Bovine spongiform encephalopathy (BSE), referred to as "mad cow disease," is a chronic degenerative nervous system disease affecting cattle. The disease was first diagnosed in 1986 in Great Britain. BSE is so named because of the spongy appearance of the brain tissue of infected cattle when sections are examined under a microscope. Affected animals may display changes in temperament, such as nervousness or aggression, abnormal posture and difficulty in rising, decreased milk production, or loss of body weight despite continued appetite. Affected cattle die or are killed. The incubation period (the time from when an animal becomes infected until it first shows signs of disease) is from 2 to 8 years. Following the onset of clinical signs, the animal’s condition deteriorates until it dies. This process usually takes from 2 weeks to 6 months. Currently, there is no test to detect the disease in a live animal; veterinary pathologists confirm BSE by postmortem microscopic examination of brain tissue or by the detection of the abnormal form of the prion protein.
Since November 1986, over 178,000 head of cattle have been diagnosed with BSE in Great Britain. The epidemic peaked in January 1993 at approximately 1,000 new cases reported per week. Agricultural officials in Great Britain have taken a series of actions to eradicate BSE, including making BSE a notifiable disease, prohibiting the inclusion of mammalian meat-and-bone meal in feed for all food-producing animals, prohibiting the inclusion of animals more than 30 months of age in the animal and human food chains, and destroying all animals showing signs of BSE and other animals at high risk of developing the disease.
The identification in 2003 of a BSE case in Canada, and the subsequent identification later that year of a BSE case in the United States that had been imported from Canada led to the concern that indigenous transmission of BSE may be occurring in North America. In response, the US Department of Agriculture (USDA) implemented additional safeguards to minimize the risk for human exposure to BSE and on July 1, 2004, initiated a 12- to 18-month-long intensive testing program for BSE among cattle at relatively high risk for the disease (e.g., non-ambulatory cattle). A US-bred cow was found to be BSE-positive in June 2005 in Texas.