By Maggie Fox, Health and Science Editor

WASHINGTON, Sept 12 (Reuters) - A rare genetic mutation may underlie some cases of mad cow disease in cattle and its discovery may help shed light on where the epidemic started, U.S. researchers reported on Friday.

The mutation, in an Alabama cow that tested positive in 2006 for bovine spongiform encephalopathy, or BSE, is identical to one that causes a related brain-wasting disease in humans and that suggests BSE may sometimes arise spontaneously in cattle.

The finding also may lend credence to a 2005 theory that the BSE epidemic in cattle could be traced to feed contaminated with either cattle or human remains scavenged from India's Ganges River, the researchers report in the Public Library of Science journal PLoS Pathogens.

BSE or mad cow disease swept through British dairy herds in the 1980s, forcing the destruction of millions of animals. No one ever found where it came from but most experts thought it may have come from cattle feed that contained the remains of sheep infected with a similar disease called scrapie.

Cattle were never known to develop BSE before the epidemic, but some experts had argued they may have. This report lends credence to that idea.

BSE, scrapie and a human version called Creutzfeldt-Jakob disease, or CJD, are all brain-destroying illnesses called transmissible spongiform encephalopathies. In some cases, animals or people that eat brain and nervous system material from victims of these disease can develop them, too.

It is passed along by misfolded infectious protein fragments called prions.

A very rare disease called variant CJD has been found in people who ate infected beef products. Fatal and incurable, it has affected just 167 people so far.

SPONTANEOUS CASES

CJD is also known to pop up spontaneously in the human population. A genetic mutation causes the disease in one in a million people globally.

U.S. Department of Agriculture researchers Jurgen Richt and Mark Hall of Kansas State University tested the brain of the Alabama cow and found a mutation identical to the prion gene mutation that causes some cases of CJD.

It is probably rare in cattle, found in fewer than one in 2,000, they said in the report, published at http://dx.plos.org/10.1371/journal.ppat.1000156.

But the animal passed along its mutation to its heifer, which suggests it is inherited.

The finding shows that cattle can develop BSE without ever eating contaminated feed, the researchers said. Cattle with similar mutations can be expected in cattle herds worldwide and might be discovered during BSE testing, they said.

The finding supports at least two theories about the origin of the BSE epidemic -- that British cattle that developed BSE before the disease was discovered were fed to other cattle, or that infected feed from India was the source.

"It is well known that large amounts of mammalian protein material were imported from India to the U.K. during the relevant time period (late 1970s and early 1980s)," the researchers wrote.

Most countries now ban the use of meat and other parts from mammals in food for cattle.

It will be important to continue measures meant to protect people who eat beef products, they said. These include banning the use of brain, spinal cord and other tissues known to carry the infectious prions. (Editing by Bill Trott)

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