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Pathology of Viruses

Giovanni Maga maga at vetbio.unizh.ch
Sat Apr 8 09:02:56 EST 1995

In article <Bk28wW7.ksturts at delphi.com>, Keith Sturts <ksturts at delphi.com>

> Can anyone explain to a layman what the pathology mechanism is that
> creates 'symptoms' of virus invasion?
> In my limited pathology knowledge, I can think of two possibilities:
> a) the virus disturbs the normal metabolism of the cell
>     (perhaps this is two broad to define without delimiting)
> b) the metabolic waste of virus presence and activity overloads the cell(s)
> thus causing sick symptoms in the host.
> It is suprisingly hard to find physiological explanations as to why viruses
> effect animal life.  There is no shortage of symptomatic description,
> however...
> Thanks

Your point a is basically correct. Different mechanisms are involved in the
occurence of symptoms after virus infection. In general, symptoms are
related with the cytopatic effect that the viruses have on the cell they
infect. Since viruses are intracellular parasites, they use the cellular
metabolism in order to meet their needs (replicate their DNA, synthesize
their proteins, build-up new virions). Different viruses do it in different
ways. Herpes viruses for example have their own set of enzymes to replicate
DNA, whereas papillomaviruses rely almost entirely on the DNA replication
machinery of the cell. The most dramatic effect is obviously the breakeage
of the cell which occurs in lytic infections when the mature virions are
released out from the infected cell. This cause, for example, the symptoms
of some herpetic infections like herpes simplex or varicella-zoster, which
infect mainly epithelial cells. Since different viruses show tropism for
different cell types, the symptoms are different as well, depending on the
organ or the body's district they infect. For example, influenza viruses
have a particular tropism for the mucosa of the respiratory tract, whereas
some Adenoviruses affect the digestive tract, often causing enterites. Some
symptoms are as well due to the mechanisms of defense of the body. The
simplest case: fever, can be considered the response of the body to an
infection (mediated by molecular signals) since many viruses fail to
replicate at temperatures above 37 C degree. But also inflammation can
arise as a consequence of the action of the cell-mediated immune response.
The effect on the cell can be also much more sophysticated. Think about the
oncoviruses. In the case of papovaviruses (for example), there are
specialized viral proteins which act as transcription factors and
replication factors, antagonizing the correspondent cellular factors and
driving the cell toward malignancy. 
As far it concerns your point b, it seems to me that fits better to
bacterial infections, since both exo and endotoxins are well known to be
the major responsable of symptoms in some cases (C.botulinum, B.pertussis,
C.tetani for example or the various toxins of Staphilococci and
I apologize if this is a *very* poor list, but I think that it is not so
difficult to find out physiological explanations of the symptoms caused by
viruses (at least some of them) if you look at the right place. One place
would be a basic textbook of medical virology that you can find in almost
every university or hospital library. Then you can ask someone with a
knowledge in medical science or biological science to help you to clarify 
points that are obscure.
Regards, G.Maga PhD
Biochemistry, University of Zuerich (CH)
maga at vetbio.unizh.ch

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