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Pathology => Human Diseases => Bovine Spongiform Encephalopathy
Bovine Spongiform Encephalopathy
Bovine Spongiform Encephalopathy or Mad Cow Disease (BSE), degenerative brain disorder of cattle. Symptoms in cows include loss of coordination and a typical staggering gait. Affected animals also show signs of senility, for example, lack of interest in their surroundings, the abandonment of routine habits, disinterest in feed and water, or unpredictable behavior. Affected cattle show symptoms when they are three to ten years old.
First identified in Britain in November 1986, over 170,000 cases have since been recorded there. Sporadic incidences have been confirmed in other European countries, with Switzerland (over 260 cases) and Ireland (over 260 cases) identifying the largest number. It has also been recognized in Canada, where cases are confined to dairy cows imported from Britain. BSE has not been officially confirmed in the United States or any other major milk-producing country.
Autopsies of affected cattle reveal holes in the brain tissue that give it a spongy, or spongiform, texture. Similar spongiform diseases have been recognized in humans (for example, Creutzfeldt-Jakob disease or CJD) for over a century and in sheep (scrapie) for over 200 years. The cause of BSE is unproven, although there is strong evidence that prions, which may be infective proteins, are the agent. Other hypotheses suggest that prions work with an as yet undetected virus to cause the infection.
Recycled animal tissue, which had been routinely fed to British dairy cows as a protein supplement, was identified as the source of the infection. The European Commission's Scientific Veterinary Committee and the world control body, the Fédération Internationale des Epizooties (FNE) believes that BSE was originally spread from sheep's brains infected with scrapie and that its spread was accidentally accelerated by the ingestion of brain tissue taken from cows that had become infected with BSE.
Following through with this fodder transmission theory, the British government introduced compulsory destruction of suspect animals and their carcasses beginning in 1988. The feeding of animal tissue to cows was banned in Britain in July 1988 and since mid-1992, monitors working for the United Kingdom Ministry of Agriculture have recorded a persistent decline in the number of confirmed cases. It is estimated that the program will eradicate BSE in Britain by the end of 1999.
Since the initial report of the disease, there has been fear and speculation that it might be transferable to humans through milk or beef products. The appearance of CJD in several dairy farmers in Britain in the early 1990s heightened the alarm. The medical community was aware of the similarity of CJD symptoms to those of BSE and the documented fact that a related disease, known as kuru, was spread by ritualistic cannibalism among New Guinea tribesmen. In late 1990, consumer concern over the transmission of BSE to humans triggered a temporary drop in British beef consumption. A similar scare struck Germany in mid-1994.
In late March 1996 the British Ministry of Health announced the discovery of 10 cases of a newly described type of fatal CJD (new variant CJD, or nvCJD) in which the victims had distinct brain tissue symptoms, were all under the age of 42, and had no hereditary record of the disease. The government admitted that the victims may have contracted the disease through contact with BSE-infected cattle before the eradication of suspected animals had taken effect. The announcement represented an about-face in the stance of the government, which had previously denied any possible link between BSE and human disease.
Following the British government's announcement, beef consumption plummeted in Britain and the European Union banned British beef imports worldwide. At least 16 countries from Europe, the Middle East, Africa, and Asia initiated independent bans. In the United States, the Department of Agriculture made plans to step up its testing of cattle for BSE, although the disease has never been reported in U.S. cattle. British beef has been banned in the U.S. since 1989.
Although the hypothesis of the cattle-to-human transmission path is not yet proven, it has been strengthened by the results of two studies released in 1997. Laboratory mice injected with brain tissue from BSE-infected cows and another group injected with brain tissue from nvCJD-infected humans both developed the same symptoms of brain degeneration, such as walking abnormally; moreover, for both groups, the infection was ultimately fatal. In addition, researchers found the same prion strain in both groups of mice. In one of the studies, researchers injected a third group of mice with tissue from humans who had died of classical CJD; these mice developed no symptoms and survived the trial.
In early 1998 scientists found that the suspected disease agent, prion protein, appears not only in the brain tissue of infected humans but also in other organs and in blood. Subsequently there has been a growing concern that nvCJD might be transmitted through blood products-for example, components of human serum used for vaccines in children. In 1998 the British government announced that a large number of blood products will be tested to determine whether they might be infected with the disease. A ban on the export of British blood products, as well as of a blood protein called albumin, may eventually follow.
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