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What happens to viruses outside the host?

  1. Oct 27, 2014 #1

    anorlunda

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    I read that viruses outside the host remain infectious only for a short time; hours or days. What happens to them? How are they damaged?

    A second related question. All living organisms require an external source of energy to survive (correct?) The words live and die are hard to apply to viruses, but do viruses need external energy to "survive" ?
     
  2. jcsd
  3. Oct 27, 2014 #2
    I've read some viruses where able to survive for many years in a frozen enviroment and then become infectious again. I guess this depends on a sort of virus (but I'm not expert)
     
  4. Oct 28, 2014 #3
    Most viruses live [sic!] in water, and I have no idea how longlived they are. It could depend on how easy it is for them to travel from host to host.

    On land, destruction seems to be the case. Flu viruses are airborne, and most such viruses (flu, ebola) lasts for about 15 minutes. I would assume the sensitive mechanism they depend on to infect the host, which is usually fueled by immense internal pressures to inject the virus genome out of the virus capsid into cells, is destroyed.

    Those destroying mechanisms could be either moist (flu virus doesn't transmit well in winter, and I have read this as a hypothesis why) or absence of moist (i.e. maybe longer lived viruses dry out and lose that inner tension). Unfortunately, seeing how vital it could be for combating virus disease, I don't think this is well researched.

    "Life" is seldom a simple or useful definition.

    More generally, "life" could be the process of evolution (the process of all life), and then its objects are populations rather than individuals. Under such a definition, viruses are life, while the last isolated sexual animal could be "residual life" or "non-life" or "rather extinct" (no partners).

    If you use "life" as a means to identify individuals, you get something like the NASA definition for astrobiological search: an evolving, metabolizing organism. E.g. cellular organisms. And yes, such life need energy to metabolize. In fact, it may be the other way around, it can be free energy that once organized metabolism. (See "battery" or "alkaline hydrothermal emergence" theories for emergence of life.)

    Secretly viruses needs energy too. They are parasites on cells, and use their metabolism for procreation. The best way to look at a virus is perhaps that the viroid (encapsulated virus) is the infectious inert spore, and the "molecular organism" that injects/insert and inhabits an infected cell is the living, metabolizing, adult parasite stage. It even works out if you see the inner pressure injection mechanism of phages as metabolic derived potential energy.

    Some viruses form more or less cell-like "viral factories" instead of distributed "molecular organisms" inside the infected cell. Since some, or perhaps all, viruses seems to originate as parasitic cellular organisms* that as parasites can undergo simplification of "body plan" but complexification of "life cycle", that shouldn't surprise us.

    *Even the capsid proteins hint at this. They root in cellular membrane protein pore complexes, exactly what you would expect to see kept but modified in parasites that insert (break down cell walls with excreted muralytic enzymes) or inject (use something like similar bacterial pore constructs) stuff into cells.
     
    Last edited: Oct 28, 2014
  5. Oct 28, 2014 #4
    I suppose a virus can be considered "alive" so long as it is not damaged and still has the potential to infect a host.
    Different viruses have different vulnerabilities.
    Here's a good reference for Ebola (with ample citations): http://www.phac-aspc.gc.ca/lab-bio/res/psds-ftss/ebola-eng.php
    Quoting from that document:
    The next section titles "Survival Outside Host" may be particularly interesting to you. It's about 4 times longer that the section I quoted.
     
  6. Oct 29, 2014 #5
    .Scott, thanks for that reference! Apparently I got my numbers from an unreliable source (my memory).:s
     
  7. Oct 29, 2014 #6

    anorlunda

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    My original question had more to do with what happens at the molecular level. I ask that because I have come to think of viruses as elaborate organic molecules rather than "organisms".

    Suppose a virus needs to be hydrated to "survive". When exposed to air, is it exposure to O2, or N, or UV radiation, or something else that damages the "virus molecule" ? I'm looking for a physical chemistry answer.

    Sorry, I struggle to find proper nomenclature in this context.
     
  8. Oct 29, 2014 #7
    Coincidently, this just showed up on phys.org this morning: http://phys.org/news/2014-10-scientists-resurrect-year-old-viruses.html#ajTabs

    However, I don't think that this would quite qualify as a virus "surviving". The description in the article suggests that the scientists used RNA from one virus and DNA from another to construct the whole genomes - and then "resurrected" the DNA one based on the genome. So they may have been dealing with multiple viral fragments - each incomplete.
     
  9. Oct 29, 2014 #8
    I don't think hydration itself is an issue, but oxidation certainly is. Here is an article:http://www.ncbi.nlm.nih.gov/pubmed/22718628
     
  10. Oct 30, 2014 #9
    Ah. Let us drop the problem of infectious in practice (maybe flu viruses can be "resurrected" in the right humidity) then.

    I also realized later that the viruses that insert themselves in the host, or the viruses that trick eukaryote host cells to engulf them as potential food or as defense (in (in amoeba phenotypes such as our immune defense phagocytes) that I forgot to mention, can potentially free their innards by capsid disassembly. I know too little about viruses to know. (My interest is astrobiological, so viruses as potential remnants of and constraints on cellular life.) So we can drop the internal pressure issues too.

    Remains other structural and all kinds of molecular damage. RNA heteropolymers hydrolyze with a halflife of about 4 years. Double-stranded RNA viruses and RNA virus capsids may slow that down, but at a guess not by an order of magntitude.

    DNA heteropolymers have hydrolysation - water molecules breaking them apart - halflifes of millions of years, part of the reason we have them is their stability. But I think people have found that in practice (mummies) chromosomes destructs with halflifes of 100s of years. (So not in conflict with Scottt's 700 year virus genome reference, as there could be non-hydrolyzed pieces left.)

    Cell's have O2 (OO) stress enzymes that protects them, that are rooted in NO stress defense molecules - the atmosphere before it oxygenated produced NO at low levels (CO2 and NH3 around volcanoes would do that), an even more potent oxidant than O2 - but most or all viruses have not. So that is a potential destruct mechanism as noted.

    Potentially a virus DNA/RNA could happen to be repaired by some host's repair mechanisms before the eventual resulting RNA strands would be recognized as foreign (as I remember it the bacterial/archaeal DICER like immune defense works on RNA), but I doubt that is common.
     
  11. Nov 28, 2014 #10
    Since there is usually at least some amount of water vapor in air, virus particles in the general environment might well retain closely bound water molecules required to stay folded correctly. That would be a function of the actual partial water vapor pressure and the non-covalent binding constant for water molecules on the surface of viral capsid proteins. I have no idea whether this is part of the issue. Viruses with phospholipid membranes may be more sensitive to dry environments.
     
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