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Comments on the Vermont sheep and TSE

Originally published in the June 2001 issue of Marker, quarterly magazine of the National Colored Wool Growers Association.

This is a very controversial issue, and in spite of what the media has portrayed, there is a different side to the story. I know that many of you will not want to listen to what I have to say here. However, I sincerely hope some of you will.

Transmissible spongiform encephalopathies, or TSE's, are caused by prions (pronounced pree-ons). Prions are extraordinary agents that cause a number of infectious, genetic, and spontaneous disorders. They are resistant to heat, radiation and common disinfectants.

The known prion diseases in animals are: Scrapie/sheep, TME/mink, CWD/elk, and BSE/cows. Human diseases include: CJD, GSS, FFI, Kuru, and Alpers syndrome. Prion diseases are called 'transmissible spongiform encephalopathies' because the brain develops large vacuoles or holes, lesions, and protein deposits. They are always fatal.

Like from a science-fiction movie, this infectious agent is just a small protein particle, just a little piece of protein. It's not a bacteria, it's not a virus; a prion is different. It contains no known nucleic acids, no DNA or RNA, and the host's immune system fails to make detectable antibodies. A prion is a modified form of normal cellular protein (PrPc). For some reason this normal protein can be changed or mutated into an infectious form, PrPsc for scrapie for instance; so far there are 15 different 'strains' found in sheep. These mutations can increase the rate of spontaneous conversion of PrPc to PrPsc or increase susceptibility to infection.

Some forms of PrPc are more easily converted to infectious forms than others. When prions are passed from one species to another, disease develops only after a very long incubation time, if at all; but when passed on to other members of the new specie the incubation time can decrease quickly.

In the laboratory as little as 1/2 a gram (that's 0.018 ounce) of BSE-infected cow brain tissue fed orally to sheep caused BSE! This is orally. When scientists study TSE's in mice they inject the brains of mice with infected material, but that's different. These sheep infected with BSE showed the same signs of scrapie and normal tests can't differentiate between the two. This is truly a frightening disease. Remember, while we've known about scrapie for a couple of hundred years, BSE didn't exist before 1986 and it's only been since 1994 that we learned BSE could infect humans. Problems seem to have begun when we started feeding animals to animals, like the cannibalistic tribes in New Guinea with Kuru. It's the mutations and spontaneous events that make these agents so alarming, plus the long incubation periods.

In 1952 the USA closed its doors to the importation of sheep from countries affected by scrapie. The only exception was Canada. The Vermont sheep were imported during a brief period of time in 1996 when imports were allowed only from certain countries. A total of 65 East Friesian milk sheep were imported from Belgium and the Netherlands to farms in Vermont. Importations were immediately stopped with the publication about experimental oral transmission of BSE to sheep.

Then nine of the originally imported Vermont sheep showed changes in the brain, four were found positive for TSE, all were probably exposed to feed contaminated with BSE agents in Europe.

USDA officials did not act on their own concerning the Vermont flocks. They consulted with scientists and prominent researchers in the TSE field - all concurred and supported USDA's actions. Many believe that any possibility of risk from these sheep should be stopped by purchasing the sheep and destroying the carcasses.

To my knowledge the four animals were tested by four different methods. The first two tests were run in 1999 and second two in 2000.

  1. The first was a histopathology test. In this test thin slices of brain tissue are examined under the microscope. These sheep did show typical degenerative neurological changes. While an indication of an abnormality, this method is not diagnostic for TSE.
  2. Both the brain tissue and third eyelid tissue were tested by immunochemistry methods which detect abnormal prion proteins indicative of TSE. Both tissues were negative.
  3. The Western-blot analysis is the approved test authorized by APHIS. This method detects abnormal prion proteins in a different way. Western-blotting, also called immunoblotting, is a universal tool used in research world-wide. It is used to identify specific antigens recognized by certain antibodies. The sheep all tested positive for TSE.
  4. The capillary electrophoresis assay, a new and still experimental method of separating components of blood, can supposedly detect abnormal prion proteins. The four sheep tested positive.
Some asked why a test by Prionics wasn't used as suggested. Prionics is a company in Switzerland that makes a product called Prionics-Check. This product is a commercial surveillance tool that detects BSE-specific prion proteins in cattle brain tissue and stem cells in the slaughterhouse. This product supposedly "combines the reliability of the western-blot technique with the requirements for mass screening", the company's words. So it's basically the same thing - sort of like baking a cake from scratch versus using a boxed product.

It will take 2-3 years of testing in mice to tell us which TSE disease was in these sheep. Several generations of mice must be infected with samples from these sheep to see distinctive patterns in lesion profiles, incubation times, and definitive pathology. In a couple of years when the mouse testing is complete, hopefully the results will show that the sheep were just infected with scrapie, but will it be one of the 15 strains that we know in this country or a different one, maybe a more virulent one? But then what if it is BSE? And worse yet - what if it's a different TSE than either scrapie or BSE.

I was motivated to enroll in the voluntary scrapie program almost five years ago because of what one of the veterinarians here at work said to me at coffee one day. He said that there were several sheep maladies that were making the headlines of late, including OPP and Johne's, but none were as frightening as scrapie. His words haunt me every day...

For further reading:

Prusiner, Stanley. 1995. "The Prion Diseases". Scientific American. Volume 272, No.1, pages 48-57.

Balter, Michael. "On the hunt for a wolf in sheep's clothing". Science Magazine, 2000, Volume 287, pages 1906-1908.

The NOVA program The Brain Eater; Do Prions exist?

Richard Rhodes, "Deadly Feasts", 1997, Simon & Schuster; NY, NY. Excellent reading!

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