Preventive Medicine and
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:
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.
Evidence suggests that the agent that causes
BSE has already spread to at least some animals in the U.S.
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.
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
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
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
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
BSE in the U.S.
The factors that led to the appearance of
BSE in Britain are also present in the U.S.:
Scrapie is present in US sheep.
Sheep and cattle remains were fed to U.S.
cattle until 1997.
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.1 Although 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,
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
As surprising as it may sound, that even
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
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.
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
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
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
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
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
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
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
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.
23. Feeds and Nutrition, op. cit., pp. 184-5 and 258.