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BSE - Mad Cow Disease
BSE - Mad Cow Disease

Introduction to BSE 

What is BSE?

Bovine Spongiform Encephalopathy (BSE) is the bovine (cattle) form of a group of diseases called transmissible spongiform encephalopathies (TSEs). TSEs are chronic degenerative diseases that affect the central nervous system of the infected animal. They are known to occur in 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 U.S. 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 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.

Information from this section comes from the following sources:

  • Veterinarian Office of Michigan State University
  • “Emerging and Exotic Diseases of Animals” – Iowa State Univ – College of Veterinary Medicine

BSE – the risk to humans 

USDA, one of the federal agencies responsible for the safety of certain meat and poultry products, as well as animal health, maintains the risk to human health that might stem from eating BSE-infected animals is extremely low in the United States. This is due in large part to the programs they have had in place for several years to detect the presence of BSE, to prevent the spread of the disease through the cattle population, and to reduce the likelihood that animal products harboring the infectious agent make their way into the human food supply. More information about these regulatory programs, which also involve the U.S. Food and Drug Administration, is provided on the Federal Initiatives page.

As BSE is the bovine (beef) form of a transmissible spongiform encephalopathy (TSE), there also are several TSEs expressed in humans: Creutzfeldt-Jakob disease (CJD), variant-CJD (vCJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), fatal familial insomnia (FFI) and Kuru. Since 1996, strong evidence has accumulated for a causal relationship between outbreaks of BSE in cattle and the variant Creutzfeldt-Jakob disease (vCJD). Both disorders are invariably fatal brain diseases with unusually long incubation periods measured in years and are caused by an unconventional transmissible agent (neither virus nor bacteria). The U.S. Centers of Disease Control (CDC) provides comparison of the naturally occurring CJD and the BSE-induced vCJD on its Web site, though it is important to note that these two diseases ARE NOT the same and that classical CJD has not linked to consumption of beef.

Since variant CJD was first reported in 1996, a total of 221 patients with this disease from 11 countries have been identified. As of October 2009, variant CJD cases have been reported from the following countries: United Kingdom, France, Ireland, the United States, Spain, the Netherlands, Portugal, Canada, Italy, Japan, and Saudi Arabia. Two of the three U.S. cases, two of the four cases from Ireland, and the single cases from Canada and Japan were likely exposed to the BSE agent while residing in the United Kingdom. One of the 23 French cases may also have been infected in the United Kingdom. There has never been a case of vCJD that did not have a history of exposure within a country where this cattle disease, BSE, was occurring.

It is believed that people who have developed vCJD became infected by consuming cattle products contaminated with the BSE agent. Research has shown that the agent accumulates in specific tissues: brain, spinal cord, retina, dorsal root ganglia (nervous tissue located near the backbone), distal ileum (part of the small intestine) and the bone marrow. Thus, muscle meat (steaks and hamburger) from infected animals are believed to be safe for human consumption. Still, an animal suspected of being infected with BSE is prohibited from entering the human food supply. The BSE agent has not been detected in blood, milk or dairy products

Information in this section came from the following sources:

  • Centers for Disease Control


Systems that protect public health and the beef industry 

When the first native-born North American case of BSE was diagnosed in Canada in May of 2003, the United States closed its northern border to cattle and beef products; cattlemen, dairymen and others directly involved with the cattle industry took an interest. U.S. consumers became more interested in December 2003 when BSE was discovered in a Holstein that had been slaughtered in Lake Moses, Washington. BSE was discovered in this cow because of an existing BSE surveillance program in which suspect animals (including downer cattle) were tested for presence of the prion agent. By the time tests confirmed presence of BSE, the affected dairy cow’s meat had entered the food supply, its risk materials had been rendered, and the meat products distributed. The discovery led to public awareness of the systems and programs that have long been serving to protect both public health and agricultural industries from foreign animal disease.

While a number of cases (18 in Canada and three in the U.S) have been found in North America since 1993 and confirm that BSE is present, the actual number of infected animals present in the cattle population is believed to be extremely low. The United States and Canada have conducted surveillance for BSE since 1992 at rates that have met or exceeded international standards. There is no doubt that the steadily increasing intensity of surveillance has contributed to the probability of finding a case of BSE.

The Federal government has a number of programs to protect public health and the beef industry from BSE, most dating from the early 1990s.

Preventing the spread of BSE to, and in, the cattle population:
Ruminant feed ban: The primary initiative for preventing the spread of BSE to U.S. cattle and within the cattle population, should it get into the country, is the ruminant feed ban. The inclusion of rendered mammalian protein in feed for ruminants was prohibited in August 1997. FDA extended the feed ban to prohibit the inclusion of high risk cattle materials in feed for all animal species from October 26, 2009.

Keeping BSE prions out of human food:
Prohibition on “Downers”: Until December 30, 2003, “downer” cattle could be slaughtered for human food once they had passed inspection for signs of disease by a USDA veterinarian, while a sample of brain tissue was taken for BSE analysis as part of the surveillance program. Under the newer rule, cattle that are downers for any reason may not be slaughtered for human food.

Getting Central Nervous System (CNS) tissue off the table: Three primary methods of accomplishing this are

  • banning the brain and spinal cord tissue of animals 30 months of age or older from the human food supply,
  • banning the use of air-gun stunning, which can cause CNS tissue to move into and to contaminate muscle tissue, and
  • requiring processors to show routinely that tissue separated from the carcass using advanced meat recovery systems (AMR) does not contain CNS tissue.

Detecting BSE in the US cattle population:  After the first US case of BSE in 2003, the government developed an enhanced surveillance program to determine the probability of BSE in United States cattle. Under the enhanced surveillance program, 647,045 samples were collected from 5,776 unique sites across the United States between June 1, 2004 and March 17, 2006. Of these samples, two were confirmed positive (0.0003% of the sampled population). The USDA used this data to develop a maintenance surveillance program testing approximately 40,00 animal per year.

Click here to learn more about the USDA Enhanced Surveillance Program and the monthly totals of animals tested.

Responding efficiently to the detection of BSE: The federal government has a BSE Response Plan that defines the actions agencies will take when BSE is detected – to contain the disease, trace the source, identify potential spreaders, indemnify farmers and protect public health. The degree of their success has a direct impact on beef economic markets.

The highly integrated nature of the North American beef industry requires that the United States, Canada and Mexico adopt a coordinated approach to address both the regulatory and trade aspects of BSE challenges. Maintaining consumer confidence in the safety of beef is fundamental to the management of BSE and remains a top priority among the three governments endeavoring to improve the international approach to BSE.

For more information and resources on this topic, please visit Federal Initiatives.

National Animal Health Laboratory Network: This laboratory network was developed in 2002 as a way to improve the testing and surveillance of domestic animal diseases, such as BSE, that are a potential threat to animal and public health and the national economy. For more information and resources on this topic, please visit the EDEN NAHLN page

Information in this section came from the following sources:

  • Canadian Food Inspection Agency


Educational resources 


Last Updated:4/30/2012 11:30 AM

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