Transmissible spongiform encephalopathies (TSEs) are chronic degenerative diseases that affect the central nervous system of the infected animal. They are known to occur in cattle (bovine spongiform encephalopathy or BSE), sheep and goats (scrapie), deer and elk (chronic wasting disease), mink (transmissible mink encephalopathy) and domestic cats (feline spongiform encephalopathy).

Cattle suffering from BSE are often irritable and can react violently to stimuli that normally do not impact healthy animals. As the disease progresses, animals become dizzy and disoriented and eventually lose the ability to walk. This erratic behavior is responsible for the disease being commonly referred to as “mad cow disease.”

Unlike foot and mouth disease (which spreads rapidly from animal to animal and from herd to herd), there is no evidence that BSE is contagious or spreads by contact between cattle or by contact between cattle and other species. BSE is not believed to be caused by a bacteria, virus, parasite, fungus, toxin, or chemical. Currently, the most accepted theory is that BSE is caused by a modified form of a normal nerve-cell surface component known as a “prion protein.”

If eaten, these modified prion proteins can accumulate in the brain and other tissues, including the spinal cord, subsequently causing normal prion proteins to change to the modified form. These modified proteins continue to accumulate in the brain to the point where they damage brain cells, eventually leading to neurological disease and death. Why or how this substance changes to become disease-producing is still unknown.

Whether normal or abnormal, prion proteins primarily are found in neurological tissue, including the brain and spinal cord. Thus, the disease spreads when a susceptible animal eats the brain, spinal cord or nervous tissue of an infected cow where the abnormal prion protein has accumulated, or when a susceptible animal eats proteins rendered from these tissues.

BSE was first diagnosed in the United Kingdom in 1986, and was likely caused when cattle were fed rendered protein that contained prions from the carcasses of scrapie-infected sheep or cattle with a previously unidentified transmissible spongiform encephalopathy (TSE). The practice of using products such as meat-and-bone meal as a source of protein in cattle rations had been common for several decades. Restrictions on ruminant protein in feed for ruminant animals were first imposed in England in 1988. In August 1997, the US Food and Drug Administration (FDA) established regulations that prohibit the feeding of most mammalian proteins to ruminants. Similar restrictions were put in place in Canada at the same time. For more information, visit US FDA Federal Initiatives. It is believed that the BSE infected cow in Washington state was exposed to feed containing the prion proteins before the feed bans went into place in North America.

Educational resources

Land-Grant University Web Pages
Iowa State University Extension — BSE Information Sources
NDSU Extension Service — BSE Frequently Asked Questions

Federal Agency and Association BSE websites

USDA BSE Surveillance in the United States

USDA Animal and Plant Health Inspection Service (APHIS)

USDA Food Safety Inspection Service (FSIS)

Food and Drug Administration (FDA)

Centers for Disease Control (CDC) — BSE and Creutzfeldt-Jakob Diseases (vCJD)

International Agency BSE Web Pages

BSE – British Department of the Environment, Food and Rural Affairs (DEFRA)
BSE Statistics from Britain
BSE – Canadian Food Inspection Agency
BSE – Geographical Distribution (OIE)