From: TSS ()
Subject: USDA INCONCLUSIVE RARE STRAIN OR ONE BIG FAIRY TALE FED TO PRESS
Date: June 16, 2005 at 4:46 pm PST
i think this strain story reflects one huge confused fairy tale that
has been fed to the press.
-- there are many dozens if not hundreds of human strains of CJD, one
for each of the many mutation types in the human prion gene and one for
each animal strain that has crossed over. some of these strains may
superficially resemble each other at the phenotypic level or on a
simple-minded gel but they are readily separated by more careful
comparative biochemistry. many publications have done just that.
-- the same will be true for cows. sheep. cervids. etc. they too will
have many strains of familial TSE due to mutations in their prion gene.
a US population of 100 million cows means that every year some calves
will be born with relevent mutations in their prion gene, predominantly
due to the CpG effect and expansion of the octapeptide repeat.
disease-causing variants that arise in this way are tabulated on the
mad cow web site.
-- when massive numbers of animals with the same prion genotype are
exposed by the same route to the same massively distributed strain (eg,
UK BSE, that strain can become abundant. however other strains will
arise by various mechanism, for example the Stetsonville, WI strain of
bovine TSE. thus it is very very implausible that any connection exists
between french/italian and US strains.
-- if the new texas cow has not been exposed, directly or indirectly,
to UK feed, it is probably just a US-specific strain that has arisen
from one of the usual sources. this cow may or may not be a founder --
there could be an earlier founder cow or sheep or cwd deer rendered
into feed and giving rise to a local south Texas mini-epidemic of which
only this current cow has been detected.
-- sporadic cjd and sporadic tse are not too be confused with
spontaneous prion disease. the former is a catch-all term for prion
disease of currently unknown cause; the latter is a speculative concept
of undemonstrated occurence. usda invented and promotes "spontaneous"
because it is no-fault -- no consequences for feed suppliers, public
just needs to get used to it like breast cancer or SUV rollover.
-- there is no such thing as a "weak" positive. only prpsc interacts
with these antibodies. prpsc is only present in an infected animal.
there is no such thing as low or natural background level of prpsc. it
is in all cases a progressive condition (though an animal may die first
from unrelated conditions). what the guy at Ames apparently means is
diffuse-staining prpsc deposits of a normal amount of infectivity
(smeared less-focused band due to slight molecular inhomogeneities)
rather than a "weak" infection.
-- low levels of obex prpsc may only mean early phase of infection or
late phase of some strain that does not accumulate in the obex
particularly. the subsequent infectivity of this strain could be much
higher than UK BSE both to cows and humans. or lower. or the same. or
different after transmission. no one has measured the infectious titre
(# infectious doses per mg) of this cow's strain.