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Bse: the end of a mystery?; at last, scientists think they know why 'mad cow' disease began and why in the uk. Now they just have to prove it

BSE: the end of a mystery?;
at last, scientists think they know why 'mad cow' disease began and why in the UK.
Now they just have to prove it

July 27, 2001 The Independent (London) by Steve Connor


It is one of the most puzzling scientific stories of the day. What was the cause of "mad cow" disease, and why did it originate in Britain? Bizarre as it may sound, given the depth of research into the disease, these questions have never been fully answered. Now, a new and plausible explanation has emerged that might finally resolve the conundrum of why BSE happened when it did and where it did.

Trawling through mountains of scientific literature, and using the report of the BSE Inquiry as a guide, an officially appointed committee of six scientists has found evidence to support a theory that BSE came about as a result of feeding artificial solid food to very young calves. It could explain, they believe, why in the 1980s, Britain was the only country to become affected by a disease that is now synonymous the world over with death and disaster.

There have been many attempts at explaining the origin of BSE over the years. Some have blamed the use of organophosphates in sheep dip, others have suggested it is a bizarre autoimmune disease, and still another theory is that it was the result of abnormal intakes of trace elements in the diet. None of these, however, has withstood serious scientific scrutiny.

Two theories have continued to vie with each other for respectability. One, originally proposed more than a decade ago, is that BSE is in effect sheep scrapie in cattle. Scrapie, a brain disorder that has been known about since the 18th century, is said to have jumped the species barrier into cattle as a result of contaminated sheep carcasses being rendered down into meat and bonemeal fed to cows. This is called the scrapie hypothesis.

The competing theory is that BSE originated as a genetic mutation in a single cow sometime in the 1970s. The mutation in the cow's prion gene generated infectious prion proteins that were then transmitted to other cattle when that cow's carcass was rendered into meat and bonemeal. This is the spontaneous-mutation hypothesis.

Both ideas invoke meat and bonemeal as the means of sustaining the epidemic, which is now beyond dispute. John Wilesmith, a veterinary epidemiologist at the Government's Central Veterinary Laboratory, was the first to establish the role of meat and bonemeal in a seminal scientific paper published in 1988. He explained how the epidemic had spread within the cattle population - from eating the infected remains of other cattle - but the study did not explain how BSE got into the cattle population in the first place.

Pinpointing the origin of BSE is not the same as finding the cause of the epidemic. One is about finding how BSE infected the first cow, the other is about how this first cow managed to pass on the infection to others. Finding the cause is no arcane matter of little relevance to today. Knowing the cause means that scientists can be absolutely sure that BSE cannot happen again.

The six scientists on the government-appointed committee, chaired by Professor Gabriel Horn of Cambridge University, identified how important it is to resolve the issue. The committee's Review of the Origin of BSE, published last week, says: "The BSE epidemic has already caused untold human suffering; it has severely damaged the farming and associated industries in the UK, and also put these industries in other countries at risk. It is important for future generations that we understand and learn from the lessons of the recent past."

The committee also identified the enigma that lies at the heart of the BSE story. "Two of the major puzzles of the BSE epidemic are that the disease probably began sometime in the 1970s to early 1980s, and that it began in Britain."

The Horn committee points out that neither the scrapie hypothesis nor the spontaneous-mutation one really explains: why Britain and why then? Scrapie -contaminated sheep must have been fed to cattle for decades since rendering was introduced in the 1920s, and in many other countries other than Britain. Equally, the chances are that spontaneous genetic mutations leading to BSE must have occurred elsewhere. America, for instance, has 10 times the number of cattle, so why did the US, which also recycled cattle remains in feed, not get BSE?

A clue that might resolve the problem was found in the same scientific paper of John Wilesmith that pinpointed meat and bonemeal as the cause of the epidemic. Wilesmith's study found that the risk of exposure to the BSE agent was about 30 times greater for calves than for adult cows.

The second clue came from scientific papers dating back 30 years that detailed changes to the feeding practices of young calves of two weeks old, which Britain almost alone seems to have pioneered.

The Horn committee has established that Britain began to introduce meat and bonemeal to these very young calves as a cheaper replacement to the powdered milk and soya bean supplements commonly used in other countries. "During the 1970s, feed compounders in the UK began to introduce MBM (meat and bonemeal) into the high-protein pelleted rations fed to artificially reared calves from the dairy herd, typically beginning in the first two weeks of life," the Horn report says.

Only in Australia was there a similar move to feeding meat and bonemeal on weaning calves. And, as the report points out, Australia is scrapie- free. "These considerations imply that the UK was the only country in which scrapie was endemic, where significant amounts of meat and bonemeal were fed to very young calves, and this practice began in the 1970s," the report says.

Another factor that may have played a part was a change in rendering practices, involving lower temperatures and the use of solvents. Changes in rendering practices were long held as a possible reason for why sheep scrapie should have suddenly got into meat and bonemeal, but this idea was rubbished by the BSE Inquiry. In fact, the Inquiry dismissed the entire scrapie hypothesis as "fallacious".

The Horn committee, however, does not take this approach, arguing that the spontaneous-mutation hypothesis favoured by the Inquiry does not in itself make much sense. "There are several reasons for treating with caution the evidence on which this conclusion of the Inquiry is based," it says.

In summary, the scientists led by Professor Horn believe that an "an unusual concatenation of events" occurred in the UK during the 1970s and 1980s: "The diet of many calves was changed so that meat and bonemeal was included in their starter rations. Furthermore, the meat and bonemeal is likely to have included a relatively high level of scrapie-infected material. Changes in rendering processes may have resulted in a small but clinically significant increase in the degree of infectivity of this material in meat and bonemeal."

Young calves of just two weeks of age, which would normally still be suckling, might be unusually susceptible to the BSE agent and this should be tested in experiments, says the Horn committee. If this is proven, it might be the evidence that finally clinches the theory.

It would be sublime vindication for John Wilesmith, who was criticised by the BSE Inquiry for pursuing the scrapie hypothesis even though he was the first scientist to identify correctly that meat and bonemeal was the means by which BSE was spread. Although the Horn committee has not proved the origin of BSE, the six scientists have come closer than anyone since Wilesmith to nail the ultimate cause of the disease.


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