Bovine Spongiform encephalopathy (BSE) is a
progressive neurological disorder of cattle that results from infection by
an unconventional transmissible agent. Strong scientific evidence has seen
for a relationship between outbreaks in Europe of disease in cattle called
bovine spongiform encephalopathy (BSE, or “mad cow disease”) and a disease
in humans originally called new variant Creutzfeldt-Jakob disease (vCJD).
Both disorders are fatal brain diseases with unusually long incubation
periods measured in years. Although there is very strong evidence that
the agent responsible for the human disease is the same agent responsible
for the BSE outbreaks in cattle, the specific foods that might be
associated with the transmission of this agent from cattle to humans are
unknown. However, bioassays have identified the presence of the BSE agent
in the brain, spinal cord, retina, dorsal root ganglia (nervous tissue
located near the backbone), distal ileum, and the bone marrow of cattle
(called Specific Risk Materials or SRMs) experimentally infected with this
agent by the oral route.
Through
the end of November 2003, more than 183,000 cases of BSE were confirmed in
the United Kingdom alone in more than 35,000 herds. The outbreak may have
resulted form the feeding of scrapie-containing sheep meat-and-bone meal
to cattle. There is strong evidence and general agreement that the
outbreak was amplified by feeding rendered bovine meat-and-bone meal to
young calves.
The
nature of the transmissible agent is unknown. Currently, the most
accepted theory is that the agent is a modified form of a normal cell
surface component known as PRION protein. The pathogenic form of the
protein is both less soluble and more resistant to enzyme degradation than
the normal form.
On
December 23, 2003, the U.S. Department of Agriculture announced a
presumptive diagnosis of bovine spongiform encephalopathy (BSE, or “mad
cow” disease) in an adult Holstein cow from Washington State. Samples
were taken from the cow on December 9 as a part of USDA’s BSE surveillance
program. The BSE diagnosis was made on December 22 by histopathology and
immunohistochemical testing at the National Veterinary Service Laboratory,
Ames, Iowa. The diagnosis was confirmed by an international reference
laboratory in Weybridge, England, on December 25. Preliminary trace-back
based on an ear-tag identification number suggests that the BSE-infected
cow was imported into United States from Canada in August 2001.
Strong
evidence indicates that BSE has been transmitted to humans primarily in
the United Kingdom, causing a variant form of Creutzfeldt-Jakob disease (vCJD).
As of December 1, 2003, a total of 153 vCJD cases had been reported
worldwide; of these, 143 cases had occurred in United Kingdom. The
Centers for Disease Control and Prevention (CDC) monitors the trends and
current incidence of CJD in the United States.
An
experimental study reported in June 1996 showed that three cynomologus
macaque monkeys inoculated with brain tissue obtained from cattle with BSE
had clinical and neuropathological features strikingly similar to those of
variant CJD (Nature 1996; 381:743-4). An experimental study involving
inoculation of a panel of inbred mice with the agents causing BSE and
variant CJD substantially increased the strength of the scientific
evidence for a causal association between variant CJD and BSE (Nature
1997; 389:498-501). In this study, groups of inbred mice and a group of
cross-bred mice inoculated with brain homogenates from variant CJD cases
were reported to have had latency periods and lesion profiles consistent
with the BSE pattern.
Preventive
Measures: Public health control measures, such as enhanced BSE
surveillance, the culling of sick animals, and bans of specified risk
materials (SRM), have been instituted in countries of Europe and United
States to prevent potentially BSE-infected tissues from entering the human
food chain. The most stringent of these control measures, including an
“Over Thirty Months Scheme” that excludes all animals older than 30 months
from the human food and animal feed chain, have been applied in the United
Kingdom and appear to be highly effective. In June 2000, the European
Union Commission on Food Safety and Animal Welfare strengthened the
European Union’s set of control measures in relation to BSE by adopting a
decision requiring all member states to remove SRMs from the animal feed
and human food chains. Milk and milk products from cows are not believed
to pose any risk for transmitting the BSE agent.
Reference:
(1) Journal of the American
Medical Association
November 8, 2000
(Volume: 284, No.18, pp.2322-3).
(2) Clinics of Laboratory
Medicine
December 2002
(Volume 22, pp. 849-62).
(3) Nature 1996; 381:
743-4.
(4) Nature 1997; 389:
498-501.
(5) Proc National Acad Sci.
1999;96 : 15137-42
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