Bovine Spongiform Encephalopathy (BSE), also known as "mad cow disease," is a progressive and fatal neurologic disease of cattle. It is caused by an abnormal prion protein that accumulates in the nervous tissue of infected animals. There are two forms of BSE: classical BSE and atypical BSE.

• Classical BSE: This form of BSE develops as a result of foodborne exposure to prions via contaminated animal-source proteins (meat and bone meal) in cattle rations. It is the most well-known form of BSE and was first diagnosed in the UK in 1986. During the peak of the UK outbreak in 1992, 37,280 cases were reported in a single year. Most cases of classical BSE are diagnosed in cattle 3-6 years old, and the incubation period after exposure is approximately 2-8 years.

• Atypical BSE: This form of BSE occurs spontaneously at very low levels in all cattle populations, particularly in older cattle, usually eight years of age or older, and does not appear to be associated with contaminated feed. Bioassay data support the hypotheses that these strains are biologically distinct from classical BSE and might not pose a risk to humans.

BSE is not a contagious disease and therefore is not spread through casual contact between cattle or with other species. The primary route of spread of classical BSE infection in cattle is feed contaminated with the infectious agent. The BSE agent is extremely resistant to heat and to normal sterilization processes, and it does not evoke any detectable immune response or inflammatory reaction in host animals. There is no treatment or vaccine to prevent BSE, and affected animals may display changes in temperament, abnormal posture, incoordination and difficulty in rising, decreased milk production, or loss of condition without noticeable loss of appetite.

An ongoing, comprehensive interagency surveillance program for BSE has been in place in the U.S. since 1990. The program targets cattle populations with the highest probability of BSE detection, including animals with neurological symptoms consistent with BSE and cattle 30 month...