Mad cow disease, or bovine spongiform encephalopathy (BSE), is a fatal brain disorder that occurs in cattle and is caused by some unknown agent. In BSE, the unknown agent causes the cow's brain cells to die, forming sponge-like holes in the brain. The cow behaves strangely and eventually dies. The connection between BSE and humans was uncovered in Great Britain in the 1990s when several young people died of a human brain disorder, a new variation of a rare brain disorder called Creutzfeldt-Jakob Disease (CJD), which typically strikes elderly people. The new variation was called new variant Creutzfeldt-Jakob Disease (nvCJD), was similar to BSE and its connection to BSE was based on the following findings:
- The nvCJD victims had lived in areas where outbreaks of BSE had occurred in cattle years earlier. No victims were found in areas without BSE outbreaks.
- The brains of nvCJD victims had proteins called prions (pronounced "pree-ahnz") that were similar to those from the brains of BSE-infected cows, but different from those found in victims of classic CJD.
- The time between the BSE outbreaks and the deaths of the victims was similar to the time that it takes for Creutzfeldt-Jakob disease to develop.
- Brain tissue from BSE-infected cows caused experimental animals to develop symptoms and brain tissue disorders similar to those of the nvCJD victims.
The British government concluded that BSE was probably the cause of nvCJD, and that the victims contracted the disease probably by eating meat from BSE-infected cows.