Bovine Spongiform Encephalopathy

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Also known as

Mad cow disease


Bovine spongiform encephalopathy (BSE) was first found in cattle in the United Kingdom, and is similar to a disease in humans called variant Creutzfeldt-Jakob disease (vCJD). Both disorders are fatal brain diseases caused by a prion--an altered form of protein. The brain of an affected cow (collected at necropsy) looks spongey under a microscope.

Bovine spongiform encephalopathy (BSE) is a progressive (continually worsening) neurological disorder of cattle. The current theory is that the infective agent is a modified form of a normal protein. For reasons not yet understood, the normal protein changes into a pathogenic (harmful) form that damages the central nervous system.

This disease has a very long incubation period. It usually takes four to six years from the time a cow is infected with the abnormal prion to when it first shows signs of BSE. There is no way to tell that a cow is incubating BSE by looking at it, but once the animal starts to show signs, it gets continually sicker until it dies, usually within 2 weeks to 6 months.

The first cases of BSE probably occurred in the United Kingdom during the 1970’s with two cases positively identified in 1986. BSE possibly originated as a result of feeding meat-and-bone meal to cattle—containing infected products from a spontaneously occurring case of BSE or from scrapie-infected sheep products. Scrapie is a prion disease of sheep. There is evidence that the disease then spread throughout the United Kingdom cattle industry because of the common practice of feeding rendered carcasses as bovine meat-and-bone meal to young dairy calves as a protein supplement—and it was sometimes prion-infected.

The BSE epizootic (disease temporarily widespread) in the United Kingdom peaked in January 1993 at almost 1,000 new cases per week. During the eradication program about four million cows were killed in the UK.

Since then, annual numbers of BSE cases in the United Kingdom have dropped sharply—from 14,562 cases in 1995 to only 2 cases in 2015—thanks to efforts to make sure no cattle feeds contain body parts of other cattle. Only four cases were reported globally in 2017, and the condition is now considered to be nearly eradicated.

Efforts to prevent the disease in the UK also include not allowing any animal older than 30 months to enter either human food or animal feeds.  Because of the long incubation period, young animals’ brains are not yet affected. In continental Europe, cattle over 30 months must be tested (brains checked) if they are intended for human food. In North America, tissue of concern (brain, spinal cord and intestines), known as specified risk material, cannot be added to animal feed or pet food.

There is probably an association between a new human prion disease called variant Creutzfeldt-Jakob disease (vCJD) that was first reported in 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 human population to potentially BSE-contaminated food (1984-1986) and the onset of initial variant CJD cases (1994-1996) is consistent with incubation periods for the human forms of prion disease.

Most 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 became infected when they were living or traveling overseas.


  • Weight loss,
  • Incoordination,
  • Difficulty getting up (downer cow),
  • Nervousness,
  • Abnormal behavior,
  • Violent irrational behavior


Cattle are believed to become infected when fed meat-and-bone meal containing either the remains of cattle that spontaneously developed the disease or scrapie-infected sheep products. Traditionally, the parts of a cow not eaten by people were cooked, dried, and ground into powder, used for several purposes, including 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. If a cow gets BSE, it most likely ate the contaminated feed during its first year of life, and won’t show signs of BSE until it is five-years-old or older.

Prions are misfolded proteins that can transmit their misfolded shape onto normal variants of the same protein. Prions cause several fatal neurodegenerative diseases in humans and many other animals.

It is not known what causes the normal protein to misfold, but these abnormal three-dimensional structures are suspected of collapsing nearby protein molecules into the same shape.

The word prion is from proteinaceous infectious particle. The role of a protein as an infectious agent is very different from all other known infectious agents such as viruses, bacteria, fungi and parasites, all of which contain nucleic acids (DNA, RNA or both). The abnormal prion agent is not destroyed even if material containing it is cooked or heat-treated.

Prions replicate by causing normally-folded proteins of the same type to take on their misfolded shape, which then go on to do the same, leading to an exponential chain reaction. Eventually, these prions aggregate into dense plaque fibers that damage brain cells.

The brain cells begin to die off in massive numbers, eventually leading to the microscopic appearance of holes in the brain, degeneration of physical and mental abilities, and ultimately death.

Prion variants of the prion protein are thought to be the cause of transmissible spongiform encephalopathies (TSEs), which include scrapie in sheep, chronic wasting disease (CWD) in deer, bovine spongiform encephalopathy (BSE) in cattle and Creutzfeldt–Jakob disease (CJD) in humans.

All known prion diseases in mammals affect structure of the brain or other neural tissue; all are progressive, and always fatal. The body of the sick cow does not even know the abnormal prion is there, and cannot fight off the disease.   

Neither vCJD nor BSE is contagious. A person (or cow) cannot get the disease from being near a sick person or cow. Research has shown that people cannot get BSE from drinking milk or eating dairy products, even if the milk came from a sick cow. The meat is also safe to eat if not contaminated by brain or spinal cord tissue.

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

Only six cows with BSE have been found in the U.S. The first was reported in 2003 and the most recent case was found in August 2018.

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.

There are several strains of BSE: the typical or classic BSE strain responsible for the outbreak in the United Kingdom and two atypical strains (H and L strains).

The BSE strain responsible for most of the BSE cases in Canada is the same classic or typical strain linked to the outbreak in the United Kingdom. It is known to be preventable through elimination of BSE contaminated feed.

This typical strain has not yet been identified in any U.S.-born cattle.  All five U.S.-born BSE cases and two of the 20 Canadian-born BSE cases were caused by atypical BSE strains.


High-risk cow parts--that have the highest chance of being infected with the abnormal prion—are brains and spinal cords from cows 30 months of age or older. Since August 1997, the FDA has not allowed most parts from cows and certain other animals to be used to make food for cows.

This protects healthy cows from getting BSE by making sure their food is not contaminated with the abnormal prion. The USDA prevents high-risk cows and cow products from entering the U.S. from other countries.  

To prevent BSE from entering the U.S., restrictions were placed on 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.

Public health control measures, such as surveillance, culling sick animals, or banning specified risk materials, have been instituted in many countries, particularly in those with indigenous cases (in native animals) of confirmed BSE, to prevent potentially BSE-infected tissues from entering the human food supply.

The most stringent control measures include a UK program that excludes all animals more than 30 months of age from the human food and animal feed supplies. This program appears to be highly effective.

Other control measures include banning the use of mechanically recovered meat from the vertebral column of cattle, sheep, and goats for human food and BSE testing of all cattle more than 30 months of age destined for human consumption.

Because ruminant tissue in ruminant feed was probably responsible for the BSE outbreak in the United Kingdom, and evidence of possible transmission of BSE to humans, the U.S. Food and Drug Administration instituted a ruminant feed ban that became effective in 1997.

Then in 2009, FDA established an enhanced BSE-related feed ban to harmonize BSE feed control measures in the U.S. with those in Canada.

Canada’s was aimed at more effectively preventing and quickly eliminating BSE from that country. 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 cattle feed.

Applying the same measure to pet food and fertilizer materials addresses the possible exposure of cattle and other susceptible animals to these products.


There is no treatment for this disease; affected animals are euthanized.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.