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Peter W Pappas ppappas at magnus.acs.ohio-state.edu
Wed Dec 21 06:33:18 EST 1994

In article <199412202315.PAA29253 at net.bio.net>,  <CGE at CU.NIH.GOV> wrote:
>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

I agree with Graham that "throwing out" virulence is not appropriate - we just 
need to me careful on how it's defined.  Moreover, virulence is one parameter 
that is important (perhaps most important) in determining cost:benefit ratios 
for programs designed to control or eradiacate parasitic diseases.  This is 
something I try to impart upon my parasitology students when they ask WHY there
are no programs to control disease X, Y, or Z in country A, B or C.  The answer
often boils to simple economics -- considering the virulence, would a program 
to control or eradicate that disease be cost-effective (considering the 
economic and human resources at hand)?
Peter W. Pappas, Professor/Chairperson, Department of Zoology,
The Ohio State University, Columbus, OH  43210  USA
E-mail: pappas.3 at osu.edu; FAX (614)-292-2030,
PHONE (614)-292-8088

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