BOVINE SPONGIFORM ENCEPHALOPATHY

(BSE) also called mad cow disease a fatal neurodegenerative disease of cattle. Bovine spongiform encephalopathy is caused by an infectious agent that has a long incubation period, between two and five years. Signs of the disease include behavioral changes, such as agitation and nervousness, and a progressive loss of muscular coordination and locomotive function. In advanced stages the animal frequently loses weight, shows fine muscular contractions over its neck and body, walks in an abnormal and exaggerated manner, and may isolate itself from the herd. Death usually follows within a year of the onset of symptoms. No treatment or palliative measures are known. First recognized in cattle in the United Kingdom in 1986, BSE became epidemic there, particularly in southern England. Cases also were reported in other parts of Europe and in Canada. The disease is similar to the neurodegenerative disease of sheep called scrapie. It is thought to have arisen when cattle were fed high-protein supplements made from ruminant carcasses and offal (the trimmings of butchered animals). Although animal remains had been used as a source of dietary supplements for several decades without problems, modifications to the rendering processspecifically, reduction in the temperatures used and discontinuance of certain solventsin the early 1980s were followed by the outbreak of BSE. The timing of events suggested that the modified process no longer incapacitated the infectious agent. In 1988, on the basis of this inferred connection, the British government banned the use of animal-derived protein supplements. BSE, scrapie, and similar diseases in other species, such as Creutzfeldt-Jakob disease and kuru in humans, are categorized as transmissible spongiform encephalopathies. They are so named because the brain tissue of organisms with the disease becomes pitted with holes in a spongelike pattern. The cause of these diseases is attributed to an unusual infectious agent called a prion. The prion is a modified form of a normally harmless protein found in the brain of mammals and birds. In its aberrant form, however, the prion protein builds up in nerve cells as it multiplies. This accumulation somehow damages these cells and leads to the characteristic neurodegeneration. After the emergence of BSE, concern grew over a possible relationship between the animal disease and the occurrence of Creutzfeldt-Jacob disease in people. Beginning in the mid-1990s a new variant form of Creutzfeldt-Jacob disease (nvCJD) took the lives of dozens of people in Europe. In experiments with mice, researchers found that prions from human cases of nvCJD caused a disease pattern similar to that caused by prions from cows with BSE. The result suggested that the human infection is linked to BSE.