Viral virulence

[ryan at mbcf.stjude.org]

In article <95051421563534 at spock.colsf.edu>, stuhansenmic at SPOCK.COLSF.EDU writes:
> I'm an undergrad bio major who has been lurking here for some time.  With
> all this talk of Ebola, it seems to me that a virus that destroys its host
> very quickly is certainly not helping itself.  Would'nt it more beneficial
> for the virus to remain latent for a long period of time to allow it to
> be transmitted further and replicate more? Am I way off track in this idea?
> Thanks in advance for any responses.
> 
> Mike Hansen

Hi Mike, good question.

If any virus has only one possible host species, then it might enjoy only
limited success by a strategy of killing its hosts in short order (thereby
losing opportunities for transmission to new hosts). Each infectious agent must
achieve its own peculiar balance between aggressively replicating and 
spreading through a host's tissues, and permitting infected hosts to survive 
long enough for transmission. Many different combinations of infectivity & 
pathogenicity can make an infectious agent successful.

Keep in mind Ebola, like many viruses, is not limited to a single host species.
I presume this virus has a natural animal host species from which intermittent
human infectious episodes emerge. I expect that Ebola pathology in that host is
much less rapid and/or pronounced. My guess is that the dramatic pathology of
Ebola infections in humans is due to its poor adaptation to using humans as
hosts. If humans are incidental to the natural history of Ebola (that is, if
Ebola has evolved in a stable relationship with some other host), then this
virus's limited success in human populations is not a problem for the virus.
While infections are devastating for infected people, the virus should not
suffer because of its poor adaptation (eg limited transmissibility) to the 
human host. In its natural host, presumably, Ebola life goes on happily.

A virus can't do something just because it would be beneficial, it
can only do what it has evolved the capability to do (in a specific host). It
would be very beneficial for me personally to get out of my virology lab and
make a lucrative living on the PGA tour. Sadly, I don't have this capability,
and I would probably function very poorly and with little success in that new
environment. Roughly similar constraints may limit the success of a virus in
an unaccustomed new host.
 
Cheers, Kevin.
-- 
--
       	==========================================
	Kevin W. Ryan
	Department of Virology & Molecular Biology
	St. Jude Children's Research Hospital
	Memphis, Tennessee 38101-0318, U.S.A.
	
	phone: (901) 522-0411
	fax:   (901) 523-2622
	Internet: ryan at mbcf.stjude.org
	==========================================