cover it in depth.