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Clues to Scrapie-Mad Cow Link
30 April 2001 7:00 pm
An as yet unpublished study may lend new credence to the theory that bovine spongiform encephalopathy (BSE), or "mad cow disease," could have originated in sheep.
Scientists are anxious to understand the relationship between BSE and two other related neurodegenerative diseases: scrapie, which afflicts sheep, and variant Creutzfeldt-Jakob Disease, which has killed nearly 90 people in the United Kingdom and three in France. All three diseases have been linked to abnormal proteins called prions, and some researchers have speculated that cattle got the disease from feed containing ground-up sheep with scrapie, which has infected sheep herds for decades.
Last October, however, this so-called "scrapie hypothesis" was dismissed in a major report from a U.K. panel chaired by Andrew Phillips (Science, 3 November 2000, p. 911). The report threw its weight behind the hypothesis that BSE arose from a spontaneous mutation in cattle, creating a new form of prion.
The new work, first reported on 27 April in The Independent, is from a team led by veterinarian Danny Matthews, chief of prion disease research at the U.K.'s Veterinary Laboratories Agency in Weybridge. In July 1999 his team injected the cerebrums of 10 calves with brain tissue from sheep that had died from scrapie before 1975, well before the BSE epidemic got going in the early 1980s. (That way, the researchers were sure they were injecting scrapie, and not some form of BSE transmitted back to sheep.) A second group of calves was injected with brain matter from sheep that had died after 1990. So far, one calf from each group has died from a neurodegenerative disease resembling BSE. Tests to unmask the disease-causing agent are still under way.
If it turns out that the scrapie agent is the killer, says prion researcher Moira Bruce of the Institute for Animal Health in Edinburgh, it would be the first confirmation that scrapie can cause BSE-like symptoms in cattle, and that would strengthen the scrapie hypothesis. But, Bruce cautions, "it would not prove" the link. Indeed, says epidemiologist Peter Smith, acting chair of the U.K.'s Spongiform Encephalopathy Advisory Committee, "it is going to be very difficult to sort out the origins of the epidemic."
Matthews agrees his findings do not necessarily rehabilitate the scrapie hypothesis. But, he says, any insights into the possible relationship between scrapie and BSE could help prevent future epidemics: "The nearer we get to finding the origins, the better we can refine future policy."