'Mad cow' diseases still baffle scientists

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'Mad cow' diseases still baffle scientists

 

 

28 May 2003

By Maggie Fox, Reuters

 

 

WASHINGTON — Mad cow disease still mystifies scientists because it is not

caused by a virus, bacteria, or other microbes; does not alert the immune

system;

and can jump from species to species. The case of a single Canadian cow

infected with mad cow disease — known formally as bovine spongiform

encephalopathy

(BSE) — has panicked meat traders, closed cross-border cattle trade, and set

off a flurry of investigations to find out how it happened.

 

At least one U.S. expert believes there will be no mystery behind the case.

Dr. Paul Brown of the National Institute of Neurological Disorders and Stroke

says it is likely the 6-year-old animal got BSE just as British animals did:

from eating contaminated feed.

 

" The most plausible explanation is that sometime in the late 1990s this cow,

and probably other cows in the same herd at the same time, were exposed to

contaminated feed imported from a country that didn't get BSE until later, "

Brown said. " It is conceivable that a spontaneous case of BSE occurred, but

nobody

has ever seen that. "

 

BSE is a member of a class of diseases called transmissible spongiform

encephalopathies (TSEs). They are believed to be caused by misfolded proteins

called prions. Since these proteins occur naturally in the body, they do not

alert

the immune system as a virus or bacteria would to prompt a fight against the

disease.

 

Recent research suggests the " bad " prions can cause others to go bad simply

by touching them.

 

Proteins are made by cells following instructions laid out in the genes. But

like a cardboard box, a protein must be folded to function, and they

sometimes get folded into the wrong shape. Usually a cell will recognize this

and

cause it to be broken up.

 

But Susan Lindquist of the Whitehead Institute at the Massachusetts

Institute of Technology and colleagues found in late 2002 that this does not

always

happen. They discovered that if the misfolded prions are not broken up quickly

enough, they accumulate and alter the cell's metabolism, killing it.

 

In the brain this kills neurons. When the neurons die they break apart,

releasing more prions into the system.

 

There is a genetic susceptibility to TSEs. For example, human

Creutzfeldt-Jakob disease (CJD) occurs randomly in about one in 1 million

people. There is a

gene mutation that runs in families and causes 5 to 10 percent of cases of

CJD. CJD, which is incurable and always fatal, can also be passed on by tissue

transplants and, in theory, in blood.

 

People can get a form of CJD called vCJD from eating infected beef products.

 

" In contrast to the traditional forms of CJD, vCJD has affected younger

patients (average age 29 years, as opposed to 65 years for CJD), has a

relatively

longer duration of illness (median of 14 months as opposed to 4.5 months), and

is strongly linked to exposure ... to BSE, " the World Health Organization said

in a statement.

 

TSEs affect cats, mink, elk, deer, cattle, sheep, and a range of other

animals. Rabbits, horses, and dogs seem to be resistant, but it is not clear

why.

 

TSEs can be passed on in food. Cattle are believed to have become infected

with BSE when they were fed the rendered remains of sheep, which have their own

TSE called scrapie.

 

Rendering or processing food can destroy the prions but only with

difficulty. Just cooking will not do the trick.

 

 

 

 

Source: Reuters