Scrapie may not be the cause of BSE

The leading theory on the emergence of BSE has been that cattle initially
contracted the disease by eating feed containing sheep material that was
infected with scrapie. However, attempts to duplicate this process by giving
scrapie to cows in laboratory conditions have failed. In addition, many
countries have included sheep remains in cattle feed without creating BSE. This
has led some scientists to speculate that BSE arose as a rare spontaneous
condition in cattle, which spread to other cows when they ate these animals’
remains.

This theory appears to have been strengthened by Italian
research into the rare cattle prion disease
bovine amyloidotic spongiform encephalopathy, or BASE, which affects older
animals and is distinct from BSE.

BASE mutates into BSE
If BASE can mutate into BSE, this could well be how BSE
emerged. “I think BASE is a natural prion disease of older cattle, which turned
into BSE,” says Tagliavini Tagliavini from the Carlo Besta Neurological
Institute

in Milan.
Another possibility is that BASE turned into BSE after
cattle remains were fed to sheep. In preliminary research, Hubert Laude of the
French national agricultural research agency INRA, says that he got BASE to turn
into BSE in mice engineered to carry sheep PrP, but not in mice with cow or
human PrP. This would suggest that sheep could have got BSE from cattle infected
with BASE. Cattle would then have got BSE by being fed with BSE-infected sheep
material.

Contaminated feed
New cases of BSE in UK cattle born after the
reinforced feed ban in 1996 (BARB cases)

continued to appear in 2006, although numbers have fallen since the peak
in 2003. To date 160 BARB cases have been detected and proportionally similar
numbers have been found in other EU countries. BASE has not been discovered in
British cattle yet. The BARB cases are thought to be the result of very low
level contamination of feed that is either imported or comes from feed bins on
farms containing residual amounts of old feed. If this is the case, then very
small amounts of the infectious agent could cause BSE, and the tail of the
epidemic could continue for a number of years.

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