Virulence versus Pathogenicity - (Oct/13/2005 )
I see the two terms used more or less interchangeably, yet I am sure that there is an important difference, at least in some contexts. I'm just not quite sure I've grasped it. Could someone explain it?
Pathogenicity is he ability of a microorganism to cause disease.
Virulence is degree of pathogenicity of a organism. it is indicated as fatality rates or ability to invade a host tissue and cause disease.
I take it then that virulence should be regarded as a more global term, one which includes pathogenicity as well as transmissibility, whereas pathogenicity refers specifically to the damage done to an individual host ?
There is an interesting history to this...
Ancient Roman physicians understood "virus" to mean "a poison of animal origin", and the diseases these poisons caused were called "virulent". During the Renaissance, it was recognized that there were non-contagious diseases caused by poisons and contagious diseases caused by infectious agents; the name "virus" was reserved for the latter.
In the nineteenth century, microbiologists recognized that bacteria were the agents responsible for many diseases, and bacteria were referred to as the "viruses of disease".
As microbiological techniques improved, it was found that some of these infectious agents could not be cultivated and would pass through any filter available at the time (like Pasteur found was true for rabies, for example). This lead to the distinction of "viruses" and "filterable viruses", with the latter term fading over time, and the former term meaning what we recognize it for today.
"Virulent" is also used in phage work to differentiate from "lysogenic" -- a lytic phage is virulent, a lysogenic phage (until it enters a lytic cycle) is not.
Thus, the term "virulent", to me anyhow, carries with it the ability to be transmitted from host to host; thus the causative agent of a disease can be pathogenic without being virulent (if it's unable to be transmitted). Also, with a nod to the original Roman definition, perhaps the disease is virulent (if it's transmissible), its causative agent is pathogenic.
Given the history of the term, it might also be a bit more correct to use "virulent" when speaking of viruses or viral diseases, and "pathogenic" when speaking of bacteria or bacterial diseases.
I agree the terms are used interchangeably now, but common use doesn't make it correct -- witness such misuse as "weakly" or "highly" homologous...
And thank you, HomeBrew.
That 's about the most informative take on the matter I've seen yet. It helps... some.
I guess I'll just quit worrying about it.
I found an explanation that cleared this up completely for me, and just thought I'd pass it along: