On 15 Sep 1995, Graham Clark wrote:
> <snip, snip, cut, cut>
> To which Stephen Kayes replied:
>> -Katy:
> - 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.
> As might be expected from a glib generality, there are exceptions
but there are interesting relations that must be considered. Graham is
correct. But as regards the disease caused by Entamoeba, liver abscess
from this organim is far less common than is amebic hepatitis which is
caused by a toxin released from the parasite which is then transported to
the liver by the hepatic portal system. E.h. is after all an intestinal
parasite that can invade any tissue of the body. The liver "just happens
to be connected to the GI track and thus, when E.h. invades the submucosa
of the bowel wall and enters the blood stream the ameba are swept to the
liver where they can then resume eating tissue cells, such as hepatocytes
and can elicit some rather stricking histopathological responses.
> 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.
> Again I agree with Graham but the fact that the malaria parasite
has a tropism for hepatocytes (translated: there are receptors on the
liver cell surface that attach to the malaria organism and result in its
internalizaton. The liver is where the few invaders from the mosquito
undergo their first rounds of asexual multiplication. And what better
place to do so but in the liver which is where the body stores much of
its glycogen (remember carbohydrates are the easiest substrate to use for
energy) derived from ingested food, in the gut which travels by the
portal system to the liver. Did natural selection choose liver cells for
this aspect of the life cycle. Makes sense to me based on the very large
amount of blood flowing through the liver at any one time and the fact
that the sinusoids are lacking continouus basement membranes which allow
the parasites to actually touch hepatocytes without having to leave the
blood stream.
> 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
> disease.
>
Again, Graham is correct but I ask why do Schistomes (at least
mansoni and japonicum) choose the inferior and superior mesenterics?
Because they drain the intestines and can steal from the host's raw
nutrients. These parasites too, are reproducing at intense rates and
need a good source of raw materials. After the infections become
established and the bowel wall becomes fibrotically scarred, eggs can no
longer penetrate and pass to the outside (to keep the life cycle going)
so they back up in the parasites local environment and blood flow sweeps
them into the hepatic portal system from the mesenterics and into the
liver. I still vote that this gut-liver axis is an extremely beneficial
site for endoparasites to practice their crafts. And Mother nature (in
the guise of natural selection) would promote such a busines plan.
> 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?
>> Graham
End of Graham's text with my comments inserted.
The bottom line is that both Graham and I have a certain gestahlt
of the internal relationships of host-parasite interactions. Our
differing interests and backgrounds flavor how we think about the same
things. Thats why there is no one single way to train a parasitologist.
You never know who might just see the obviously missed insight into the
host-parasite relationship that all the clasically trained people just
passed over and over. In closing, Katy, your rather simplistic question
is really excellent and I hope it continues to promote a dialogue on site
selection by parasites.
Steve Kayes
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