Mad Cow Disease Investigation
Mad cow disease investigation (Forensic Science)
Bovine spongiform encephalopathy (BSE) is one of several spongiform encephalopathies that may be heritable or transmissible. The infectious agent that causes BSE in cattle and the related variant form of Creutzfeldt-Jakob disease (vCJD) in people is novel in that it does not contain any nucleic acid (DNA or RNA) and it does not elicit an immune response, making it difficult to detect other than at autopsy, when spongelike lesions are seen in the brain. The infectious agent appears to be a protein called a prion. Heritable spongiform encephalopathies are caused by mutations in the prion gene, producing abnormal prions that adopt an unusual conformation and clump together over time to cause the brain pathology and neurological symptoms characteristic of the diseases. The diseases can be transmitted to susceptible animals or persons through the insertion of fragments or extracts from diseased tissue into the brain or bloodstream or, much less efficiently, by ingestion.
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Spongiform Encephalopathies and Human Risk (Forensic Science)
Scrapie, found in sheep, is the earliest known spongiform encephalopathy. First noted during the 1730’s in Britain, it is present in most countries of the world, except for Australia and New Zealand. It is passed from animal to animal, probably through contact with saliva, urine, or feces. The infectious agents, abnormal prions, decay slowly in the environment and may take years to dissipate from a contaminated pasture. Scrapie has not been known to infect humans, either through husbandry or through consumption of meat or milk.
Chronic wasting disease is a transmissible spongiform encephalopathy of deer, elk, and moose. It was first observed in 1967 in a captive mule deer in Colorado and has been identified in thirteen U.S. states and two Canadian provinces. No transmission to humans has been documented.
Since appearing in 1986, BSE has been diagnosed in twenty-nine countries. It occurs in cattle between two and eight years of age and is fatal. In the course of the disease, the animals lose coordination and show extreme sensitivity to sound, light, and touch. Although BSE may be transmitted from mother to calf, the major cause of the BSE epidemic in Britain was cattle feed containing contaminated ruminant-derived protein. Sick or dead animals, as well as parts of the carcasses of slaughtered animals not sold for human consumption, are routinely converted to a protein-rich supplement....
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Perspective and Prospects (Forensic Science)
The BSE epidemic arose in Britain because protein supplements fed to cattle were derived from cattle that contained the abnormal prions. In the United States, the protein supplements fed to cattle come primarily from soybean meal. The origin of the infectious agent is likely to have been a spontaneous mutation of bovine prions that led to an abnormal conformation. The abnormal prions were amplified through the feeding of ruminant-derived protein back to cattle. It would be possible to create a BSE epidemic by incorporating the infectious agent, which is difficult to detect and to destroy, in an area’s food, water, or blood supplies. Manufacturing the agent would be a daunting task, however, and malicious use of the agent would not offer instant gratification in view of the long incubation period (years to decades) necessary for the disease to manifest itself.
Continued monitoring of BSE and vCJD is warranted. In addition, surveillance of the food supply should persist to prevent consumption of meat from BSE cattle. Bans on feeding ruminant-derived protein to cattle must be maintained and enforced until methods can be developed to ensure the destruction of the abnormal prions they may contain.
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Further Reading (Forensic Science)
Harris, D., ed. Mad Cow Disease and Related Spongiform Encephalopathies. New York: Springer, 2004. Collection of essays by international experts in their fields provides an introduction to prion biology and up-to-date reviews of BSE in cattle, vCJD in humans, and chronic wasting disease in deer.
Packer, Richard. The Politics of BSE. New York: Palgrave Macmillan, 2006. Former head of the British Ministry of Agriculture, Fisheries and Food provides a full and candid analysis of the BSE outbreak in Great Britain and responses to it. Includes an informative chapter on the official BSE inquiry.
Prusiner, Stanley B. “Detecting Mad Cow Disease.” Scientific American 291 (July, 2004): 86-93. The discoverer of prions describes new tests that may permit rapid detection of abnormal prions in cattle and humans.
_______. “The Prion Diseases.” Scientific American 272 (January, 1995): 48-57. Explains the history of prion diseases, including mad cow disease, and the basis for the prion theory, which Prusiner developed and for which he was awarded the Nobel Prize in Medicine in 1997.
Schwartz, Maxime. How the Cows Turned Mad: Unlocking the Mysteries of Mad Cow Disease. Berkeley: University of California Press, 2004. Provides a history of prion diseases from the description of scrapie in the eighteenth century to modern transmission among animals and humans.
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Mad Cow Disease Investigation (World of Forensic Science)
Bovine spongiform encephalopathy (BSE, also popularly known as mad cow disease) and Creutzfeldt-Jakob disease (CJD, which occurs in humans) are ailments in which the functioning of the brain is progressively impaired.
Beginning in the 1980s in the United Kingdom, mad cow disease has been a sporadic concern in that country and others. By 1992, three cows in every 1,000 in Britain were estimated to have the disease. Then in the winter of 1997, another outbreak led to the slaughter of 100,000 cattle as a measure to stop the spread of the disease. In more recent incidents, detection of the disease in the Canadian province of Alberta in 2003 led to a ban on imports of Canadian beef to the United States. As of mid-2005, the ban is still in effect, although it is anticipated to be lifted before year's end.
The detection of mad cow disease and the determination of the extent of the disease involved a large, coordinated epidemiological (disease-tracking) and forensic science investigation.
Initially, a cow may be suspected of being infected because of behavioral changes, including loss of coordination, clumsy gait, and even the appearance of foam at the mouth (hence the origin of the mad cow moniker). Typically, the suspect cow will be removed from the herd and slaughtered. Then, examination of tissues and fluids commences. These examinations can involve microscopy of tissue sample and the use of antibodies to identify the causative agent.
Mad cow disease is associated with visually abnormal pinpoints (or plaques) in the brain, and in a changed texture of the brain tissue. These alterations are detected when the brain tissue is microscopically examined as part of an autopsy of a cow suspected of having the disease. The brain tissue, particularly in the cortex and cerebellum, becomes filled with large open spaces (vacuoles) and becomes spongy in texture. The "spongiform" part of the BSE descriptor comes from this texture characteristic.
In Canada, cattle have been tagged with an identifying code since 2001. The identifier enables the movement of cattle to be tracked from the herd (and from herd to herd) to the slaughterhouse. This enables the pattern of an illness outbreak, including mad cow disease, to be better investigated.
Mad cow disease, CJD, and maybe even other diseases such as scrapie, transmissible mink encephalopathy, fatal familial insomnia, and kuru, are thought to have a common cause, namely prions. Prions are particles that are made solely of protein. Even though they lack genetic material, they are infectious.
Mad cow disease causes a progressive neurological deterioration in cattle that is similar to the course of CJD in humans. Infected cattle are more temperamental, have problems with their posture and coordination, have progressively greater difficulty in rising off the ground and walking, produce less milk, have severe twitching of muscles, and lose weight even though their appetite is undiminished. The suspected incubation period, the time from when the animal is first infected until symptoms appear, ranges from two to eight years. After appearance of symptoms, deterioration is rapid and the animal dies or is destroyed within six months. The disease is one of a group of related diseases called transmissible spongiform encephalopathies (TSEs) in animals.
Bovine spongiform encephalopathy was confirmed as a disease of cattle in November 1996. Since then, with the exception of cases in Canada and a single case in the United States in 2004, almost all reported cases have been in cattle born in the United Kingdom. Other countries in Europe and Asia have reported BSE, but in far fewer numbers than in the U.K. As of November 2001, the total number of confirmed cases of BSE in U.K. cattle was just over 181,000. In 1993, a BSE epidemic in the U.K. peaked at almost 1,000 new cases per week. While the cause of this near-exclusivity has yet to be conclusively determined, a common practice in the United Kingdom was to feed cattle "offal," the ground up waste from the slaughter process. Cattle feed was also prepared from the ground bones and tissues of sheep, cattle, and other animals, providing a means of delivering prions from infected animals to healthy ones. The exact origin of the prions is not known. Sheep, susceptible to a similar disease called scrapie, known for many years, are considered a likely source.
Until the 1900s, scientists thought that the transmission of the BSE agent to humans did not occur. However, several post-mortem, forensic studies (autopsies and brain tissue examination) conducted in the 1990s debunked this assumption. In 1994, cases of young people (median age was 26 years) with a CJD-type disease began appearing in the U.K., often in related geographical areas. As CJD affects mostly people over age 65, and symptoms differed slightly and developed more slowly in those affected in the new outbreak, the disease was given the distinct name of variant CJD, or vCJD. An intensive investigation was launched that eventually revealed vCJD as most likely caused by eating beef from cattle infected with BSE. As of 2005, 105 cases of vCJD have been identified in young adults mostly in the U.K., with three cases occurring in France and one in Ireland. The largest number of cases occurred during 1999 (27), and has decreased to less than five cases per year afterward, suggesting that the outbreak of the disease is waning. Chances of contracting vCJD by eating beef in the U.K. are very small as of 2005, due to measures implemented more than a decade earlier (longer than the usual vCJD incubation period) to protect the food supply from BSE-infected beef.
As well, studies on mice published in 2004 have cast doubt on the previous view that the infectious agent of mad cow disease is localized exclusively in only the brain, spleen, spinal cord, and lymph tissue. Prions were additionally detected in the kidney, pancreas, and liver tissues of infected mice. This finding has profound forensic implications, since typically an investigation of mad cow disease focuses on examining samples from the brain and the other traditional locations. The presence of prions elsewhere would be overlooked. As there is no conclusive diagnostic test for variant CJD while an affected person is alive, other than a costly and invasive brain biopsy that will offer no benefit for the outcome of the disease, forensic examination of brain tissue at autopsy is the usual method of providing a definitive diagnosis of CJD and variant CJD in humans, and BSE in cattle.
SEE ALSO Animal evidence; Autopsy; Prions.