virulence

ctfaulkn at utkvx.utk.edu ctfaulkn at utkvx.utk.edu
Tue Dec 20 23:07:57 EST 1994


In Article <199412202315.PAA29253 at net.bio.net>
CGE at CU.NIH.GOV writes:
>
>First, pathogenicity is NOT a relative term. An organism is either a
>pathogen or it is not.
>
	I have to disagree on this point because I accept the premise that
pathogenicity varies for particular infections as a result of host and
environmental factors, dose of the infective inocula, route of entrance of the
infection, and source of the infection.  The example given in Mausner & Bahn
(1985:268) is that staphylocci are not pathogens when located in the rectum,
but when found in the peritoneal cavity or meninges serious disease results. 
I can think of other examples of clinical parasitism where this also occurs,
for example Trichomonas gallinae generally causes no problems unless it gets in
a different host (pigeon, for example) or is able to leave the digestive tract
and invade new sites.  I don't think anyone will disagree that T.gondii has the
potential to cause disease. However, many people are infected (25 to 50 % of
the adult population, i this is an overestimate though) but few actually
develope clinical toxoplasmosis, and even fewer die.  I would have to state
that given these definition T.gondii (the average run of the mill isolate) is a
parasite of low pathogenicity and low virulence in people.  The picture is
different ,however, in kangaroos and lemurs where health animals secumb to
clinical illness with almost any isolate. 

>Second, virulence IS a relative term - the relative capacity of a
>pathogen to cause disease under a defined set of circumstances. It
>HAS been clearly defined.
>
	I agree.  Or put another way, virulence is the proportion of 'clinical
cases' resulting in severe clinical mainfestations (including sequela).  I was
in error to imply that mortality had to be a necessary term in the definition. 
The CFR (case fatality rate) is simply one way to quantify virulence.

>The mechanisms are not being addressed, only the observed effect of
>infection with a pathogen. This will be measured differently for each
>pathogen. Host mortality should not enter in to it, as many pathogens
>do not cause death. 

	I don't think Ewald is concerned about how an etiologic agent causes
disease, only the observed effect ethier is does or does not.  Concern then has
to be directed at the population rather than the individual agent-host
interaction.
	I think mortality is a useful attribute because its absolutness lends
itself to statistical analyses. Again there is no ambiguity about assigning
ones who are alive to one group and dead ones to another. Perhaps its better
suited for outbreak investigations with plague or cholera ;).

>I doubt if anyone will question that Giardia can
>cause disease of varying severity, but I am not aware of any deaths
>directly linked to infection with this pathogen.
>
	Then I would say that Giardia is a parasite with low virulence,
recognizing that its association with diarrhea and dehydration in neonates
would cause us to modify this to recognize its increased pathogenic potential
and increased viulence in this population under these particular circumstances.

>Only an organism that never causes disease should be called
>'non-pathogenic'. 

	non-pathogenic under what circumstances?  

>
>I whole-heartedly agree that this terminology needs to be used
>carefully and that individuals should define how they are using the
>terms if they deviate from those in the dictionary. But throwing out
>a word because it is often used incorrectly is not the answer. 

	I agree. We just need to develope a consenus on definitions.

>the terms are quite simply defined:
>
>Disease is a deviation from the normal condition.
>A pathogen is an organism capable of causing disease.
>Virulence is the relative capacity of a pathogen to cause disease
>under defined conditions.
>
>The biggest problem is how virulence is to be measured for a given
>organism. Mortality is just one endpoint of a spectrum. I don't agree
>that the terminology per se is problematic.

	My only problem is with pathogencity being an either or category.


**********************************
*      Charles T. Faulkner       *   When you don't know where you're
*  Univ of Tennessee, Knoxville  *   going any road will take you there.
*   (ctfaulkn at utkvx.utk.edu)     *                            Alice
*********************************                                




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