Mad Cow Disease

Preventive Medicine and Nutrition

Bovine spongiform encephalopathy (BSE, also known as mad cow disease) has surfaced in Great Britain, France, Switzerland, Canada, and other countries. Health authorities consider it to be the most likely cause of a new variant of Creutzfeldt-Jakob disease, a fatal brain disease that recently killed several young people in Britain.

Americans are concerned as to whether BSE may present a health threat in the U.S. Today, we present a review of scientific evidence that supports three conclusions, showing that, indeed, that concern is wholly justified:

  1. .The conditions that led to the emergence of BSE in Britain are present in the U.S. Current U.S. livestock rendering and feeding practices are similar to those present in Britain at the onset of the BSE epidemic.

  2. Evidence suggests that the agent that causes BSE has already spread to at least some animals in the U.S.

  3. Between 1979 and 1990, 2,614 Americans died of Creutzfeldt-Jakob disease, and the possibility that BSE played a role in some of those deaths cannot be ruled out.

Brain Disease in Cows

Bovine spongiform encephalopathy is a fatal central nervous system disease first identified in the U.K. in 1986. Affected cows show increased apprehension, poor coordination, difficulties in walking, and weight loss. The infections that cause BSE apparently existed for several years before the disease was recognized.1-3

BSE is not limited to Britain. It has been found in native cattle in France, Switzerland, Northern Ireland, the Republic of Ireland, the Channel Isles, and the Isle of Man. Cattle exports carried the disease to Canada, Denmark, Oman, and the Falkland Islands.1,4 Moreover, at least 100,000 cattle whose BSE status is unknown have been shipped from the U.K. to other countries. Most of these animals are unaccounted for.

Scientists put the blame for BSE on the practice of feeding cattle the remains of sheep infected with scrapie, a disease with many similarities to BSE and whose name comes from the fact that affected animals develop a persistent itch that causes them to scrape off their wool or hair. It has been recognized in Britain since at least 1730, and an estimated one-third of British flocks are currently affected by scrapie.1 It is also endemic in U.S. sheep. The disease persists in cattle herds when they are fed the remains of BSE-infected cattle, as was common in the U.K. and continues in the U.S.

The infectious agent in scrapie and BSE is not a virus or bacterium, but a faulty protein, called a prion (pron: pree’ on), discovered by Stanley B. Prusiner of the University of California at San Francisco. Disease-causing prions are proteins that are normal in their molecular make-up, but abnormal in their shape, like springs that have been bent out of their normal configuration. They can distort the normal proteins in brain and nerve cells, turning them into disease-carriers as well. A teaspoonful of tainted cattle feed is believed to be all it takes to give a cow BSE.5

Cattle remains in animal feed in the U.K. have also spread encephalopathic diseases to domestic cats, captive wild cats (puma, cheetah, ocelot), and captive wild ruminants (nyala, gemsbok, eland, Arabian oryx, greater kudu, scimitar-horned oryx).6 An infected mother kudu apparently passed the infection to her newborn, who came down with symptoms at 19 months, never having had contact with contaminated feed or other sick animals.4

Brain Disease in Humans

Prions that cause bovine spongiform encephalopathy are believed to be the culprits in some cases of Creutzfeldt-Jakob disease (CJD), the most common transmissible encephalopathy in humans. CJD was first identified in the 1920s. It can take as long as 30 years for the disease to manifest, but it proceeds quickly once symptoms start. In weeks to months, affected individuals lose mental faculties and muscle coordination, pass into a coma, and die. CJD has been found in the U.S., Britain, France, Italy, Chile, Czechoslovakia, Hungary, Israel, and Japan.

The disease is similar to kuru, which was identified in 1957 in the Fore highlanders of Papua, New Guinea, who apparently contracted it as a result of eating the brains of the deceased. Cannibalism has stopped in New Guinea, and kuru has virtually disappeared.3

There are also two inherited forms of human prion diseases. Gerstmann-Stäussler-Scheinker disease manifests with dementia and a lack of coordination. In fatal familial insomnia, dementia is preceded by difficulty sleeping. Some cases of CJD may also be inherited, and some may arise spontaneously as a result of genetic mutations causing proteins to lose their normal configuration.3

Several studies have linked CJD with exposure to animals. The risk of CJD appears to be higher in those who have had contact with rabbits, mink, ferrets, deer, or other wild animals, or have eaten pork, rare meat, or the brains of sheep or wild game.7 The cases receiving the most recent attention in the U.K. are those of young people (ten cases so far) and dairy farmers (four so far), all of whom had eaten beef products in the preceding ten years, although one later became a vegetarian in 1991. British medical authorities believe that “the most plausible interpretation” of events is that consumption of meat from affected cattle is the cause of these recent cases.8 The new cases appear to be slightly different from typical CJD, which supports the concern that a new variant of CJD is afoot.

CJD has also been accidentally transmitted from person to person in the course of medical treatments. It was given to 2 patients through brain electrodes and 23 patients by injections of growth hormone taken from cadavers, and has been passed in transplanted tissues. Three pathology technicians became infected, presumably by contact with human tissues. Brain and nerve tissues seem to be the most dangerous tissues, but other organs and body fluids are also potentially infective.7

The prions that cause CJD are not destroyed by typical chemical or heat disinfectant methods used in hospitals. Heating to 134 degrees Celsius (273 degrees Fahrenheit) does not reduce its infectivity.7

Figures from 1979 to 1990 show that CJD was listed on death certificates of 2,614 people in the U.S. The great majority are over 50 (the mean age was 67), but 23 were in their 30s and 3 were in their 20s.9 About 10 percent of cases are hereditary, passing as a dominant trait. The reported cases are probably underestimates, as many cases are probably misdiagnosed and CJD is not a reportable disease.9

Foodborne cases of CJD that might occur in the U.S. will not necessarily mirror the clinical features of their British counterparts, as spongiform encephalopathies exist in several forms, and prions occur in different strains.4,10 Emerging cases of CJD in Britain affect a younger age group and have somewhat different clinical signs and electroencephalographic tracings.

Evidence of BSE in the U.S.

The factors that led to the appearance of BSE in Britain are also present in the U.S.:

  1. Scrapie is present in US sheep.

  2. Sheep and cattle remains were fed to U.S. cattle until 1997.

  3. Cattle remains appear to have transmitted spongiform encephalopathies to other animals.

Details on each of these issues are presented in the following paragraphs.

Scrapie was first reported in U.S. sheep in 1947, and infected sheep flocks have been identified in virtually all parts of the U.S. where sheep are raised and are not limited to Western states.4 A report drafted by scientists from the Veterinary Pathology Department of Iowa State University and the U.S. Department of Agriculture stated: “The existence of scrapie in U.S. sheep flocks is a persisting potential source of infection for BSE in cattle and spongiform encephalopathies in other animals if the British model is correct....[I]t is likely that scrapie will be present in the United States for some time.”11

Similar diseases have been found in other species in the U.S. and elsewhere, including chronic wasting disease in deer and elk, and transmissible mink encephalopathy.7

There are approximately 100 million cattle in the U.S., including about 10 million dairy cows. More than 75 percent of beef cattle pass through a feedlot, where they are routinely fed the remains of slaughtered animals, commonly referred to as offal (pronounced like “awful”).1 Likewise, chickens, pigs, and other animals are routinely fed animal remains, feces, and even municipal wastes.

A 1990 Department of Agriculture survey of the 309 known rendering plants found that 13 percent processed adult sheep, usually simply mixing their remains with other rendered species. Eight percent included sheep heads. The techniques used—a continuous rendering process with no use of solvent—mirrored those used in Britain.12

The 1990 survey showed that nearly all feed manufacturers used animal remains in their products, and three-fourths did not require that any animal species be excluded from those to be used in feeds. Fifty-six percent of beef cattle feeds and 66 percent of dairy feeds incorporated offal from unknown species that did not exclude sheep.12

Of the 398,000 tons of rendered animal protein produced in the U.K. in 1989, 43 percent came from cattle, 25 percent came from poultry, 18 percent came from hogs, and 14 percent came from sheep. Much more rendered animal protein is produced in the U.S., reflecting the larger number of cattle slaughtered. Of the 3.3 million tons produced annually, 59 percent comes from cattle, 21 percent from poultry, 20 percent from hogs, and 0.6 percent from sheep and lambs.1Although the remains of ruminant animals (cows, sheep, and goats) are no longer used for livestock feed in Britain, animal remains are still routinely used in animal feed in the U.S.

The Animal Protein Producers Industry (APPI) of the National Renderers Association, Inc., has recommended that lamb remains from flocks believed to be free of scrapie continue to be used for animal feed, but that older sheep not be used, and that rendered sheep remains be diverted to feeds other than dairy or beef cattle feed.2 In April 1996, APPI and other industry groups jointly recommended that sheep and goat remains not be used in feed for ruminant animals, but they did not move to restrict the feeding of cattle remains to other cattle or of ruminants, including sheep, to nonruminant species.

In the decade between 1975 and 1985, the percentage of feed consumed by livestock and poultry that came from pasture, range plants, hay, straw, and other roughage—in other words, unprocessed plant material—dropped from 66 percent to 59 percent. The amount that came from concentrates, part of which consists of the remains of dead animals, increased from 34 percent to 41 percent.13

The text, Feeds and Nutrition, reports:

In 1984, 2.9 million tons of tankage and meat meal were fed to livestock. Tankage and meat meal are made from the trimmings that originate on the killing floor, inedible parts and organs, cleaned entrails, fetuses, residues from the production of fats, and certain condemned carcasses and parts of carcasses....[T]he raw materials from which tankage is made should not contain hair, hoof, horn, manure, stomach contents, and hide trimmings, except in such traces as might occur unavoidably in good factory practice.14

This same text reports, “Virtually everything that goes into a slaughterhouse can be processed for some purpose. Feathers, bones, connective tissues, organs, blood meat scraps, and hoofs are used as either protein, vitamin, or mineral supplements.”15

As surprising as it may sound, that even includes manure:

In recent years, considerable attention has been devoted to the feeding of livestock manure and litter. In the past, feedlots and large poultry operations were often hard pressed to find ways of disposing of the mountainous volumes of manure and litter (manure with absorbent material). Today, these producers are turning this material—once thought to be of value only as fertilizer—into valuable feed.16

Poultry manure, along with feathers, dead birds, eggshells, and unsalable eggs, find their way into livestock feed: “In addition to the by-products of the poultry processing industry, poultry manure and litter are now being processed to produce a palatable, high-protein feed.”17

Manure is not cooked or sterilized. It is treated either by piling it up for several weeks, in a process called “deep-stacking,” which causes the temperature to rise typically to 160 degrees Fahrenheit, which is presumed to kill disease-causing bacteria, or by a fermentation process, called “ensiling.” Neither practice can be expected to have any effect on prion proteins, nor on residues of drugs in manure.18

The extremes to which livestock ranchers will go are described in the same text in a section entitled “Municipal Garbage”:

Vast quantities of foods unfit for human consumption, as well as garbage resulting from the wastage of food, pose serious disposal problems in many urban areas. One of the solutions to the problem is the cooking of garbage, so as to alleviate the hazard of trichinosis, and feeding it to hogs.16

In weighing the factors that caused the BSE epidemic in Britain, Kevin D. Walker, Ph.D., and his colleagues at the U.S. Department of Agriculture Animal and Plant Health Inspection Service concluded: “The United States has all of the necessary factors for BSE; that is the presence of scrapie in the sheep population, similar rendering technology allowing for the survival of the scrapie agent, and inclusion of meat and bone meal in cattle concentrate feeds.”1

Just as cannibalism led to kuru in humans, forcing cattle to eat the remains of other cattle—something that never happens in nature—carried risks of disease transfer.

Cattle Imports from BSE Countries

Cattle have been imported to the U.S. from countries where BSE is endemic. According to the Department of Agriculture, 499 cattle were imported from the U.K. between 1981 and 1989. As of January 22, 1996, 341 were known to be dead, most of whom had apparently been slaughtered and their carcasses used for meat and offal.19 There is no apparent record of the distribution of meat or offal from these animals. Of the remainder, 8 were again exported, 34 were unaccounted for, and the remaining animals were killed and incinerated in 1996.

In addition, 12 tons of animal protein products were shipped from the U.K. to the U.S. in 1984 and 1985,1 aside from products intended for human use.

Transmissible Mink Encephalopathy

There is evidence that transmissible encephalopathies have been present in feed derived from cattle remains in the U.S. Mink fed offal from cattle have developed a disease, called transmissible mink encephalopathy (TME), which is remarkably like BSE. Five outbreaks of transmissible mink encephalopathy have occurred, taking place in 1947, 1961, 1963 (two outbreaks), and 1985.20 The disease has also been found in Canada, Finland, Russia, and Germany.2

Veterinary researchers believe that the most likely origin of the disease is the routine practice of feeding cattle remains to mink on fur farms. Often the cattle had been “downers,” that is, unable to stand, due to disease or injury.2 Department of Agriculture researchers estimate that the brain and spinal cord of a single BSE-infected cow could easily contain enough infective material to cause disease in 1,000 mink.20

Mink have contracted other diseases from feeding on cattle remains, including anthrax, botulism, black leg, brucellosis, tuberculosis, and urinary lithiasis apparently resulting from diethylstilbestrol exposure.2

After the 1985 TME outbreak in Stetsonville, Wisconsin, experimenters went so far as to inject diseased mink brains into Holstein cattle, finding that the cattle developed spongiform encephalopathy. Remains of these cattle were then fed to healthy mink, who soon developed encephalopathy.21

University of Wisconsin researchers who investigated the infectivity of transmissible mink encephalopathy noted that their findings were consistent with the mink disease being caused by the practice of feeding “downer cow” tissues to mink. They concluded, “If this is true, there must exist an unrecognized bovine spongiform encephalopathy (BSE)-like infection in American cattle.”21 Moreover, it indicates that BSE may have been present in U.S. cattle for decades.20

Will BSE Turn up in the U.S.?

If a form of BSE is the culprit in mink encephalopathy and possibly in some cases of Creutzfeldt-Jakob disease, why has the disease not yet been identified in U.S. cattle? One reason is because there has been no effort to test substantial numbers of U.S. cattle carcasses for BSE.

Even in the best of circumstances, BSE surveillance programs are not designed to protect either cattle or humans, but rather to provide information on the status of an epidemic, if there is one. It is virtually impossible to detect the initial cases of an emerging disease, and the incubation period for BSE is so long that it can exist in cattle and be passed to other cattle or to humans long before the brain abnormalities are detected.2

U.S. cattle are typically slaughtered at an age when signs of BSE would not likely be recognized. Dairy cattle are slaughtered when their milk production declines. Only 46 percent of U.S. dairy cattle live to four years of age or older, compared to 70 percent of dairy cows in the U.K.1 Although cattle may well carry the diseased prions earlier, the symptoms do not usually become noticeable until about five years of age.4 Similarly, U.K. feedlot steers have not shown signs of BSE, presumably because they are slaughtered before symptoms begin.1

Also, conditions encouraging BSE have been in place for a shorter time in the U.S. than in Britain. R.F. Marsh and R.A. Bessen of the University of Wisconsin noted in 1993 that, although the feeding of animal remains to other animals has occurred throughout this century, the practice became widespread only recently:

Contrary to Great Britain, animal protein has only been used extensively in American cattle feed for the past five to six years. Currently we are feeding a minimum of 14 percent of all rendered cattle back to other cattle. This practice certainly increases the possibility for transmission of a putative BSE-like agent.21

The Wisconsin researchers also note, “We should not expect that a BSE-like disease in the United States would necessarily possess all the features of BSE in Great Britain. The existence of distinct biological strains of these transmissible agents is well documented.”

BSE is not likely to be found until a reasonable number of cattle carcasses are tested for it. While young cattle may harbor disease-causing prions, they are not so likely as older cattle to show pathological brain changes. The increased use of offal in feeds in the U.S. suggests that the risk of BSE is greater now than in the recent past.

Not a Time to Eat Chicken

While cattle have been considered the most likely species to have passed transmissible encephalopathies to humans, they are not the only species under scrutiny. As The Lancet reported on April 6, 1996, other animals have routinely been fed offal containing rendered cow and sheep remains:

[S]heep, pigs, and chickens have also been exposed to BSE and scrapie-contaminated feed. Whilst there is no evidence of natural transmission of BSE to these species, it would be prudent to remain open-minded about dietary exposure.22

Indeed, a brain disease called Crazy Chick disease (encephalomalacia) has been blamed on feeds deficient in vitamin E that also contain rancid fats. While polyunsaturated vegetable oils easily become rancid, the high fat content of some animal remains encourages rancidity.23

While it is unlikely that encephalomalacia in chickens has any relationship to disease in other animals, the poultry industry merits close scrutiny for its contribution to cattle diseases, because chickens are fed cattle remains and, in turn, chicken manure is routinely fed to cattle.

In summary, the conditions that led to the emergence of BSE in Britain are present in the U.S. Scrapie has been endemic is the U.S. for decades, and livestock rendering and feeding practices have allowed scrapie prions to occasionally end up in offal fed to cattle. Outbreaks of disease in mink suggest that a BSE-like agent has, in fact, been present in animal feed in the U.S. The possibility that a BSE-like agent played a role in some of the 2,614 cases of Creutzfeldt-Jakob disease that have occurred in the U.S. between 1979 and 1990 cannot be ruled out.

It should be recognized that the consumption of livestock products is clearly linked to a much higher risk of serious and sometimes fatal diseases, apart from the risk of transmissible encephalopathies. These diseases include coronary artery disease, colon and possibly other forms of cancer, diabetes, hypertension, obesity, and infection with salmonella, campylobacter, and E.coli O157:H7, among others. Making meat “safe” is not a realistic or attainable goal. Ironically, while the feeding of animal remains to other animals is now acknowledged as a dangerous practice that is restricted in some countries, the feeding of animal remains to humans is encouraged by government programs and massive industry efforts.



1. Walker KD, Hueston WD, Hurd HS, Wilesmith JW. Comparison of bovine spongiform encephalopathy risk factors in the United States and Great Britain. JAVMA 1991;199:1554-61.
2. Marsh RF. Bovine spongiform encephalopathy: a new disease of cattle? Arch Virol 1993;7(Suppl):255-9.
3. Prusiner SB. The prion diseases. Scientific American, Jan. 1995, pp. 48-57.
4. Bleem AM, Crom RL, Francy B, Hueston WD, Kopral C, Walker K. Risk factors and surveillance for bovine spongiform encephalopathy in the United States. JAVMA 1994;204:644-51.
5. Collee JG. A dreadful challenge. Lancet 1996;347:917-8.
6. Tabrizi SJ, Scaravilli P, Howard RS, Collinge J, Rossor MN. Creutzfeldt-Jakob disease in a young woman. Lancet 1996;347:945-8.
7. Steelman VM. Creutzfeld-Jakob disease: recommendations for infection control. Am J Infect Control 1994;22:312-8.
8. Will RG, Ironside JW, Zeider M, et al. A new variant of Creutzfeldt-Jakob disease in the UK. Lancet 1996;347:921-5.
9. Holman RC, Khan AS, Kent J, Strine TW, Schonberger LB. Epidemiology of Creutzfeldt-Jakob disease in the United States, 1979-1990: analysis of national mortality data. Neuroepidemiology 1995;14:174-81.
10. Marsh RF, Bessen RA. Physicochemical and biological characterizations of distinct strains of the transmissible mink encephalopathy agent. Phil Trans R Soc Lond B 1994;343:413-4.
11. Miller LD, Davis AJ, Jenny AL. Surveillance for lesions of bovine spongiform encephalopathy in US cattle. J Vet Diagn Invest 1992;4:338-9.
12. Centers for Epidemiology and Animal Health, Animal and Plant Health Inspection Service, US Department of Agriculture. United States rendering and feed-manufacturing industries: evaluation of practices with risk potential for bovine spongiform encephalopathy. USDA, Fort Collins, CO, February 1990.
13. Feeds and Nutrition. Second Edition, Ensminger Publishing Co, Clovis, California, 1990, pp. 250-1.
14. Ibid, p. 410.
15. Ibid, p. 259-60.
16. Ibid, p. 260.
17. Ibid, p. 412.
18. Ibid, p. 545.
19. United States Department of Agriculture. Questions and answers about bovine spongiform encephalopathy (BSE). 1996.
20. Robinson MM, Hadlow WJ, Huff TP, et al. Experimental infection of mink with bovine spongiform encephalopathy. J Gen Virol 1994;75:2151-5.
21. Marsh RF, Bessen RA. Epidemiologic and experimental studies on transmissible mink encephalopathy. Dev Biol Stand. Basel, Karger 1993;80:111-8.
22. Collinge J, Rossor M. A new variant of prion disease. Lancet 1996;347:916-7.
23. Feeds and Nutrition, op. cit., pp. 184-5 and 258.