MadSci Network: General Biology |
Hi Kristin,
This is an interesting question but with a quite complex answer.
Bacterias, viruses and parasites can be quite specific for their host.
Some will infect animals without getting them sick while others are able
to infect and kill many species. It depends mostly on the type of
pathogen studied. To understand how different hosts can react differently
to a pathogen, you have to understand how the pathogen interacts with its
host. When a blood pathogen gets in the bloodstream, it will usually try
to replicate itself very rapidly. If the pathogen is a bacteria, it can
do so by itself. If it's a virus or a parasite, it needs some cells or
some specific host factors to be able to replicate itself. For a virus
(like HIV) or an intracellular parasite (like leishmania for example) to
infect a host cell, it needs a very specific receptor on the surface of
its host cell. Even if there is probably an equivalent receptor on the
surface of the same cell type for many hosts, some small differences
between them will lead to a very specific interaction with the virus
(human and mouse for example both have a CD4 molecule on the surface of
the cells infected by HIV but HIV can only infect the human cells because
the human CD4 molecule is a little bit different to the mouse CD4 and it
is enough to give the virus its specific infection of humans and not
mice). For many pathogens, the host differences between cellular
receptors explains why they infect some species and not others or that
they kill some species and do not affect others even if they are infected.
Even when a pathogen infects a host without getting it sick, it helps the
pathogen by creating a reservoir of infected but healthy animals. This
reservoir can then transfer its pathogen to another host via blood-sucking
or biting insects (sand flies, tse-tse flies, mosquitoes, fleas, etc)
which are then called disease vectors.
Most mammals have the same general components in their blood but each
mammal show some small differences between one and the other which
explains the specificity of blood (and other) diseases.
The next question could then be "how is it that a pathogen can infect a
host without getting it sick?"
The answer then is more a reason of efficient replication of the
pathogen. An intracellular pathogen need some very specific cellular
factors for its replication and since most factors are a little different
between mammals in general, this can explain why a specific disease could
be able to infect and replicate but not at its optimal rate. The immune
system of the reservoir host could then be able to keep the pathogen in
check while in another host it could not. The reservoir host will live
and stay healthy while the other one will get sick and could die.
I hope this answered your question. I didn't want to get in too much
detail but you can look on the internet about some blood-borne diseases,
their cellular receptors and their specific cellular factors.
Here are some web sites you could look at:
http://www2.umdnj.edu/e
ohssweb/bbp/index.htm
http://www.virology.net/cour
seware.html http://www.biosci.ohio-
state.edu/~parasite/leishmania.html
http://www.cdc.gov/nci
dod/hip/BLOOD/blood.htm
http://w
ww.cdc.gov/ncidod/dpd/parasiticpathways/insects.htm
http://www.cdc.gov/ncidod/
dvbid/index.htm
Hope these will help,
Bye!
Mike
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