Ebola: antibody seroprevalence & virulence

Ann K. Reed reeda at erols.com
Thu Mar 14 00:24:17 EST 1996


I have been puzzled by information in what appears to be (I haven't checked
the sources, but the citations are to reputable journals and the tone is 
rational) a solid research summary by Miles Axton (from fellow amateur David 
Ornstein's site).  The Axton piece indicates that a number of surveys have 
found a very high seroprevalence rate to a variety of filoviruses, including 
antibodies specific to Ebola (varieties known before this year).  The data 
included studies of populations in several African countries, with 
seroprevalence in the 10-30% range, if memory serves (varied with location 
and occupation; rates in Madagascar, of all places, were particularly high -- 
perhaps even, for some subgroups, over 50%); and of seroprevalence in other 
countries as well (I remember, in particular, lists of rates in Germany, 
presumably in light of the lab-based Marburg outbreak.  Rates there were on 
the order of 3-5%, I think.)  (Sorry to be operating from memory, but I am, 
if anything, understating most of the figures).  The Axton summary did not 
indicate what the survey size was in any of the quoted studies.

I have been wondering how to square this information -- assuming it is
accurately represented and assuming the original studies themselves stood up 
-- with the death rates we are accustomed to seeing from outbreaks that have 
come to worldwide attention.  Could we just be ignoring subclinical cases, or 
cases in which a person becomes sick, but does not develop hemorraghic 
symptoms (and so is not readily identifiable, even in the context of an 
outbreak, as an ebola (or other filovirus) case?  Could means of transmission
affect both the seriousness of the illness and the nature of its course
(as for example contrasting bubonic and pneumonic plague); perhaps so
that risk of getting more virus into the body than it can hold in check
is relatively small unless a hemorrhagic patient, or a direct injection, is 
the source of contagion?  Could it be repeat infections with related viruses 
(but then why would hospital contacts who have no other connection be so 
quick to die?  and age make so little difference))?  Could it be related to 
the apparent phenomenon asserted by some observers of decreasing virulence
as the virus passes through successive generations of victims, so that
transmission of attenuated versions is fairly common?  Or is it more
likely that there are benign related versions that explain the antibody
results (that may be the same as the last question, I know), or other
contaminants to which the supposed seropositives were in fact
responding?

Whatever the answer, it doesn't seem possible that 10-30% of Africans in
certain regions have been exposed to a virus that kills 70% or so of
those who catch it, without anybody noticing all those who died and
noticing similarities in the circumstances.  I know infectious disease
is rampant in many of the infected areas, but multiply those 10-30% by the 
numbers presumed dead and you've got some pretty extraordinary figures that I 
doubt can be attributed to a disease we basically hadn't noticed before 1976 
(especially as the average age is somewhere around 15, so that most of these 
deaths would have occurred in the last 20 years or so).

I apologize for troubling the pros with this, particularly if sometime when I 
was not lurking this article was exposed as a fraud, but I did spend 
considerable time about 6 months ago trying to figure this out on my own, 
without success.  Now that I can post as well as read, I thought I'd see if 
anyone could enlighten me.  (also, I apologize if my terminology is a little 
off).

Thanks, 
Ann (reeda at erols.com)



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