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virulence and pathogenicity

Thu Dec 29 15:20:05 EST 1994

joaccigh at w203zrz.zrz.tu-berlin.de (Joachim Dagg) asks for
clarification of my use of terminology in the pathogenicity/
virulence debate.

I use the term pathogen to describe any organism that has the
capacity to cause disease. Thus, not all parasites are pathogens
to my mind - organisms like Endolimax nana and Entamoeba coli,
to which no disease has been ascribed, I consider nonpathogenic

I use the term virulence to describe the *relative* capacity of a
pathogen to cause disease. Virulence can be measured in many ways
and ranges from fatality at one extreme to avirulent at the other.

One of the major differences between the usages of Charles Faulkner
and myself is that he uses the terms to describe populations while
I use them more to describe individual infections. Population
studies are required to establish whether an organism is a pathogen
or not. However, within an infected population some individuals
may have virulent infections while others will have avirulent
infections (so-called asymptomatic carriers). I can accept a
population-based measure of virulence that is derived from an
average of individual virulence values but I don't think that is
how Charles is using it.

As I understand it, Charles Faulkner's virulence is a subset of
mine - my virulence applies to all infections with a pathogen while
his only applies to symptomatic infections.

Steve Kayes says avirulent pathogens don't exist, since if we could
measure all metabolic variables these organisms could prove to be
having an effect on their host. This is probably true, but I
prefer to stick with effects we can measure or observe at this
time, and allow for future reclassification should effects be shown
in the future.

I hope that this has helped clarify rather than confuse!

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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