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Tue Dec 20 18:15:59 EST 1994

In Article <3d4qkt$h8a at martha.utk.edu> ctfaulkn at UTKVX.UTCC.UTK.EDU

-When this discussion began I made a point of looking up the
-distiniction between pathogencity and virulence in Mausner and Baum's
-"Epidemiology", which I believe most regard as the functional
-equivalent of "Clinical Parasitology", or Manson's "Tropical
-Medicine", pathogenicity is the ability of an etiologic agent to
-induce clinical disease in a host, virulence is mortality resulting
-from cases of clinical disease. Virulence can be expressed as the
-Case Fatality Rate. As all of us know, simple infection with a
-parasite is not synonomus with clinical disease so we distinquish
-between infections which are asymptomatic and those which result in

and in Article <01HKTXLOIVXY95N6SI at CBE.AB.CA> Derek A. Zelmer writes:

-The potential of a parasite to cause harm is its pathogenicity.
-The damage caused is defined as pathology. Parasite induced host
-mortality and basic reproductive rate require no further definition
-or labeling. The same degree of host harm can come about through two
-very seperate mechanisms, either by a slowly reproducing, highly
-pathenopgenic parasite, or a relatively non-pathenogenic parasite
-with a high reproductive rate. Both mechanisms are very different,
-and would result from very different evolutionary and ecological
-mechanisms. Grouping all phenomena resulting in host harm under the
-blanket of virulence does nothing more than cloud the analysis of
-potential selective factors and evolutionary mechanisms.

I have several problems with the definitions used above.

First, pathogenicity is NOT a relative term. An organism is either a
pathogen or it is not.

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.

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 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.

Only an organism that never causes disease should be called
'non-pathogenic'. An infection with a pathogen that is asymptomatic
and shows no pathological changes indicates that in *this infection*
the organism is avirulent.

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. As far
as I am concerned 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.

C. Graham Clark, Ph.D.
Laboratory of Parasitic Diseases,
National Institutes of Health,
Bethesda, MD 20892

Ph.: 301-496-4740
FAX: 301-402-4941
e-mail: cge at cu.nih.gov

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