SHEEP may harbour the twisted proteins called prions that cause BSE in cows,
says Stanley Prusiner, who received the Nobel Prize in Medicine for his theory
of how prions can cause disease. If they do, the infectious agent could still be
at large, ready to bounce back to cows.
Prusiner told the Birmingham meeting that Mike Scott, his colleague at the
University of San Francisco, believes that sheep carry two strains of prions.
One causes scrapie in sheep. The other causes BSE if it spreads to cows, Scott
believes.
Scrapie has long been known to infect sheep, but BSE appeared in cows in the
1980s, following a change in the way dead animals were 鈥渞endered鈥� before being
fed back to livestock. This may have allowed the BSE-causing prion to reach
cattle. Once there, the prion spread via feed to other cattle, and then to
humans.
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Scott thinks that the scrapie prion is somehow 鈥渄ominant鈥�, preventing the BSE
strain of prions in sheep from infecting cattle and people when both strains are
present, Prusiner said. The change in the rendering process may have destroyed
the scrapie prion in feed, leaving the BSE prion free to infect cattle.
Prusiner stresses that there is not yet any proof for this hypothesis. And
even if it were true, it would not justify the mass slaughter of sheep that is
being considered in the US
(see 鈥淪ave the Vermont 376!鈥�).
The agent has always been in sheep and probably always will be, says
Prusiner. 鈥淟urking out there in small numbers in any sheep may be the BSE prion,
and under the right circumstances, the BSE prion emerges,鈥� he says.
But BSE experts in Britain, such as John Collinge of the Medical Research
Council鈥檚 Prion Unit at St Mary鈥檚 Hospital, London, believe the hypothesis is
flawed. 鈥淭here鈥檚 no evidence [BSE] came from sheep, and if the prion that causes
BSE is in there, we can鈥檛 find it,鈥� he says. 鈥淚t must be quite rare if it鈥檚
there at all.鈥� It is not clear that there were any major changes to rendering
processes, he adds.