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Animal Disease and Human Health Risk



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Is There Risk of Mad Cow Disease in the U.S. or Other Non-European Countries?

Even in the U.S., a country that officially states that there is no evidence of BSE, there have been reports that are less than comforting. One piece of evidence comes from American cases of Transmissible Mink Encephalopathy. Five outbreaks of this disease affecting thousands of mink have been documented in the U.S. The disease bears a striking similarity to both BSE and CJD, causing rapid dementia and death with spongiform brain changes.

Three of these five outbreaks occurred in large mink farms that had a practice of feeding their mink "downer" or "non-ambulatory" cattle. These synonymous terms refer to cows that die in the field or fall down and are unable to rise and stand up without support. Deemed unfit for human consumption, such cows have been used as a food source for mink and other livestock. (Of note, diseased cattle may have been food sources for the mink in the other two outbreaks as well).53 In one outbreak, the only animal protein that had been consumed by the mink had come from a U.S. downer cow and almost the entire mink herd died from a spongiform encephalopathy.54

Dr. Mark Robinson and colleagues at Washington State University have demonstrated that mink can get a spongiform encephalopathy by eating BSE-infected cattle tissue.55 The original research report describes the transmission as occurring with "relative ease" even by the oral route. In contrast to this, it has not been possible to infect mink by feeding them scrapie-infected sheep tissue. One chilling possibility is that U.S. cows are harboring the BSE prion and that the mink had then contracted the disease from this source.


Government Search for Prion Diseases Has Narrow Focus

How could BSE exist in cattle in view of all of the U.S. Department of Agriculture (USDA) reassurances to the contrary? Some are suggesting that the USDA is being far too narrow in its focus. Current BSE surveillance in the U.S. largely focuses on recognizing the British form of BSE. To this end, the U.S. government has attempted to provide thorough education to veterinary practitioners, diagnostic labs, and veterinary colleges. The USDA has distributed videotapes of British cattle with "mad cow" disease and microscopic slides showing BSE infection as it appears in England. They have even sent specialists to Great Britain to get first hand experience in disease recognition.56 The USDA admits to what may have been an oversight up to this point: as most BSE surveillance in the U.S. is based on the presumption that clinical signs and neuropathology would be the same as that seen in Great Britain.

There is growing evidence that the USDA surveillance methods may be misguided. The research suggests that BSE may cause other types of illness besides the "mad cow" syndrome afflicting British cows. Some of this research comes from USDA researchers themselves. Dr. Cutlip and colleagues made some remarkable observations when they infected American cattle with scrapie from American sheep. They were able to transmit the disease by injecting a suspension of scrapie-infected sheep brain tissue into the brains of calves. The calves did develop BSE but it was very different from that seen in England. The animals did not show the typical "mad cow" signs that cattle display in Britain. There was no aggressiveness, increased excitability, or accentuation of sensory reactions that are typical in the British variant. Furthermore, when the brains of the affected cattle were studied microscopically, they did not show the usual spongiform changes. The diagnosis could be made with certainty only by virtue of special tests that demonstrated prions in their brains. The authors concluded that "undiagnosed scrapie could contribute to the 'downer-cow' syndrome and could be responsible for some outbreaks of transmissible mink encephalopathyˇ¦"57

There is other indirect evidence that supports the possibility that BSE has infected American cattle but looks different from the British variant. Studies in other animal species have now demonstrated that different strains of prions do exist. For example, a British researcher found that when he injected the prions from an infected group of goats into mice, those mice developed an encephalopathy (brain disease) characterized by drowsiness. The prions from a different group of infected goats also caused brain disease when injected into mice; however, instead of becoming lethargic, the diseased mice turned hyperactive!58 The message was clear: there were apparently at least two different strains of prion-related illness in goats. Although the disease looked the same in the goats, when it was transmitted to another species (in this case the mice) they would develop one of two very different diseases.

Such research has raised an important question. If a prion disease in goats could give rise to different-appearing diseases in mice, could sheep scrapie result in more than one type of cow disease? As we have already seen, the answer seems to be yes. One strain or type of BSE may surface in the U.S. as a cause of "downer cow" disease. Another BSE strain is present as mad cow disease in England.


References
53 Robinson MM, Hadlow WJ, et al. Experimental infection of mink with bovine spongiform encephalopathy. J Gen Virol 1994 Sep;75 ():2151-2155.

54 Marsh RF, Bessen RA, et al. Epidemiological and experimental studies on a new incident of transmissible mink encephalopathy. J Gen Virol 1991 Mar;72 ( Pt 3):589-594.

55 Robinson MM, Hadlow WJ, et al. Experimental infection of mink with bovine spongiform encephalopathy. J Gen Virol 1994 Sep;75 ( Pt 9)():2151-2155.

56 USDA: APHIS (Animal and Plant Health Inspection Service). Bovine Spongiform Encephalopathy: Implications for the United States. A Follow Up. February 1996. Centers for Epidemiology and Animal Health. Fort Collins, Colorado. p. 13.

57 Cutlip RC, Miller JM, et al. Intracerebral transmission of scrapie to cattle. J Infect Dis 1994 Apr;169(4):814-820.

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



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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