Bovine Spongiform Encephalopathy (BSE)

I

Introduction

Bovine Spongiform Encephalopathy (BSE) or Mad Cow Disease, degenerative brain disease of cattle linked to a variant form of Creutzfeldt-Jakob disease (CJD), a fatal degenerative brain disease in humans. The clinical signs of BSE in cattle include loss of coordination, a staggering gait, difficulty in rising, decreased milk production, and weight loss. Affected animals also show signs of behavioral changes, such as nervousness, aggression, and a lack of interest in their surroundings. The period from exposure and infection to the onset of the disease, known as the incubation period, is from two to eight or more years. Once clinical signs develop, animals progressively deteriorate and die within several months.

II

Origins of BSE

BSE was first identified in the United Kingdom in November 1986. By 2003 more than 185,000 cases of the disease had been confirmed worldwide, the vast majority of them in the United Kingdom. Studies have suggested that more than 1 million animals were likely infected during this period and entered the human food supply. These animals went undiagnosed or were slaughtered before clinical signs developed. The appearance of BSE in native-born cattle has been recorded in many other European countries, including Austria, Belgium, Czech Republic, Denmark, France, Germany, Greece, Ireland, Italy, Luxembourg, The Netherlands, Poland, Portugal, Slovakia, Slovenia, Spain, and Switzerland. Cases in native-born cattle outside of Europe, although rare, have been confirmed in Canada, Israel, and Japan. BSE has also been recorded in the Falkland Islands (Islas Malvinas) and Oman in cows imported from the United Kingdom. In 2003 the first case of BSE was confirmed in the United States in a dairy cow that originated in Alberta, Canada.

Autopsies of affected cattle reveal telltale holes in the brain tissue that give it a spongiform, or spongy, appearance resembling Swiss cheese. Similar spongiform brain changes have been recognized in humans, as in the case of CJD, for more than a century. Similar changes have been seen in sheep for more than 200 years due to the brain disease known as scrapie. Together these diseases and a few others are called the spongiform encephalopathies, and they are caused by prions, normal proteins that fold into abnormal shapes and become infectious and pathogenic (disease-causing). As prions accumulate in the brain, they cause in still largely unknown ways the spongiform change to neurons (nerve cells) and produce, also in largely unknown ways, the characteristic clinical signs of the disease.

Health officials identified animal feed containing recycled animal tissue as the source of the infection that led to the BSE cattle epidemic in the United Kingdom. This type of animal feed was routinely fed to dairy cows as a protein supplement for most of the 20th century. But in the 1980s rendering (the cooking method used to process hide, bones, and other inedible tissue after slaughter) changed in a way that may have enabled the survival of the BSE infectious agent.

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There are two theories regarding the origin of the BSE prion. One is that scrapie prions were introduced into cattle feed from animal feed containing sheep brains and other sheep byproducts. Subsequently, brain tissue taken from cows infected with BSE was included in protein supplements, and this worsened the epidemic. The second theory is that the BSE prion originated via a spontaneous mutation. Such a mutation would have led to a protein that folded into the abnormal form. This abnormal prion protein would have built up in the central nervous system of the cow and then would have spread when tissues from this cow were included in protein supplements fed to other animals. The epidemic widened when the brains and other rendered products from infected cattle were used in protein supplements distributed throughout Britain and then elsewhere across Europe and in Japan and Canada.

III

Preventive Measures in Britain

To prevent the spread of BSE, the British government introduced compulsory destruction of suspect animals and the burning of their carcasses beginning in 1988. The feeding of rendered animal tissue to cattle was banned in the United Kingdom in July 1988. As a result of these efforts, monitors working for the United Kingdom Ministry of Agriculture recorded a persistent decline in the incidence of BSE after 1992, when the number of confirmed cases peaked at more than 37,000. By 2002 fewer than 1,150 cases of BSE were confirmed in the United Kingdom, and by 2003 this number had fallen to 159.

After the initial report of the disease, there was fear and speculation that it might be transferable to humans through beef products. The medical community was aware of the similarity of CJD symptoms to those of BSE and was also aware that a related disease, known as kuru, was spread by ritualistic cannibalism among New Guinea tribes.

IV

BSE and Human Disease

In March 1996 the British Ministry of Health announced the discovery of ten cases of a unique type of CJD, called variant CJD (vCJD). This type of CJD differed from the classical form in that all the patients were under the age of 42. (The classical form of the disease typically develops around age 65.) The patients displayed unusual psychiatric problems, distinct brain tissue changes identified during autopsies, and the patients had no family history of CJD. The British government found that the patients contracted the disease by eating BSE-infected meat products. The government had previously denied any possible link between BSE and human disease.

Since 1996 a number of studies have confirmed that BSE in cattle can be transmitted to humans and cause vCJD. By 2004 studies had linked BSE in cattle to more than 145 human cases of vCJD in Europe, mostly in Britain. Several lines of evidence confirmed this causal relationship: Molecular markers of vCJD prions from humans were shown to be the same as BSE prions from cattle. When scientists injected monkeys and mice with brain tissue from BSE-infected cows or brain tissue from vCJD-infected humans, the animals developed the same unique type of brain degeneration. This degeneration is distinguishable from the degeneration following injection with brain tissue from cases of the classical form of CJD.

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