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Graham Shepherd muhero.nospam at globalnet.co.uk
Sat Mar 22 07:35:56 EST 2003

"Joanne" <joann3remove at remove.tm.net,my> wrote in message
news:3e7bee09_1 at news.tm.net.my...
> no. i'm sorry. i'm still very confused on this. i'm not very good at
> microbiology, but its a prerequisite in my course. i surfed the net just
> to find out how they would describe the pathogenecity of a virus, and i
> something like this:
> "PATHOGENICITY: Infection, occurs mostly in children, causing mild,
> nonfebrile, viral disease with erythematous eruption characterized by a
> striking erythema of the cheeks and a lacy red rash on the trunk and
> symptoms resolve in 7-10 days; 25% of the infections are asymptomatic;
> severe complications are unusual, but anaemic patients may develop
> aplastic crisis; intrauterine infection may cause fetal anaemia; severe
> anaemia in the immunosuppressed; protection conferred with development of
> antibodies. "
> i had an idea that it was supposed to be something like salmonella having
> exotoxins and sideropores to invade the host, or e.coli having
> that can deregulate cyclic nucleotide activity, as all this contribute to
> the fact that the bacteria is able to produce the disease in an animal.
> according to the description here, it means i have to describe the
> signs produced by the particular pathogen. so i am very confused.
> but thanks for replying anyway.


It's a very big subject, and your own thoughts and everything you've been
told in this thread are relevant - but to break it down to fundamentals:

Pathogenicity describes a relationship between a host and an infecting
agent, eg a virus.
There are several possibilities:

(1)    A virus cannot infect a host -  eg I don't think humans can be
infected with tobacco mosaic virus. So this virus is not pathogenic for
humans. It is pathogenic for tobacco plants.

(2)    A virus can infect a host but causes no signs of infection. This is
normally called a subclinical infection. Eg Epstein Barr virus in humans in
many cases does not produce an illness. (When it does, it mainly causes
glandular fever).

(3)    A virus can infect a host and cause signs of infection. This is
called a clinical infection. Viruses like measles, mumps, herpes usually
produce clinical infection.

All of the above apply to any infecting agent - virus, bacteria, fungi,
protozoa etc.

The following does not generally apply to viruses:

An organism (eg bacteria) can live in close association with a host and
cause no damaging side effects; and in many cases the association is
beneficial. The bacteria that live in your mouth and throat help to exclude
potential pathogens (eg Candida) from infecting you. The bacteria that live
in your gut can provide essential nutrients.

When an organism is pathogenic for a host, the mechanisms of pathogenicity
can include toxin production, attachment organelles and many other specific
factors that are involved in the interaction between host and invader. The
mechanisms of host resistance are also involved and a lot of the clinical
signs, eg fever, are part of the resistance mechanism.

You will come across the term "virulence factor" at some point. Depending on
how you view the ecology of the host and the invader, this term may or may
not have meaning. It is important to remember that many "pathogens" are
organisms that exist naturally in specific environments in essentially
non-damaging associations (eg soil bacteria) and only cause infections when
introduced into a host (human or animal). Medical microbiologists focus on
the damaging relationship and interpret the organism's characteristics in
that light. From the evolutionary point of view, something that a medical
microbiologist classifies as a "virulence factor" may have an entirely
different role in the organism's normal environment. It's important to avoid
the teleological flaw that "this organism evolved a virulence factor to
enable it to damage a host". Most well-evolved parasites do little damage to
their host - you don't burn down the house you live in, do you?


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