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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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