Mad cow scares and confusion: how much we still do not know about causes and risks

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The Independent Online
Who's in danger?

Cows? Certainly; by eating just one gram of BSE-infected feed. The disease takes about five years to show symptoms. Since 1986 there have been more than 161,000 cases in the UK. Of those, 28,400 were in cattle born after the July 1988 ban on using cow and sheep offal in animal feed and 4 per cent were born to BSE-infected mothers.

Calves? Yes, either by eating infected feed or, as announced yesterday, from mothers. A study suggests that up to 10 per cent of calves born to BSE-infected mothers would become ill. However, unless there is also transmission between cows - "horizontal transmission" - this figure is low enough to mean that the BSE epidemic in cattle will die out. Good news? Yes; but now we have something new to worry about: we don't understand how BSE passes from cow to calf. Milk? No, that's been ruled out.

Sheep? Yes, in experiments. They also may have caught it on farms, because they, too, would have eaten BSE-infected feed in the 1980s. But BSE in sheep would be hard to distinguish from scrapie - a similar, endemic disease. BSE is more infectious - and so could pose a greater risk.

Can it pass to humans? As of today, the answer is a resounding "perhaps". Twelve cases of a "new variant" of CJD among Britons in two years triggered fears that the BSE agent had infected humans. But, despite fears of a plague, cases have not increased. The ban on the use of cows' brains, spinal cords and other materials for human consumption in 1989 means the greatest risk of eating infected food is probably in the past. Still, the long incubation period may mean more human cases are on the way.

Fact or fiction?

"Fact": BSE poses no risk to human health - something repeatedly said by the Government. But our real knowledge is hazier. Members of SEAC, the independent committee of scientists advising ministers on BSE and CJD, think it is possible that 12 cases of "new variant" CJD among Britons in the past two years were caused by exposure to BSE.

"Fact": BSE in cows came from scrapie in sheep, after the process of making cattle feed out of ground-up animals changed in the late 1970s. That's what the ministry says. But some SEAC members think that BSE didn't "come" from anywhere. It may simply be an endemic disease in cattle - like CJD in humans.

"Fact": BSE is very rarely passed from cow to calf - if at all. Another ministry favourite. But results issued yesterday suggest that cows in the last six months of the five-year incubation period of BSE have a 10 per cent chance of passing it on to their calves.

"Fact": BSE-induced CJD will reach epidemic proportions, killing thousands of people every year, some time in the next decade. Thus numerous doomsayers, triumphalist vegetarians and over-excited journalists. But no link between eating BSE-infected food and "new variant" CJD has been established, and experimental results that may prove a link will not be ready before next year. The removal of beef offal from human food in November 1989 has significantly reduced the risk of passing on the disease, if there is a link. The dozen cases recorded by the CJD Unit in Edinburgh do not show the rapid rise that might be expected.