Prion diseases: clues found for jump to new species

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Because we have further confirmed that prion disease can adapt to new species, and because we’ve shown that process is slow and difficult to detect, it may be time to rethink the practice of giving animals feed made from the byproducts of other animals.”

– Richard Race, research veterinarian


WASHINGTON – Although scientists believe mad cow disease did spread from cattle to people in a few instances in the Great Britain, they don’t really understand how that might have happened.

Some clues have been discovered, however, in a study designed to bring the mad cow transfer into sharper focus by examining the process by which scrapie is transferred from hamsters to mice.

“We found that the adaption is a prolonged and subtle process, and the early stages of it are very difficult to detect,” said Dr. Bruce Chesebro, who lead the team studying the problem at the National Institute of Allergy and Infectious Diseases’ Rocky Mountain Laboratories in Montana.

The results of are reported in the current issue of the “Journal of Virology.”

Scrapie and mad cow are examples of the rare, mysterious, and fatal brain diseases known as transmissible spongiform encephalopathies.

Also known as prion diseases, they include chronic wasting disease in deer and elk and Creutzfeldt-Jakob disease in humans.

Clump in brain. The hallmark of TSE diseases is misshapen protein molecules that clump together and accumulate in brain tissue.

Normal forms of these molecules, called prion protein, reside on the surface of brain cells, although no one knows their proper function. Abnormal prion proteins are the likely cause of the brain damage that occurs in TSE diseases.

Scientists believe the misshapen prion proteins somehow induce normal prion proteins to form incorrectly. These abnormal molecules may spread the disease to new individuals.

Alternatively, some scientists still believe that the disease may be initiated by a virus.

Dr. Chesebro and his team first inoculated mice with a strain of hamster scrapie, then watched the mice closely over a period of years.

Not detectable. The infected mice never became sick, but the scientists found that the hamster scrapie agent persisted in them mice for years at levels too low for standard lab tests to detect.

When the injected brain extracts from the mice into hamsters, the hamsters all got scrapie.

“We can’t be certain these results would apply to other forms of TSE disease and other species,” Chesebro said, “but the fact that it did happen suggests TSE diseases may be more widespread than we thought.

He indicated that more sensitive diagnostic tests are now required, and noted that researchers around the world are already working on such laboratory tests.

Eventually adapted. The scrapie research also showed that, under the right conditions, scrapie did gradually adapt over a period of one to two years to cause illness in mice.

The scrapie agent never caused illness in the original group of mice, but when transferred to additional groups, the disease grew stronger over time, making the newly infected mice sick.

“The scrapie seemed to learn how to deal with this new species, and it worked much better,” said veterinarian Richard Race. “In additional rounds of mice it replicated faster, and even became more lethal to them.”

Varying symptoms. Furthermore, the scrapie adapted in different ways in individual mice. Incubation periods varied widely, and the disease affected different parts of the brain.

“It was not always the same pattern of adaptation,” Chesebro said. “As the disease spread, there was a fanning out of many possibilities.”

The researchers say their results might lead public health officials to reconsider the practice of giving animals feed made from the byproducts of other animals.

Some evidence suggests that cattle in Great Britain contracted mad cow when they ate feed made from the bone meal of sheep infected with scrapie.

“Because we have further confirmed that prion disease can adapt to new species, and because we’ve shown that process is slow and difficult to detect, it may be time to rethink this feeding practice,” Race said.

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