Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative disease, caused by a prion, that mainly affects cattle. Other ruminants, cats, nonhuman primates and humans are occasionally affected; this disease is called feline spongiform encephalopathy (FSE) in cats, and variant Creutzfeldt-Jakob disease (vCJD) in people. BSE is primarily acquired by eating prion-containing tissues from an infected animal. Cooking and standard disinfection procedures do not destroy this agent. Infected animals or people do not become ill for years; however, the disease is always progressive and fatal once clinical signs develop. BSE was first reported in the United Kingdom in the 1980s. Its origins are unknown; however, the recycling of ruminant tissues into ruminant feed amplified BSE prions and caused an explosive epidemic in the U.K. This epidemic peaked in 1992, with almost 1,000 new cases diagnosed each week. BSE also spread to many European countries, North America, parts of Asia and possibly other areas of the world. Control measures, including restrictions on ruminant feed, have now greatly decreased its prevalence, and cases have become uncommon or rare in many areas. Many countries have also passed new regulations to prevent BSE-containing tissues from entering human food supplies.

As a result of increased surveillance, BSE prions that differ from the prion causing ‘classical’ BSE have been identified at very low levels in cattle populations. The leading hypothesis, at present, is that these atypical prions arise spontaneously in cattle. Some experiments suggest that an atypical prion might have given rise to the BSE epizootic when it was amplified in cattle feed.