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Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, is
a fatal, neurodegenerative disease of cattle, which infects by a mechanism that
surprised biologists upon its discovery in the late 20th century. In the UK, the
country worst affected, 179,000 cattle were infected and 4.4 million killed as a
precaution.
It is believed that the disease may be transmitted to human beings who eat
infected carcasses. In humans, it is known as new variant Creutzfeldt-Jakob
disease (vCJD or nvCJD), and by June 2007, it had killed 165 people in Britain,
and six elsewhere with the number expected to rise because of the disease's long
incubation period. Between 460,000 and 482,000 BSE-infected animals had entered
the human food chain before controls on high-risk offal were introduced in 1989.
A British inquiry into BSE concluded that the epidemic was caused by feeding
cattle, who are normally herbivores, the remains of other cattle in the form of
meat and bone meal (MBM), which caused the infectious agent to spread. The
origin of the disease itself remains unknown. The current scientific view is
that infectious proteins called prions developed through spontaneous mutation,
probably in the 1970s, and there is a possibility that the use of
organophosphorus pesticides increased the susceptibility of cattle to the
disease. The infectious agent is distinctive for the high temperatures it is
able to survive; this contributed to the spread of the disease in Britain, which
had reduced the temperatures used during its rendering process. Another
contributory factor was the feeding of infected protein supplements to very
young calves.
Infectious agent
Microscopic "holes" of tissue sections are examined in the lab. Source: APHISThe
infectious agent in BSE is believed to be a specific type of misfolded protein
called prion. Misfolded prion proteins carry the disease between individuals and
cause deterioration of the brain. BSE is a type of transmissible spongiform
encephalopathy (TSE). TSEs can arise in animals that carry an allele which
causes normal prions to contort by themselves from an alpha helical arrangement
to a beta pleated sheet, which is the disease-causing shape. Transmission can
occur when healthy animals come in contact with tainted tissues from others with
the disease. In the brain these proteins cause native cellular prion protein to
deform into the infectious state, which then goes on to deform further prion
protein in an exponential cascade. This results in protein aggregates, which
then form dense plaque fibers, leading to the microscopic appearance of "holes"
in the brain, degeneration of physical and mental abilities, and ultimately
death.
Different theories exist for the origin of prion proteins in cattle. Two leading
theories suggest that it may have jumped species from the disease scrapie in
sheep, or that it evolved from a spontaneous form of "mad cow disease" which has
been seen occasionally in cattle for many centuries. Publius Flavius Vegetius
Renatus records cases of a disease with similar characteristics in the 4th and
5th Century AD. The British Government enquiry took the view the cause was not
scrapie as had originally been postulated, and was some event in the 1970s which
was not possible to identify.
The BSE epidemic in British cattle
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Agropedia Portal
Cattle, like most other food animals, are normally herbivores. In nature, cattle
eat grass or grains. In modern industrial cattle-farming, various commercial
feeds are used, which may contain ingredients including antibiotics, hormones,
pesticides, fertilizers, and protein supplements. The use of meat and bone meal,
produced from the ground and cooked left-overs of the slaughtering process as
well as from the cadavers of sick and injured animals such as cattle, sheep, or
chickens, as a protein supplement in cattle feed was widespread in Europe prior
to about 1987. Worldwide, soya bean meal is the primary plant-based protein
supplement fed to cattle. However, soya beans do not grow well in Europe, so
cattle raisers throughout Europe turned to the less expensive animal by-product
feeds as an alternative. A change to the rendering process in the early 1980s
may have resulted in a large increase of the infectious agents in the cattle
feed. A contributing factor was suggested to have been a change in British laws
that allowed a lower temperature sterilization of the protein meal. While other
European countries like Germany required said animal byproducts to undergo a
high temperature steam boiling process, this requirement had been eased in
Britain as a measure to keep prices competitive. Later the British Inquiry
dismissed this theory saying "changes in process could not have been solely
responsible for the emergence of BSE, and changes in regulation were not a
factor at all."
Following an epizootic of BSE in Britain, 165 people (up until 2007) acquired
and died of a disease with similar neurological symptoms subsequently called
vCJD, or (new) variant Creutzfeldt-Jakob disease. This is a separate disease
from 'classical' Creutzfeldt-Jakob disease, which is not related to BSE and has
been known about since the early 1900s. Three cases of vCJD occurred in people
who had lived in or visited Britain — one each in Ireland, Canada and the United
States. There is also some concern about those who work with (and therefore
inhale) cattle meat and bone meal, such as horticulturists, who use it as
fertilizer. Up to date statistics on all types of CJD are published by the UK
CJD Surveillance Centre in Edinburgh.
For many of the vCJD patients, direct evidence exists that they had consumed
tainted beef, and this is assumed to be the mechanism by which all affected
individuals contracted it. Disease incidence also appears to correlate with
slaughtering practices that led to the mixture of nervous system tissue with
hamburger and other beef. It is estimated that 400,000 cattle infected with BSE
entered the human food chain in the 1980s. Although the BSE epizootic was
eventually brought under control by culling all suspect cattle populations,
people are still being diagnosed with vCJD each year (though the number of new
cases currently has dropped to less than 5 per year). This is attributed to the
long incubation period for prion diseases, which are typically measured in years
or decades. As a result the full extent of the human vCJD outbreak is still not
fully known.
The scientific consensus is that infectious BSE prion material is not destroyed
through normal cooking procedures, meaning that contaminated beef foodstuffs
prepared "well done" may remain infectious.
In 2004 researchers reported evidence of a second contorted shape of prions in a
rare minority of diseased cattle. In other words, this implies a second strain
of BSE prion. Very little is known about the shape of disease-causing prions,
because their insolubility and tendency to clump thwarts application of the
detailed measurement techniques of structural biology. But cruder measures yield
a "biochemical signature" by which the newly discovered cattle strain appears
different from the familiar one, but similar to the clumped prions in humans
with traditional CJD Creutzfeldt-Jakob Disease .The finding of a second strain
of BSE prion raises the possibility that transmission of BSE to humans has been
underestimated, because some of the individuals diagnosed with spontaneous or
"sporadic" CJD may have actually contracted the disease from tainted beef. So
far nothing is known about the relative transmissibility of the two disease
strains of BSE prion.
In 2005 a controversial paper in The Lancet introduced a theory that BSE might
have originated in British cattle when they ate imported animal feed that
included infected human remains from Hindu funeral ceremonies in India. This
paper is merely a conjecture, however, and the authors suggest only that further
investigation should occur.
UK epizootic and UK licensed medicines
During the course of the investigation into the BSE epizootic, an enquiry was
also made into the activities of the Department of Health and its Medicines
Control Agency. On May 7, 1999, in his written statement number 476 to the BSE
Inquiry, David Osborne Hagger reported on behalf of the Medicines Control Agency
that in a previous enquiry the Agency had been asked to:
"... identify relevant manufacturers and obtain information about the bovine
material contained in children’s vaccines, the stocks of these vaccines and how
long it would take to switch to other products." It was further reported that
the: "... use of bovine insulin in a small group of mainly elderly patients was
noted and it was recognised that alternative products for this group were not
considered satisfactory." A medicines licensing committee report that same year
recommended that: "... no licensing action is required at present in regard to
products produced from bovine material or using prepared bovine brain in
nutrient media and sourced from outside the United Kingdom, the Channel Isles
and the Republic of Ireland provided that the country of origin is known to be
free of BSE, has competent veterinary advisers and is known to practise good
animal husbandry." In 1990 the British Diabetic Association became concerned
regarding the safety of bovine insulin and the government licensing agency
assured them that: "... there was no insulin sourced from cattle in the UK or
Ireland and that the situation in other countries was being monitored." In 1991
a European Community Commission: "... expressed concerns about the possible
transmission of the BSE/scrapie agent to man through use of certain cosmetic
treatments." Sources in France reported to the British Medicines Control Agency:
"... that there were some licensed surgical sutures derived from French bovine
material." Concerns were also raised: "... regarding a possible risk of
transmission of the BSE agent in gelatin products."
Husbandry practices in the United States relating to BSE
Soybean meal is cheap and plentiful in the United States. As a result, the use
of animal byproduct feeds was never common, as it was in Europe. However, U.S.
regulations only partially prohibit the use of animal byproducts in feed. In
1997, regulations prohibited the feeding of mammalian byproducts to ruminants
such as cows and goats. However, the byproducts of ruminants can still be
legally fed to pets or other livestock such as pigs and poultry such as
chickens. In addition, it is legal for ruminants to be fed byproducts from some
of these animals. A proposal to end the use of cow blood, restaurant scraps, and
poultry litter (fecal matter, feathers) in January 2004 has yet to be
implemented , despite the efforts of some advocates of such a policy, who cite
the fact that cows are herbivores, and that blood and fecal matter could
potentially carry BSE.
In February 2001, the USGAO reported that the FDA, which is responsible for
regulating feed, had not adequately policed the various bans. Compliance with
the regulations was shown to be extremely poor before the discovery of the
Washington cow, but industry representatives report that compliance is now 100%.
Even so, critics call the partial prohibitions insufficient. Indeed, US meat
producer Creekstone Farms alleges that the USDA is preventing BSE testing from
being conducted .
Japan was the top importer of U.S. beef, buying 240,000 tons valued at $1.4
billion in 2003. After the discovery of the first case of BSE in the U.S. on
December 23, 2003, Japan stopped U.S. beef imports in December 2003. In December
2005, Japan once again allowed imports of U.S. beef, but reinstated its ban in
mid-January 2006 after a technical violation of the U.S.-Japan beef import
agreement: a vertebral column, which should have been removed prior to shipment,
was included in a shipment of veal.
Tokyo yielded to U.S. pressure to resume imports, ignoring consumer worries
about the safety of U.S. beef, said Japanese consumer groups. Michiko Kamiyama
from Food Safety Citizen Watch said about this: "The government has put priority
on the political schedule between the two countries, not on food safety or human
health."
Possibly due to pressure from large agribusiness, the United States has
drastically cut back on the number of cows inspected for BSE.
Sixty-five nations have full or partial restrictions on importing U.S. beef
products because of concerns that U.S. testing lacks sufficient rigor. As a
result, exports of U.S. beef declined from $3.8 billion in 2003, before the
first mad cow was detected in the US, to $1.4 billion in 2005.
On December 31, 2006, Hematech, a biotechnology company based in Sioux Falls,
South Dakota, announced that it had used genetic engineering and cloning
technology to produce cattle that lacked a necessary gene for prion production -
thus theoretically making them immune to BSE.
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