Mad cow disease, scientifically known as bovine spongiform encephalopathy (BSE), is a fatal neurological disease in cattle caused by an abnormal form of a prion protein that destroys the brain and spinal cord.
The disease was first identified in Great Britain in 1986, although research suggests that the first infections may have spontaneously occurred in the 1970s.
The disease is believed to be spread by feeding calves meat and bone meal contaminated with BSE or scrapie, a prion disease in sheep. Illnesses similar to BSE also have been seen in elk, deer and exotic zoo animals.
There were 36,000 diagnosed cases of mad cow disease in Great Britain in 1992, during that height of a mad cow epidemic there. Twenty-nine cases were diagnosed worldwide in 2011, according to the American Meat Institute.
Humans do not get mad cow disease but there is strong epidemiologic and laboratory data linking a rare, fatal, degenerative human brain disease called Creutzfeldt-Jakob disease (CJD) to eating BSE-contaminated products. Tissues most likely to carry the infection include the brain and spinal cords of sick animals.
Uncontaminated muscle meat, such as steaks, as well as ground beef and other products are considered safe.
There is no scientific evidence to suggest that milk and dairy products carry the agent that causes BSE, according to the U.S. Department of Agriculture.
There are about 200 cases per year of CJD in the United States. The disease usually appears in later life and runs a rapid course, according to the National Institutes of Health.
There are three major types of CJD, according to NIH.
Sporadic CJD is the most common type, accounting for 85 percent of cases. People affected with this type have no known risk factors for the disease.
Hereditary CJD accounts for 5 percent to 10 percent of cases and affects people with a family history of the disease or those who test positive for a genetic mutation associated with the illness.
The acquired form of CJD accounts for fewer than 1 percent of cases. It is transmitted by exposure to infected brain or nervous system tissue, usually through certain medical procedures, NIH said.
(Reporting By Lisa Baertlein in Los Angeles; Editing by Bob Burgdorfer)