Bovine spongiform encephalopathy (BSE), also known as mad cow disease, was first seen in Great Britain in 1985. It is not transmissible from cow to cow; rather, cows become infected by eating feed that includes mammalian proteins, which contain prions. Infected adult cows develop signs of the disease very slowly. It may take two to eight years from the time a cow is infected until it shows any signs of the disease. Early symptoms include a change in behavior and attitude, such as nervousness or apprehensiveness. The cow may become aggressive toward other cattle or humans, eg, kicking when being milked. As the disease progresses, the cow finds muscle control and coordination difficult, which leads to difficulty in standing and walking. Cattle that become disabled to the point where they cannot stand are called “downers.” Weight loss and decreased milk production also occur. Eventually the cow dies, if not slaughtered first.