mad cow disease

n. A fatal disease of the central nervous system in cows caused by defective proteins called prions which cause infected nerve cells to replicate the prion instead of the proteins usually manufactured within the nerve cell. These proteins cause the death of the cells and create spongy holes in the brain where cells have died. The disease has an incubation period of more than a year and usually occurs when the tissue of the nervous system is eaten by another animal. The disease became widespread in Great Britain during a period in which it was common to grind up the brains of cows in manufacturing cattle feed. The few human cases are variants of Kreutsfeldt-Jakob disease caused by eating diseased beef as prions are found in the bone marrow, blood, spleen, and lymph nodes, as well as in the central nervous system although at much reduced levels. Also called bovine spongiform encephalopathy and, in humans, variant Kreutsfeldt- Jakob disease (v-KJD).