virulence

ctfaulkn at utkvx.utk.edu ctfaulkn at utkvx.utk.edu
Fri Dec 23 00:25:58 EST 1994


In Article <199412221811.KAA07614 at net.bio.net>
CGE at CU.NIH.GOV writes:
>In Article <3dc8p7$i6o at martha.utk.edu> ctfaulkn at UTKVX.UTCC.UTK.EDU
>writes:
>
>- Relative pathogenicity is defined as:
>-
>-     number of clinical cases resulting from infection
>------------------------------------------------------------------
>-total number of infections in the population under investigation
>-
>- Relative virulence is defined as:
>-
>-   number of clinical cases w/serious manifestations (mortality)
>------------------------------------------------------------------
>-     total number of clinical cases resulting from infection
>
>The problem I have with these definitions are that they are somewhat
>subjective (what is a 'clinical' case; what constitutes 'serious')
>and that, as far as I am concerned, they describe a continuum. They
>also rely on population phenomena which may be because they are
>from an epidemiology textbook.
>

	Your point about subjectivity is true. That is why epidemiologists
place so much importance on a formal case definition (usually published in the 
MMWR).  Early work with the epidemiology of Lyme disease was difficult prior to 
the case definition of what constituted Lyme Borreliosis.  The matter of a
serious case is much harder to define explicity. I think thats why the CFR 
(case fatality rate) has been used to estimate (describe) relative virulence.


>
>Incidentally, for the past two days it has just been the two of us
>going back and forth over these definitions. Doesn't anyone else out
>there have an opinion?
>

	Apparently not. Its kind of hard to build a consenus based on n=2.


Returning to Ewald, it is interesting that his typology appears to be based
on kinds of transmission patterns and their relationship with virulence, 
rather than phylogeny.  I sort of enjoy that because its a good example of the 
artificiallity of classification schemes. We point out to the vet students that
classification schemes are functional in nature, and can be based on any
critera to suit your purposes. For example, if you are a pathologist a
classification of parasites based on their location in the host may be more
important (at least intially) than their phylogenetic relationships. 

    Perhaps other folks would like to share their views on how we might look
at parasite virulence with regard to modes of transmission, as opposed to 
a particularistc phenomena in individual host-parasite interactions. 


**********************************
*      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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