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Big death toll feared if mad-cow disease has infected sheep: study

January 9, 2002 Agence France Presse
British scientists on Wednesday said there was the risk of a major rise in the human death toll if sheep had become infected by mad-cow disease, but admitted the estimate was beset by many unknowns.

The estimate, by epidemiologists from the Imperial College of Science, Technology and Medicine in London, was published online by the weekly British science journal Nature.

Scientists are battling to find out whether bovine spongiform encephalopathy (BSE), as mad-cow disease is known, has been transmitted to sheep, possibly via feed supplements -- the suspected way in which cattle became infected.

Seeking to get an idea of the likely toll if such were to be the case, the Imperial College team devised a computer model to predict how many fatalities might occur among people who ate both infected beef and sheepmeat. Between 2001 and 2080, the number of deaths in Britain from eating only BSE-contaminated beef would generally be from 50 to 50,000, with a worst-case peak of 100,000, according to their estimate.

If sheep are also assumed to be infected, the overall toll in some scenarios would remain unchanged.

But in many other scenarios, it would be between 20-50 percent higher.

Under the worst-case scenario, 150,000 would die over 80 years, through eating infected sheepmeat and beef.

The team stressed there are many unknowns, notably about the incubation period of the agent, which, even assuming that it exists, could take an unknown number of years before causing clinical symptoms in humans.

The study assumes that because the anatomy of sheep is different from that of cattle, more of a sheep's body would be infectious rather than just the spine, brain and tonsil, as in the case of cows. This, in turn, would widen the transmission to humans.

The scenarios would vary according to the strength of this infectivity and on how quickly the government acted to tackle any scare.

If, for instance, the government quickly removed suspect tissues from public consumption and destroyed older animals likelier to be at risk, that could cut the risk by up to 90 percent, the study said.

Sheep have their own version of BSE, which is called scrapie. It has been around for centuries and has never been known to infect humans.

According to one popular theory, scrapie leapt the species barrier to cows, infecting cattle that ate the ground-up remains of sheep. In that process, the theory goes, scrapie somehow changed into a more virulent agent and became harmful to people who ate infected beef.

But an equally worrying question is whether uninfected sheep may have caught the same agent as cattle, only for this to be misdiagnosed as scrapie.

It is known that some sheep were exposed to some of the same infected protein feed which passed BSE to cattle before controls were introduced in the late 1980s and in the 1990s.

Laboratory tests have already established that BSE can be passed to sheep when the animals are injected with infected brain material from cattle or if they eat infected feed.

The latest research follows a bungled four-year British study into whether BSE exists in sheep.

Initial findings from the study last year suggested that one percent of sheep might have BSE, triggering a scare that Britain's entire flock of 40 million sheep and lambs would have to be destroyed.

But tests at the last minute showed that the brain samples being assessed had in fact come from cows.

BSE has been blamed for causing a related ailment in humans called variant Creutzfeldt-Jakob disease (vCJD), in which a rogue protein proliferates in the brain, destroying cells and causing the brain to become spongey. The disease is fatal.

Until December 3 last year, the latest date for which full official figures are available, there had been 113 cases of definite or probable vCJD in Britain. Ten patients were still alive at that date.

There has been one fatality in Ireland, and five confirmed or suspected cases in France.


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