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.

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.