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Why do many parasites affect the liver?

Graham Clark Graham_Clark at d4.niaid.pc.niaid.nih.gov
Fri Sep 15 09:46:22 EST 1995

On Thu, 14 Sep 1995, k.sinka wrote:

> Of the parasites and viruses that affect the liver
> Malaria, Scistosomiasis and all the Hepatitis strains
> is it their 'intention' to get to the liver or is it just a 
> function of the way the bloodstream goes to this
> organ that means they end up there?
> Katy S

To which Stephen Kayes replied:

-    I suspect that you hit the reason with the bloodstream going to the 
-liver.  Remember that the liver has a double blood supply, namely the 
-hepatic artery and the hepatic portal system which drains the intestinal 
-tract.  The blood contained within the portal supply contains the 
-majority of nutrients absorbed from the host's  food supply and thus, is 
-extremely attractive to parasites capable of an intravascular existance.  

I don't believe that it is that simple. I can think of three distinct
scenarios for parasites that affect the liver (somewhat simplified below):

1. Organisms like Entamoeba histolytica that end up in the liver as a 
function of the blood flow from the intestines, and when in the liver 
cause disease (in this example amebic liver abscess). The liver is not 
part of the normal life-cycle of the organism.

2. Organisms like the malaria parasites where there is a specific life-
cycle stage that occurs in the liver. In this case there is presumably a 
specific tropism for the liver without which the parasite would be cleared 
from the bloodstream.

3. (which is somewhat like '1') Organisms that affect the liver but never
actually reside there, like schistosomes. Schisto worms live in the 
mesenteric veins and shed eggs some of which end up in the liver (among
other places) via the blood stream, where they cause granulomas and 

In none of these cases is it clear that it is the nutrient richness that
determines the parasite's 'attraction' to the liver although it is likely
that the blood flow does have a lot to do with how they end up there.
Is this a reasonable conclusion?

C. Graham Clark, Ph.D.
Laboratory of Parasitic Diseases,
National Institutes of Health,
Bethesda, MD 20892-0425, USA
Tel: 301-496-4740
FAX: 301-402-4941
e-mail: gclark at nih.gov

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