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News Flash: Beef imports banned
amid Mad Cow Disease scare!
Such headlines arise when there is concern that cattle in some region may have been exposed to Mad Cow Disease. What exactly is Mad Cow Disease and why should humans care about sick cows? Let's take a closer look at this disease as well as some related ones.
Mad Cow Disease (or Bovine Spongiform Encephalopathy) is an infectious disease of cattle transmitted not by a virus or bacteria, but by an abnormal form of a type of protein called a prion. The disease is associated with progressive damage to the brain accompanied by changes in motor control and behavior. The affected parts of the brain develop a spongy appearance.
Prions are not transmitted through the air by a cough or a sneeze or other casual contact. They can be transmitted, however, by consuming or directly contacting infected tissues. Eating infected tissue? How would that happen? One theory that has gained fairly wide acceptance is that Mad Cow Disease first developed when cattle were given feed containing meat and bone meal from sheep infected with a similar prion disease called scrapies. The cattle developed worsening problems with movement and balance as well as erratic behavior. The abnormal prions seem to be nearly indestructible, able to pass on the disease even after the commercial processing of the feed, cooking, boiling, or even irradiation. Once cattle were infected then byproducts from these cattle, also used in animal feed, could also transmit the disease.
The largest outbreak occurred in the United Kingdom in 1993, when the cause and transmission of Mad Cow Disease was just beginning to be understood. About 185,000 cases in cattle have been identified in the U.K. and surrounding countries since then. Only about a dozen cases have been identified in North America in that time period. It is hoped that changes in the use of animal byproducts in cattle feed will prevent further outbreaks of the disease.
Prion diseases (or "transmissible spongiform encephalopathies") similar to Mad Cow Disease have been discovered in a wide variety of species: goats, sheep, mink, squirrels, elk, deer, cats, rhesus monkeys, lemurs, cheetahs, puma, lions, tigers, and a variety of exotic zoo ungulates like kudu and elands. Even humans can develop a prion disease known as Creutzfeldt-Jakob Disease.
Creutzfeldt-Jakob Disease (CJD) is normally a rare, but fatal brain disorder. In the United States, typically only 200-300 new cases are identified each year, mostly in individuals in their late fifties or older. About 5-15% of these cases are familial; a mutated gene leading to the production of abnormal prions is passed on from generation to generation. Most other cases are considered "sporadic CJD" for which the cause is not yet known. There may be a long incubation period before the first signs of CJD, but once symptoms appear the disorder usually progresses at a rate much faster than other dementias such as Alzheimer's disease. The spongiform changes in brain tissue cause the brain to shrink and can impair all the various functions of brain - motor coordination, memory, judgment, personality, speech, vision, and, eventually, basic life-sustaining processes.
The spread of Mad Cow Disease was associated with an unusual increase in cases of CJD. Unlike sporadic CJD, these cases appeared to be linked to the consumption of beef from cattle with undiagnosed BSE and involved individuals much younger than the typical CJD patient. This new form of CJD became known as "variant Creutzfeldt-Jakob Disease" (vCJD). Because appearance of the symptoms of CJD is sometimes delayed, additional CJD cases related to the Mad Cow Disease outbreak in the United Kingdom may surface in the future.
No treatment that can cure or prevent the progression of Creutzfeldt-Jakob Disease has yet been discovered. Currently treatment focuses on alleviating symptoms and making an individual with CJD as comfortable as possible.
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For more information about Mad Cow Disease and Creutzfeldt-Jakob Disease, visit the following Web sites:
Copyright © 1996-2007, Eric H. Chudler, University of Washington