Host preference

Giovanni Maga maga at vetbio.unizh.ch
Thu Jan 25 03:15:49 EST 1996


In article <9601250307.AA65939 at castle.UVic.CA>, leifsok at UVic.CA (Kirk
Leifso) wrote:

> I was wondering if a virus has a host preference, or if they 
> attack only one kind of cell.

To put it in a simple (or simplistic) way: viruses have *host*
specificities in the sense that they infect organisms belonging to one or
a few related species (to give you a well known example: HIV infects only
humans, SIV that is a closely related virus infect only monkeys, FIV
infects only cats). This specificity can be assigned basically to two
mechanism: cell specificity and immune system. Usually a virus infect one
type of cells (epithelial, lymphocytes, epathocytes...). This because the
viral envelope bears particular proteins (receptors) that can interact
with other proteins on the cell surface, thus allowing viral entry. Such
cellular proteins are often species-specific. Also, the virus in order to
replicate within the host cell needs to interact with some cellular
proteins. For example, SV40 virus infects naturally only monkeys. However,
in vitro it can replicate also within human cells, but not of any other
eukaryote. This is due to the fact that some cellular proteins required
for replication (either the DNA polymerase/primase or the single strand
DNA bainding protein) are conserved between human and monkeys, but not
enough between, for example, humans and calves. The immune system also
play a fundamental role. Viruses have evolved mechanisms that enable them
to escape (at leats partially) the immune response triggered by the
infection. These mechanism can involve antigenic mimetism or simply the
viruses have a rate of replication inside the infected cell that is faster
than the rate of clearing of these same cells by the cell-mediated immune
response. These mechanisms can be effective in one or few closely related
species, but not in all. Also the opposite is true: in some hosts, viral
infection has no or reduced mortality of the host. This because the immune
system can partially control (but not prevent) the viral infection. Thus,
the virus can still replicate and the host can survive enough to spread
the infection to a new host. Think about influenza virus: it does not
usually kill the host, thus it can readily spread within the population.
When, by accident, a virus infect a new host, sometimes it kills it too
fast, thus limiting the possibility that the infected organism spreads the
infection. This is probably what is happening with emerging viruses like
Ebola (another well known example...maybe even too popular). They are
likely an example of viruses well adapted to some natural host different
than humans, that when infect another host (humans) cause a devastating
effect and fast death. (These viruses are not new i.e. *emerging* from
nowhere. They probably have been around for thousands of years. They are
*emerging* as human infecting viruses, due to the increased penetration of
man within a previously incontaminated enviroment). In sum, the *host
preference* and the *target cell preference* are closely related
phenomena.
Forgive me for the oevrsemplification of the matter (and for the eventual
mistakes this could have led to). I suggest you to refer to a textbook of
virology for a more complete explanation.
G.Maga.



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