Scientists disagree on cause of infection

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http://seattlepi.nwsource.com/local/154558_prion30.html

 

 

Tuesday, December 30, 2003

 

Scientists disagree on cause of infection

 

By TOM PAULSON

SEATTLE POST-INTELLIGENCER REPORTER

 

Drowned out by calls for more testing and regulation to protect against mad cow

disease are major disagreements over what causes the illness, how it is

transmitted and even what parts of a contaminated cow may be unsafe to eat.

 

Resolving these differences between the U.S. Department of Agriculture and other

scientists may be critical to determining how the nation should respond to its

first known case of BSE, bovine spongiform encephalopathy.

 

" There are many different ways you can get the disease, " said Dr. Giuseppe

Legname, a neuroscientist who works at the University of California-San

Francisco lab of the researcher credited with discovering what many believe is

the cause of mad cow disease.

 

Mad cow disease, like the similar Creutzfeldt-Jakob disease in humans, is

strongly linked to the presence in brain tissue of an abnormal protein called a

" prion. " Whether the prion is the root cause of the disease, however, is still

hotly debated.

 

The scientist who characterized and named the prion (to mean " infectious

protein " ), Dr. Stanley Prusiner, won the 1997 Nobel Prize in medicine for

discovering " an entirely new genre of disease-causing agents. "

 

Prusiner and Legname are among those calling for testing all, or nearly all,

cattle for mad cow disease.

 

They contend the current U.S. Department of Agriculture method of testing only

the brains of select cattle using stained antibodies that react to specific

prions does not detect lower levels of infection, or all the varying " strains "

of prions.

 

The California researchers also believe the infection may be occurring

" spontaneously " -- as Creutzfeldt-Jakob does in humans -- and not necessarily

coming only from feed made of contaminated animal products, as most scientists

believe.

 

" The USDA is saying the beef muscle is safe, but we don't really know that, "

Legname said.

 

The UCSF team last year found prions in mice muscle tissue and claim to have

developed a much more sensitive, rapid test that they have made commercially

available.

 

Prions, unlike viruses or bacteria, are believed to somehow replicate without

having to inject genes into the cells they infect. Prusiner won the Nobel for

showing that these abnormal proteins bind to normal proteins and pass on their

defect, doing most of their damage in the brain where holes appear to create a

spongelike appearance.

 

While Prusiner and like-minded colleagues believe prions are the primary cause

of these spongiform diseases -- mad cow, Creutzfeldt-Jakob or scrapie in sheep

-- not everyone is convinced.

 

" I don't think we know yet, " said Dr. Bruce Chesebro, an expert on this class of

diseases at the National Institutes of Health's Rocky Mountain Laboratories in

Hamilton, Mont.

 

" The nature of the infectious agent is still not completely clear. "

 

Chesebro said it would be premature to rule out the possibility that prions are,

in fact, symptoms or co-conspirators in a yet-to-be-discovered viral infection.

 

" I think it could very well be a virus, " he said.

 

Those who claim that a virus cannot withstand the heat of cooking the meat,

Chesebro said, haven't taken a close look at some of the microbiological

residents found in the geothermal pools at Yellowstone National Park.

 

Chesebro said there are a number of findings involving " prion diseases " that

don't strongly support this infectious protein as a primary cause.

 

The evidence, Chesebro said, also does not support Legname and Prusiner's claim

that BSE may be occurring spontaneously in addition to being spread through

contaminated feed.

 

For one thing, he said, the number of Britain's human cases linked to BSE

declined after the ban on using brain-tainted feed for cattle.

 

Australia and New Zealand's success in eliminating scrapie -- the sheep

equivalent for BSE -- followed by many years with no new cases indicates the

infection can be totally eliminated, he said.

 

Chesebro said he believes sample testing of animals remains a good way to

monitor for the disease even if prions are not the primary cause of the

infection.

 

But he said the scientific community needs to continue to explore the mysteries

associated with these diseases.

 

Dr. William Hueston, director of the University of Minnesota's center for animal

health and food safety, agrees with Legname that standard tests likely miss some

infections. But he doesn't think more testing with better methods will do much

to reduce the risk.

 

" Testing more animals won't improve food safety, " said Hueston. " That just gives

a false sense of security. "

 

Japan's total testing scheme for cattle, Hueston said, is largely a public

relations effort launched in 2001 after that country's first case of BSE.

 

" The most likely source for this infection is contaminated feed, and blocking

this source will be the only effective means of blocking this infection, " he

said.

 

While there may be no direct evidence showing that BSE-tainted feed is the only

way to spread mad cow disease -- or to refute Prusiner's hypothesis of

" sporadic " BSE -- Hueston said the epidemiological evidence is overwhelming.

 

" The key to this is to get all brain and spinal cord out of the food supply, " he

said.

 

There's no good scientific reason, Hueston said, to allow human consumption of

adult cow brain by humans (or other animals) if it is considered a disease

carrier.

 

DIFFERING VIEWS OF MAD COW DISEASE

 

USDA:

Mad cow disease results from cattle eating feed contaminated with infected cow

brain or spinal cord -- a practice banned in 1997. Only limited testing (of

brain matter only, in the United States) is needed because the feed ban is the

main barrier to protect against spread of the disease. Muscle tissue, such as

steaks, is considered safe.

PRION PIONEER:

Prions are abnormal proteins believed to spread disease like viruses or bacteria

but through a unique method of infection. Dr. Stanley Prusiner, who won the

Nobel Prize for discovering them, believes prions are likely to be in the muscle

tissue of infected cattle even if not detected in standard tests. Prusiner says

mad cow disease may occur " spontaneously " just as Creutzfeldt-Jacob disease does

in humans, not necessarily from contaminated feed. He believes nearly all cattle

should be tested using more sensitive methods.

SKEPTICS:

Mad cow, Creutzfeldt-Jacob, scrapie and similar diseases involve prions, but

they may not be the fundamental cause. The actual cause is unknown. Some experts

believe a viral cause is a possibility. Testing can indicate presence of the

infection and protect against spreading the disease, but the tests likely do not

identify the primary cause.

P-I reporter Tom Paulson can be reached at 206-448-8318 or

tompaulson

 

 

 

 

 

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