Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, is a fatal neurodegenerative disease (encephalopathy) in cattle that causes a spongy degeneration of the brain and spinal cord. BSE has a long incubation period, about 2.5 to 8 years, usually affecting adult cattle at a peak age onset of four to five years, all breeds being equally susceptible. BSE is caused by a misfolded protein—a prion. In the United Kingdom, the country worst affected, more than 180,000 cattle were infected and 4.4 million slaughtered during the eradication program.