BSE (bovine spongiform encephalopathy) is a progressive neurological disorder of cattle that results from infection by an unusual transmissible agent called a prion. The nature of the transmissible agent is not well understood. Currently, the most accepted theory is that the agent is a modified form of a normal protein known as prion protein. For reasons that are not yet understood, the normal prion protein changes into a pathogenic (harmful) form that then damages the central nervous system of cattle.

Research indicates that the first probable infections of BSE in cows occurred during the 1970’s with two cases of BSE being identified in 1986. BSE possibly originated as a result of feeding cattle meat-and-bone meal that contained BSE-infected products from a spontaneously occurring case of BSE or scrapie-infected sheep products. Scrapie is a prion disease of sheep. There is strong evidence and general agreement that the outbreak was then amplified and spread throughout the United Kingdom cattle industry by feeding rendered, prion-infected, bovine meat-and-bone meal to young calves.

The BSE epizootic in the United Kingdom peaked in January 1993 at almost 1,000 new cases per week. Since then, the annual numbers of BSE cases in the United Kingdom have dropped sharply—

• 2 cases in 2015
• 11 cases in 2010
• 225 cases in 2005
• 1,443 cases in 2000
• 14,562 cases in 1995

Cumulatively, through the end of 2015, more than 184,500 cases of BSE had been confirmed in the United Kingdom alone in more than 35,000 herds. Regularly updated numbers of reported BSE cases, by country, are available on the website of the Office International Des Epizooties.

Strong epidemiologic and laboratory evidence exists for a causal association between a new human prion disease called variant Creutzfeldt-Jakob disease (vCJD) that was first reported from the United Kingdom in 1996 and the BSE outbreak in cattle. The interval between the most likely period for the initial extended exposure of the population to potentially BSE-contaminated food (1984-1986) and the onset of initial variant CJD cases (1994-1996) is consistent with known incubation periods for the human forms of prion disease.

The word BSE is short but it stands for a disease with a long name, bovine spongiform encephalopathy. "Bovine" means that the disease affects cows, "spongiform" refers to the way the brain from a sick cow looks spongy under a microscope, and "encephalopathy" indicates that it is a disease of the brain. BSE is commonly called “mad cow disease.”

BSE is a progressive neurologic disease of cows. Progressive means that it gets worse over time. Neurologic means that it damages a cow’s central nervous system (brain and spinal cord).

Most scientists think that BSE is caused by a protein called a prion. For reasons that are not completely understood, the normal prion protein changes into an abnormal prion protein that is harmful. The body of a sick cow does not even know the abnormal prion is there. Without knowing it is there, the cow’s body cannot fight off the disease.

The parts of a cow that are not eaten by people are cooked, dried, and ground into a powder. The powder is then used for a variety of purposes, including as an ingredient in animal feed. A cow gets BSE by eating feed contaminated with parts that came from another cow that was sick with BSE. The contaminated feed contains the abnormal prion, and a cow becomes infected with the abnormal prion when it eats the feed. If a cow gets BSE, it most likely ate the contaminated feed during its first year of life. Remember, if a cow becomes infected with the abnormal prion when it is one-year-old, it usually will not show signs of BSE until it is five-years-old or older.

A common sign of BSE in cows is incoordination. A sick cow has trouble walking and getting up. A sick cow may also act very nervous or violent, which is why BSE is often called “mad cow disease.”

It usually takes four to six years from the time a cow is infected with the abnormal prion to when it first shows symptoms of BSE. This is called the incubation period. During the incubation period, there is no way to tell that a cow has BSE by looking at it. Once a cow starts to show symptoms, it gets sicker and sicker until it dies, usually within two weeks to six months. There is no treatment for BSE and no vaccine to prevent it.

Currently, there is no reliable way to test for BSE in a live cow. After a cow dies, scientists can tell if it had BSE by looking at its brain tissue under a microscope and seeing the spongy appearance. Scientists can also tell if a cow had BSE by using test kits that can detect the abnormal prion in the brain.

People can get a version of BSE called variant Creutzfeldt-Jakob disease (vCJD). As of 2019, 232 people worldwide are known to have become sick with vCJD, and unfortunately, they all have died. It is thought that they got the disease from eating food made from cows sick with BSE. Most of the people who have become sick with vCJD lived in the United Kingdom at some point in their lives. Only four lived in the U.S., and most likely, these four people became infected when they were living or traveling overseas.

Neither vCJD nor BSE is contagious. This means that it is not like catching a cold. A person (or a cow) cannot catch it from being near a sick person or cow. Also, research studies have shown that people cannot get BSE from drinking milk or eating dairy products, even if the milk came from a sick cow.

Sheep, goats, mink, deer, and elk can get sick with their own versions of BSE. Cats are the only common household pet known to have a version of BSE. It is called feline spongiform encephalopathy, and the same things that are being done to protect people and cows are also protecting cats. No cat in the U.S. has ever been found to have this disease.

Prevention Measures against BSE Spread

To prevent BSE from entering the United States, severe restrictions were placed on the importation of live ruminants, such as cattle, sheep, and goats, and certain ruminant products from countries where BSE was known to exist. These restrictions were later extended to include importation of ruminants and certain ruminant products from all European countries.

Because the use of ruminant tissue in ruminant feed was probably a necessary factor responsible for the BSE outbreak in the United Kingdom and because of the current evidence for possible transmission of BSE to humans, the U.S. Food and Drug Administration instituted a ruminant feed ban in June 1997 that became fully effective as of October 1997. As of October 26, 2009, a regulation issued by FDA in April 2009 came into effect establishing an enhanced BSE-related feed ban in the U.S. This enhanced feed ban will further harmonize BSE feed control measures in the U.S. with those in Canada (see below). In addition, FDA continues to enforce its important 1997 mammalian-to-ruminant feed ban through its BSE inspection and BSE feed testing programs.

As of July 12, 2007, an enhanced BSE-related feed ban came into effect in Canada. CFIA established this ban to more effectively prevent and quickly eliminate BSE from Canada. The enhanced ban prohibits most proteins, including potentially BSE infectious tissues known as “specified risk materials” (SRM) from all animal feeds, pet foods, and fertilizers, not just from cattle feed as required by the ban instituted in 1997. The 1997 feed ban in Canada was similar to the feed ban instituted in the United States that same year. As recently reported by CFIA, removing SRM from the entire animal feed system addresses risks associated with the potential contamination of cattle feed during production, distribution, storage, and use. Applying the same measure to pet food and fertilizer materials addresses the possible exposure of cattle and other susceptible animals to these products. With this ban in place, CFIA expects BSE should be eliminated from the Canadian cattle herd by about the year 2017.

In late 2001, the Harvard Center for Risk Assessment study of various scenarios involving BSE in the U.S. concluded that the FDA ruminant feed rule provides a major defense against this disease.

BSE/TSE Action Plan of the Department of Health and Human Services (DHHS)

On August 23, 2001, the Department of Health and Human Services (HHS) issued a department-wide action plan outlining steps to improve scientific understanding of BSE and other transmissible spongiform encephalopathies (TSEs). The action plan has four major components:

Surveillance for human disease is primarily the responsibility of CDC.

Protection is primarily the responsibility of the Food and Drug Administration (FDA).

Research is primarily the responsibility of the National Institutes of Health (NIH).

Oversight is primarily the responsibility of the Office of the Secretary of DHHS.

The U.S. Food and Drug Administration (FDA) is doing many things to keep the food in the U.S. safe for both people and cows. Since August 1997, the FDA has not allowed most parts from cows and certain other animals to be used to make food that is fed to cows. This protects healthy cows from getting BSE by making sure that the food they eat is not contaminated with the abnormal prion.

In April 2009, the FDA took additional steps to make sure the food in the U.S. stays safe. Certain high-risk cow parts are not allowed to be used to make any animal feed, including pet food. This prevents all animal feed from being accidentally contaminated with the abnormal prion. High-risk cow parts are those parts of the cow that have the highest chance of being infected with the abnormal prion, such as the brains and spinal cords from cows that are 30 months of age or older.

By keeping the food that is fed to cows safe, the FDA is protecting people by making sure that the food they eat comes from healthy cows.

The FDA also works with the U.S. Department of Agriculture (USDA) to keep cows in the U.S. healthy and free of BSE. The USDA prevents high-risk cows and cow products from entering the U.S. from other countries. The USDA also makes sure that high-risk cow parts, such as the brains and spinal cords, and cows that are unable to walk or that show other signs of disease are not used to make food for people.

The steps the FDA and USDA have taken to prevent cows in the U.S. from getting BSE are working very well. Only six cows with BSE have been found in the U.S. The first case was reported in 2003 and the most recent case was found in August 2018.

It is worth noting that there are two types of BSE, classical and atypical. Classical is caused by contaminated feed fed to cows. Atypical is rarer and happens spontaneously, usually in cows 8-years-old or older. Of the six U.S. cows found with BSE, five were atypical. The only case of classical BSE in the U.S. was the first one, in 2003, in a cow imported from Canada.

What is BSE?

BSE (bovine spongiform encephalopathy) is a progressive neurological disorder of cattle; its symptoms are similar to a disease of sheep, called scrapie. BSE has been called "mad cow disease." BSE and scrapie both result from infection with a very unusual infectious agent. As of January 2004, more than 180,000 cases of BSE were confirmed in Great Britain in more than 35,000 herds of cattle. The epidemic peaked in January 1993 at almost 1,000 new cases per week. Although the origin of the disease is uncertain it may have resulted from the feeding of scrapie containing meat and bone meal (MBM) to cattle or from feeding cattle MBM derived from a cow or other animal that developed the disease due to a spontaneous mutation. There is strong evidence and general agreement that the outbreak was amplified by feeding meat-and-bone meal prepared from cattle to young calves.

What causes BSE?

The nature of the infectious agent that causes BSE and scrapie is unknown. Currently, the most accepted theory is that the agent is a modified form of a normal cell protein known as a prion. A prion is not a bacterium, parasite, or virus, and thus treatments usually used for treating or preventing bacterial infections (e.g. antibiotics) or viral infections are not effective against prions.

Where is the BSE agent found in cattle?

In cattle naturally infected with BSE, the BSE agent has been found in brain tissue, in the spinal cord, and in the retina of the eye. Additional experimental studies suggest that the BSE agent may also be present in the small intestine, tonsil, bone marrow, and dorsal root ganglia (lying along the vertebral column).

Which countries have reported BSE?

The vast majority of cases of BSE (more than 97% as of 2003) have been reported from the United Kingdom during an epidemic. However, endemic cases have also been reported in other European countries including: the Republic of Ireland, Switzerland, France, Liechtenstein, Luxembourg, Netherlands, Portugal and Denmark. The numbers of reported cases by country are available on the web site of the Office International des Epizooties. These numbers should be interpreted with caution, however, because the intensity and methods of surveillance probably vary over time and by country. In 2003 one case was reported in Canada and one in the United States (in a cow born in Canada).

How was BSE spread?

It is thought that BSE was spread via meat-and-bone meal fed to cattle. The practice of using this material as a source of protein in cattle feed has been common for several decades. In the late 1970s there was a change in the production (rendering) process used to make this meat and bone meal. One hypothesis has been that this change permitted the infectious agent of scrapie (a transmissible spongiform encephalopathy, or TSE, of sheep) to survive the rendering process, and get transmitted to other animals, such as cows, that are fed meat-and-bone meal nutritional supplements. An inquiry by the British government has however, concluded that scrapie infected MBM was not the source of BSE nor was the change in the rendering practices responsible for survival of the BSE agent. Rather, this inquiry has stated that BSE may have originated spontaneously as a result of a genetic mutation and was amplified by the feeding of contaminated MBM to cattle.

What has the British government done in response to the BSE epidemic?

In response to the BSE epidemic, the British Government instituted a series of measures to minimize the risk of disease transmission among both animals and humans. These included a ban on feeding ruminant protein (ruminants are animals, such as cows, sheep and goats) to ruminants (1988), removal of some "high risk" materials (such as brain, spinal cord and intestines) from cattle at slaughter (1989 and 1995), and a ban on cattle over 30 months of age from being used for food (1996). Following institution of these measures, Great Britain has seen a decrease in the number of cattle with BSE from a peak incidence of 36,682 confirmed cases in 1992 to 1044 confirmed in 2002.

Does BSE occur in the US?

According to the Animal Health and Plant Inspection Service (APHIS) of the United States Department of Agriculture, BSE has been detected in one cow in the United States. Following complications of pregnancy this cow was slaughtered December 9, 2003. As part of the APHIS ongoing surveillance of downer animals a brain sample was taken to test for the BSE agent. On December 25, 2003 it was confirmed that this animal tested positive for the BSE agent (on December 23, 2003 this was a "presumptive" case). Following this discovery the USDA and FDA announced additional measures to enhance the US protections against BSE (see: What measures has the US government taken to ensure that people are not exposed to the BSE agent in foods?).

What measures has the US government taken to ensure that people are not exposed to the BSE agent in foods?

The USDA is responsible for the health of US livestock. To prevent BSE from entering the country, the USDA Animal and Plant Health Inspection Service (APHIS) has, since 1989, prohibited the importation of live ruminants from countries where BSE is known to exist in native cattle. On December 12, 1997, APHIS stopped the importation of live ruminants and most ruminant products, including meat, meat-and-bone meal, offals, glands, etc. from all of Europe. FDA is responsible for animal feeds in the US. In August 1997, FDA prohibited the use of most mammalian protein in the manufacture of animal feeds given to ruminants. Following the discovery of one cow with BSE in the US, the USDA and FDA have announced additional measures to enhance protections against the spread of BSE in US cattle and to minimize human exposure to bovine materials that may contain the BSE agent. USDA has issued an interim final rule (Federal Register January 12, 2004 Vol. 69, Number 7) removing downer animals and specified risk materials and tissues from the human food chain; requiring additional process controls for establishments using advanced meat recovery (AMR); holding meat from cattle that have been targeted for BSE surveillance testing until the test has confirmed negative; and prohibiting the air injection stunning of cattle (http://www.aphis.usda.gov/lpa/issues/bse/bse.html). In January 2004, FDA proposed additional safeguards including: excluding brain, spinal cord, gut and eyes of older animals from human food and from rendered material in animal feeds, eliminating poultry litter, cow blood and processed plate waste as feed ingredients for cattle, labeling requirements for pet food, and additional control measures to prevent cross contamination of feed and feed ingredients at feed mills. In addition, since 1990, the USDA has led an interagency surveillance program for evidence of BSE in the US. USDA has tested 20,000 animals annually for each of the last 2 years, and approximately 75 percent of these were downers at slaughter. A BSE risk assessment performed by Harvard University's Center for Risk Analysis at the School of Public Health concluded that even if BSE were to occur in the US the measures already taken would largely prevent its spread to animals or humans, and the disease would gradually disappear over a number of years.

How is the BSE agent detected?

The presence of the BSE agent in tissues is generally determined by injecting animals, usually mice, with samples, then observing the mice to see if they die and have characteristic brain tissue changes. Mouse inoculation studies take a long time (up to 700 days) to detect the agent, and a negative result (that is, lack of brain tissue changes in the injected mice) may only mean that there was too little of the infectious agent to cause symptoms, not that the material was completely free of the infectious agent. It is also possible to detect the presence of the abnormal prion protein in tissues (such as brain) using special staining procedures although these methods do not allow an accurate assessment of infectivity of the infected material.

Does BSE or a similar disease occur in humans?

BSE belongs to a group of progressive degenerative neurological diseases known as transmissible spongiform encephalopathies (TSEs). TSE diseases are always fatal. The TSE diseases include scrapie, which affects sheep and goats; transmissible mink encephalopathy; feline (cat) spongiform encephalopathy; and chronic wasting disease of deer and elk. There are six TSE diseases that affect people: kuru, classical Creutzfeldt-Jakob disease (CJD) and variant Creutzfeldt-Jakob disease (vCJD), Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, and sporadic fatal insomnia. The human diseases are very rare; for example, classical CJD has been well studied and occurs sporadically worldwide at a rate of about one case per one million people each year.