(Reuters) - The following are key dates in the spread of mad cow disease in Canada and the United States.

The U.S. Department of Agriculture confirmed on Tuesday the fourth U.S. case of mad cow, formally known as bovine spongiform encephalopathy (BSE), in a dairy cow in central California.

First discovered in Great Britain in 1986, mad cow disease destroys the brain of cattle and a similar fatal disease can affect people who eat meat from infected cattle, according to scientists. More than 150 people and 184,000 cows, mainly in Britain and Europe, have died from the diseases.

The outbreak in the United Kingdom peaked in January 1993, with nearly 1,000 new cases per week. Over the next 17 years, the annual numbers of BSE cases dropped sharply, with fewer than 200 cases diagnosed worldwide in 2007 and 29 cases in 2011, according to the American Meat Institute.

The disease is believed to be transmitted when cattle eat contaminated livestock feed.

1993

Canada confirms one beef cow is infected with mad cow disease and says the animal was imported in 1987 from Britain, which had an extensive outbreak of the disease in the late 1980s. The animal and the herd it came from are destroyed.

May 20, 2003

Canada confirms its first native case of mad cow disease in an Alberta cow. The discovery prompts the United States and other nations to ban imports of Canadian cattle and beef.

December 23, 2003

USDA announces first U.S. case of mad cow disease in a Washington state dairy cow. Investigators later determine the Holstein was born in April 1997 in Alberta, Canada. The animal was exported to Washington state in September 2001 as part of a herd of some 81 dairy animals. Some 40 nations ban shipments of American beef.

January 2, 2005

Canada confirms second indigenous case of mad cow disease in an eight-year-old Alberta dairy cow. Officials try to track some 141 beef and dairy cattle that may have eaten contaminated feed between 1995 and 1997 on the same farm, including one animal that was shipped to the United States in February 2002 for immediate slaughter.

January 11, 2005

Canada confirms third native case of mad cow disease in an animal born in southern Alberta. The Charolais animal was born in the spring of 1998, after a 1997 ban on using cattle remains as a protein supplement in cattle feed.

June 2005

USDA confirms the second U.S. case of mad cow disease and the first case in an American-born animal. The disease was found in a 12-year-old beef cow in Texas.

January 20, 2006

Japan suspends imports of U.S. beef after inspectors discover three cartons of veal contain parts of spinal cord material from young calves. The USDA acknowledges that the shipment violated a U.S.-Japan trade agreement and promises to tighten U.S. meat inspection procedures so it does not happen again.

January 23, 2006

Canada announces another case of mad cow disease in a six-year-old Holstein-Hereford cow from a dairy farm in Alberta. The animal was born after the 1997 feed ban took effect.

Canada had a total of five BSE cases in 2006 and nine between 2007 and 2010, according to the U.S. Centers for Disease Control and Prevention.

March 2006

USDA announces the third U.S. case of mad cow disease. It was the second case in a native-born animal born before feed controls were implemented. The infected cow was found in Alabama.

March 2011

Canada confirms the disease in an Alberta dairy cow. The case is believed to be Canada's 18th.

April 2012:

USDA confirms the fourth U.S. case of mad cow disease.

(Reporting by Julie Ingwersen; editing by Dale Hudson and Andre Grenon)