Market Farming

[Sojourner <sojournr AT missouri.org>]

Shelley Pasco wrote:
> 
> The only part I'm confused on is the cross-species thing.
> I've heard that BSE originated as Scrapie in sheep--are
> they just two versions of the same thing?

I have to admit I'm weak on these particular details, but the last time
I extensively researched this scrapie was NOT thought to be the most
likely vector for the introduction of BSE into the beef supply in
Britain.  I'll tell you what I found out last time I went into this
extensively with the understanding that it may be outdated information
now.

In parts of Africa there are several known diseases of cattle and
relatives of cattle that are spongiform encephalopathies.  There were
several major outbreaks of one or more of these TSEs within African
bovine populations within the window of incubation/infection for the
start of the BSE epidemic.  Importation of potentially contaminated
materials for use as feed additives may have exposed British cattle to
infection via this vector.  In addition, within the window of
incubation, there was an epidemic among bovine species at the London Zoo
(I forget just which ones) that resulted in the deaths of several zoo
specimens.  It is believed that this disease was most likely a
spongiform encephalopathy (no brain autopsies were done, to my
knowledge).  These carcasses also found their way into the British
cattle feed supply.

Shortly prior to the time all this was happening, there had further been
a changeover from the old method of rendering animal carcasses that
involved a whole lot of steam and heat to a newer, quicker, cheaper
method that involved considerably lower temperatures and chemical
treatments.  Whereas the old method of rendering carcasses (whether from
scrapie-infected sheep or from bovines infected with some other form of
TSE) would have destroyed many though not all of any prions present, the
new method wouldn't have destroyed *any at all*, therefore allowing an
influx of more infectious material into the feed supply than previously
would have been possible.

Scrapie is fairly common among sheep; note that there has never been a
case of a human being contracting scrapie.  It is possible that the
greater prevalence of sheep in Great Britain (I think the ratio at the
time was something like 3 sheep to every cow, whereas here its 10 cows
to every sheep) and the consequently higher amount of possibly scrapie
infected material in British cattle feed may have caused an increased
risk of cross-species infection than we have here, which may explain why
British cows came down with BSE and ours did not, I don't know.  

At any rate, the way it works is that prions CAN jump species
boundaries, but its very hard for them to do so.  So while its possible
that BSE may be the result of the scrapie prion jumping the species
boundary it would make more sense for the original vector of infection
to be within the species; e.g., from bovine to bovine.  But then reality
doesn't always have to make sense.  Yes, its possible that BSE is the
result of scrapie becoming established in the bovine population.  I
don't know what evidence there currently is for or against that theory.

Whatever the original vector of infection, once the prion was in the
British cattle population the wide-spread practice of grinding up dead
cattle to feed back to the live ones (protein is protein is protein, is
the way the thinking went) gave the disease a vector for rapid
deployment. Every time an infected cow died and was added to the feed
that was that much more exposure to the prions (whatever their origin)
for the rest of the population.

Once this practice was stopped the rate of infections began to decline. 
Once the "backlog" of cases still in the incubation period had a chance
to be expressed, the disease went into rapid decline.  I don't know what
the current figures are for the disease in Britain, but last I checked
it was expected to be eradicated by 2003.

> I hadn't heard about being genetically susceptible either.
> You must have done an awful lot of research about this.

Well, I stumbled across a BSE fear-mongering site that was so rife with
rumors and what had to be out and out lies that it got me pissed off and
I embarked on a spree of BSE research that hasn't totally stopped yet
(it has slowed to a bare trickle though).  One of the things these guys
(and I found bunches of similar sites) were saying was that you could
get BSE from chickens and from petting your cats.

There was so much speculation going on, and newspapers were reporting
all sorts of things as fact with no supporting evidence, it was really
hard to find the truth, especially since so much of it IS in
"scientificese".  3 and a half years as a biology major and 5 years
spent doing literature searches for MDs doing research really came in
handy.

Early on they had yet to establish a connection between vCJD and BSE; in
the last couple of years they have found sufficient evidence to make the
connection more than the sheer speculation it was for awhile.  That's
one case of a rumor that happened to turn out to be true.  However, it
was never as widespread as some of the rumor mongers made it out to be.

-- 
Holly ;-D
Contrary Peasant
sojournr AT missouri.org