Many of the TSE diseases that we know
about have only appeared in the last decade, such as BSE,
FSE and ZSE,
but others such as scrapie and
CJD have been known about for a long
time. Scrapie is by far the oldest,
having been recognised in sheep for a couple of hundred years
while the first cases of CJD in humans
were diagnosed at the beginning of the 1920's. It is as yet unsure
exactly what causes these diseases, but through lab experiments
with murine modeling and
transgenic mice it has
been show that all the diseases are related.
Experiments with transgenic mice
and other research have also shown one very worrying trait of
the TSEs: there ability to by-pass
a species barrier by detouring
through another species. For example it has been found that, although
the scrapie is relatively ineffective
in crossing many species barriers,
if it was passed through cattle (producing the BSE
agent) then there were few species barriers
left completely uncrossable, giving rise to FSE
and ZSE, both new diseases, and possibly
to a new variant of CJD. It is also
possible that the disease is spreading to domesticated fowl with
isolated reports of chickens displaying BSE
like symptoms appearing on farms affected by persistent BSE
infection.
