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Animal Disease and Human Health Risk



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Prions: Frightening Carriers of Disease

One might think that processed meat and bone meal from sheep or cows could not carry any kind of serious infection. After all, carcasses are boiled as part of the process of "rendering" that ultimately converts the dead animals into feed for living creatures. It is true that the high temperatures typical in that rendering process would be sufficient to kill most disease-bearing organisms. However, the type of infectious agent that is responsible for BSE is very unusual. Unlike most illnesses that are caused by bacteria or viruses, BSE is caused by something called a prion (pronounced "pree-on"). The destructive powers of prions are outlined in Figure 3: Destructive Power of Prions.

Prions are very resilient proteins that are particularly worrisome because they are resistant to most forms of disinfection. They retain their infectivity even after normal sterilization procedures such as those using heat and ionizing radiation. In fact, prions are not even destroyed by temperatures well above the boiling point. In a laboratory test of the scrapie prion, some infectivity still remained after a full hour of exposure to dry heat at 680 degrees Farenheit. (360 degrees Celsius.).9 Prions are also impervious to freezing and drying.10 Much of the reason for their hardiness is no doubt related to their unusual composition. Prions have no genetic material and consist entirely of protein.11 They are composed of a complex combination of thousands of amino acids. Since some sterilization processes--like ionizing radiation--work by destroying an organism's genetic material, prions can not be harmed by these measures.


How Do Prions Cause Disease?

One of the most perplexing questions about prions is how an agent with no genetic material can cause disease. The renowned prion researcher, Dr. Stanley Prusiner has come up with a likely explanation based on his years of research. In an excellent article in Scientific American, Prusiner explained the current understanding of how prions do their dirty work.12 Research suggests that these prion proteins are similar in structure to proteins that occur naturally in the brains of humans and animals. The prions differ from those similar normal proteins, however, in slight differences in their three dimensional shape.13 The subtle differences allow these proteins to combine into abnormal aggregates that are responsible for the brain changes produced by a malady called Creutzfeldt-Jakob disease (CJD) and other related diseases.14 Furthermore, when abnormal prion proteins come into contact with normal brain proteins, they can influence the normal proteins to take on the prion protein's three dimensional shape. This change in shape appears to set up a chain reaction in which the changed proteins later influence neighboring proteins to do the same. The result is a progressively devastating and ultimately fatal disease that has no known treatment.

The characteristics of prions are summarized in Figure 4: Characteristics of Prions.


Humans Can Get Transmissible Encephalopathies

It is not only animals that contract the prion-induced transmissible spongiform encephalopathies. Humans can also get three such diseases, as listed in Figure 5: Prion-Induced Diseases Found in Humans.15

These rarely diagnosed diseases can be thought of as types of fast-acting Alzheimer's disease. CJD, the most common of the three, typically occurs in late middle age and generally causes death within six months of diagnosis.16 The source of most cases of CJD has not been determined.


References
9 Brown P, Liberski PP, et al. Resistance of scrapie infectivity to steam autoclaving after formaldehyde fixation and limited survival after ashing at 360 degrees C: practical and theoretical implications. J Infect Dis 1990 Mar;161(3):467-472.

10 World Health Organization Fact sheet: Bovine Spongiform Encephalopathy (BSE); Fact sheet N113; March 1996 (available via Internet at http://www.who.ch)

11 Prusiner SB. The prion diseases. Sci Am 1995 Jan;272(1):48-51, 54-57.

12 Prusiner SB. The prion diseases. Sci Am 1995 Jan;272(1):48-51, 54-57.

13 Prusiner SB. The prion diseases. Sci Am 1995 Jan;272(1):48-51, 54-57.

14 Taubes G. Misfolding the way to disease. Science 1996 Mar 15;271(5255):1493-1495.

15 Pratt K. Bovine Spongiform Encephalopathy. Update. Animal and Plant Health Inspection Services (APHIS). U.S. Department of Agriculture, 1996 p. 1.

16 Feinberg MB. Slow Virus And Retrovirus Infections. In: Scientific American Medicine (CD-ROM), 1995.


Notice of Credit
The article above is compliments of the Uchee Pines Institute, Seale, Alabama, a teaching and treatment facility devoted to natural remedies. For mor information, call 334-855-4781,e-mail: ucheepine@csi.com, or visit their Website: http://www.ucheepines.org.



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