Ebola followup NEW!

Giovanni Maga maga at vetbio.unizh.ch
Tue Jan 30 02:18:07 EST 1996


In article <hasse-2901962357370001 at hasse.dialup.access.net>,
hasse at panix.com (Hans Andersson) wrote:


> 
> "Comparing the new strain's RNA sequence with those from earlier outbreaks
> reveals wide diversity, notes Anthony Sanchez, a research officer in the
> Special Pathogens Branch at the U.S. Centers for Disease Control and
> Prevention (CDC) in Atlanta. That's not surprising, he notes, because
> Ebola, like other RNA viruses, has an error-prone replication process,
> which would boost the frequenzy of mutations and thus the emergence of new
> strains. Sanchez says that the high mutation rate increases the chance
> that the disease could someday adapt a more contagious form."
> 
> SCIENCE, Vol. 268, 19 May 1995, p. 974
> 

This quotation per se,i.e. when extrapolated from a precise context and
when not read in terms of molecular evolution, surely gives the idea that
Ebola will very probably mutate into a more dangerous form. But we know
that the mutations that appear within a population are selected by the
enviroment. They should confere an advantage to the virus, to be *fixed*
in a new strain. If they are neutral, then they will be diluted into the
gene pool, never emerging (except if they are linked to another selected
mutation). If they confers a negative phenotype, they will be lost. This
mechanism DOES NOT imply that positive mutations must go in the sense of
higher mortality. It is entirely possible, on the other way round, that
adaptative mutations in viruses will REDUCE the mortality of the infected
host and also the severity of the symptoms. This because a well adapted
virus could have more advantage not in affecting too severely its host or
killing it too fastly.
The high mutation rate per se increases the chances that new phenotypes
will appear within the population. However, these phenotypes must be
compatible with the enviroment. In case of viruses this means primarily
that every new phenotype to be positively selected must give some
advantage within the cycle virus-host (infection) and /or host-virus
(mantainance in reservoirs, replication, spreading). This gives a much
narrower spectrum of mutations available for the virus to become more
virulent.
One possible scenario depicted was that Ebola could become airborne. Given
preliminary evidences that possibly the strain called Ebola Reston (not
dangerous to humans) was airborne, this is theoretically possible.
However, this would not mean necessarily that i.e. airborne Ebola Zaire
will have the same infectivity as the wild type strain. This because
airborne transmission means aquired ability to infect through the
epithelium of the mucosa, thus developing new specific receptors (that in
turn will be much probably also new antigens) thus facing an immune
response very different from the one the virus evolved to escape from.
Moreover, mutations usually do not come alone. Thus, given the fact that
appearing of an airborne strain should involve more than one mutation and
given the fundamental differences this new strain must bear, it is as well
possible that airborne Ebola could be less a threat than the actual
strains.
Of course is precise duty of CDC to be prepared in case the worst possible
scenario will happen, thus the quoted sentence is perfectly in line with
the viewpoint of a CDC officer.
I want to stress that I DO NOT deny the possibility that Ebola could
mutate in something more dangerous. This implies that the natural history
of Ebola must be carefully followed and emergency of new strains must be
immediately registered as for any other potential pathogen. I simply
believe that also the opposite scenario is possible with almost the same
probability. 
As far as for the relevance to be given to emerging deseases vs. current
deseases as actual health dangers, as I already told you, I fully agree
with the point of view of Ian York.
My worthless thoughts of course.
G.Maga.



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