Bovine spongiform encephalopathy (BSE) — commonly referred to as “mad cow disease” — is a transmissible spongiform encephalopathy (TSE) that affects cattle. BSE first was described in cattle in the United Kingdom (U.K.) in 1986 but has been found in an additional 24 countries since that time. BSE cases in countries outside the U.K. appear to be a result of importation of live cattle that were harvested, rendered and fed back to ruminants, or importation of contaminated meat and bone meal derived from BSE-infected U.K. cattle for use as a protein supplement in cattle feed. As highlighted by the assessment of geographical BSE risk in Europe however, on behalf of the European Commission, most of the primary cases arising from such importations were never recognized. With few exceptions, cases were usually identified only after further recycling in the importing country, a decade or more after BSE was probably first introduced.
It initially was thought that, unlike some other TSE such as scrapie in sheep, there was only one strain of the disease-causing agent (prion) that caused BSE. In recent years evidence has been mounting that, in addition to classical BSE, two previously unrecognized types of BSE exist, although at an extremely low level.