WHERE did BSE come from? We still don鈥檛 know, and may never know for sure.
But the odds are that it arose by chance鈥攚hich means that it could happen
again, any time and anywhere.
The Phillips report discounts the popularly held theory that BSE arose when
cattle became infected with a spongiform brain disease from sheep. However, many
scientists are sceptical about this conclusion. The British government appears
to share these reservations, as ministers have ordered a review of the current
knowledge on the origins of BSE.
The report rejects the notion that the scrapie prion infected cattle eating
meat and bone meal (MBM) made from sheep in the early 1980s. Instead, it
suggests that BSE is a new disease that arose as a spontaneous mutation in the
prion gene in a cow or sheep in the early 1970s.
Advertisement
The new BSE prion could then have contaminated MBM. As it was an unknown
disease, cases could easily have been missed, allowing infection to cycle
silently and exponentially before it was identified in 1986.
The idea that changes in rendering practice allowed scrapie prions to be
transmitted to cows is a 鈥渃ommon misconception鈥�, says the report, as no
rendering procedure can completely inactivate infectious prions.
But Paul Brown of the US National Institute of Neurological Disorders and
Stroke in Bethesda, Maryland, argues that scrapie is still a suspect. 鈥淧erhaps
occasionally scrapie did cross to cattle earlier,鈥� he says. New, lower rendering
temperatures then boosted the amount of scrapie cows were exposed to, leading to
a visible epidemic.
Roger Morris, an epidemiologist at Massey University in Palmerston North, New
Zealand, is also sceptical about the idea that BSE cropped up spontaneously. If
true, 鈥測ou would expect BSE to have cropped up sporadically in the past in
countries with much larger cattle populations and similar rendering practices鈥�,
he says. It鈥檚 hard to believe that it would only happen once, in Britain, he
adds. He does admit, though, that the theory can鈥檛 yet be ruled out.
But Morris isn鈥檛 convinced that scrapie is the culprit either, as other
countries have scrapie and similar rendering practices to Britain. Yet none has
had a native case of BSE.
His team is working with researchers at the British government鈥檚 Veterinary
Laboratories Agency to piece together a high-resolution epidemiological map of
the outbreak. The team is trying to match data from the map to computer models
based on several theories of BSE鈥檚 origin to see which fits best.
BSE probably originated from a wildlife source alien to Britain that was fed
to cows, says Morris. This would explain why the epidemic was unique, and is
caused by a single strain of prion. The team is now focusing on a shortlist of
suspect species.
If they come up trumps, governments around the world will hastily need to
find ways to keep such animals from contaminating their own livestock.
