- 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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