BSE (bovine spongiform encephalopathy) is a
progressive neurological disorder of cattle, and has
been called "mad cow disease." Its symptoms are similar to "scrapie," a brain disease that occurs in sheep.
Cattle affected by BSE experience progressive
degeneration of the nervous system. Affected animals
may display changes in temperament, such as:
Nervousness or aggression
Incoordination and difficulty in rising
Loss of body weight despite continued
Affected cattle die and currently preventive
treatment, including a vaccine, does not exist.
The incubation period (the time from when an
animal becomes infected until it first shows disease
signs) is from two to eight years. Following the onset of
clinical signs, the animal’s condition deteriorates
until it either dies or is destroyed. This process
usually takes from two weeks to six months.
in Great Britain have occurred in dairy cows
between 3 and 6 years old.
As of November 2003, more than 183,000 cases of
BSE were confirmed in the United Kingdom in more
than 35,000 herds. The BSE epidemic in the United
Kingdom peaked in January 1993 at almost 1,000 cases per week.
What causes BSE?
BSE and scrapie both result from infection with a
very unusual infectious agent. However, the exact
causative agent of the disease has not yet been
Three main theories on the
nature of the agent have been proposed:
A prion or abnormal partiallyproteinase
K-resistant protein, devoid of nucleic
acid, capable of causing a cell to produce more
A virino or "incomplete" virus
composed of naked nucleic acid protected by host
Currently, the most accepted theory is that the agent
is a modified form of a normal cell protein known as
prions. A prion is not a bacterium, parasite, or
virus, and thus treatments usually used for treating
or preventing bacterial infections (e.g. antibiotics) or
viral infections are not effective against prions.
Currently, it is accepted that the BSE agent:
smaller than most viral particles and is highly
resistant to heat, ultraviolet light, ionizing radiation,
and common disinfectants that normally inactivate
viruses or bacteria.
Causes no detectable immune
or inflammatory response in the host.
Has not been observed microscopically.
The outbreak of BSE in the United Kingdom may
have resulted from the feeding of scrapie-infected
sheep meat-and-bone meal prepared for cattle to
Does BSE or a similar disease occur in
BSE belongs to a group of progressive degenerative
neurological diseases known as transmissible
spongiform encephalopathies (TSEs) diseases are always fatal.
The TSE diseases include:
affects sheep and goats
Feline (cat) spongiform
Chromic wasting disease of
deer and elk
There are six TSE diseases that affect
Classical Creutzfeldt Jakob disease (CJD)
Variant Creutzfeldt-Jakob disease (vCJD)
Fatal familial insomnia
Sporadic fatal insomnia
The human diseases are very rare; for
example, classical CJD has been well studied and
occurs sporadically worldwide at a rate of about one
case per one million people.
Where is the BSE agent found?
In cattle naturally infected with BSE, the BSE agent
has been found in:
Retina of the eye
studies suggest that the BSE agent may also be
Dorsal root ganglia
How was BSE spread or transmitted?
It is thought that BSE was spread via meat-and-bone
meal fed to cattle. The practice of using this material
as a source of protein in cattle feed has been
common for several decades.
In the late 1970s there
was a change in the production (rendering) process
used to make this meat-and-bone meal. One
hypothesis is that this change permitted the
infectious agent of scrapie (a transmissible
spongiform encephalopathy, or TSE, of sheep) to
survive the rendering process, and get transmitted
to other animals, such as cows, that are fed meat-and-bone meal nutritional supplements.
Does BSE occur in the United States?
On December 23, 2003, the U.S. Department of
Agriculture (USDA) announced a presumptive
diagnosis of BSE in an adult Holstein cow from
The USDA launched an
epidemiologic investigation to determine the source
of the disease. For up-to-date information access the
USDA website at www.usda.gov.
Is BSE a foodborne hazard in the United
Strong evidence indicates that BSE has been
transmitted to humans primarily in the United
Kingdom, causing a variant form of Creutzfeldt-Jakob disease (vCJD).
In the United Kingdom, where
over 1 million cattle may have been infected with
BSE, a substantial species barrier appears to protect
humans from widespread illness.
As of December 1, 2003,
a total of 153 vCJD cases had been reported
worldwide; of these, 143 cases had occurred in the
United Kingdom. The risk to human health from
BSE in the United States is extremely low.
What countries have reported BSE?
The vast majority of cases of BSE (more than 99 percent as
of 1999) have been reported from the United
Kingdom during an epidemic.
cases have also been reported in other European
Republic of Ireland
of reported cases by country are available on the
website of the Office International des Épizooties
(www.oie.int). These numbers should be
interpreted with caution, however, because the
intensity and methods of surveillance probably vary
over time and by country.
How is the BSE agent detected?
The presence of the BSE agent in tissues is generally
determined by injecting animals, usually mice, with
material believed to be infected with BSE, then
observing the mice to see if they die and have
characteristic brain tissue changes.
inoculation studies take a long time (up to 700 days)
to detect the agent, and a negative result (that is,
lack of brain tissue changes in the injected mice)
may only mean that there was too little of the
infectious agent to cause symptoms, not that the
material was really free of the infectious agent
It is also possible to detect the presence
of the abnormal prion protein in tissue (such as
brain) using special staining procedures, although
these methods do not allow an accurate assessment
of infectivity of the infected material.
What has the British government done in
response to the BSE epidemic?
In response to the BSE epidemic, the British
Government instituted a series of measures to
minimize the risk of disease transmission among
both animals and humans.
A ban on
feeding ruminant protein (ruminants are animals,
such as cows, sheep and goats) to ruminants (1988)
Removal of some "high risk" materials (such as brain,
spinal cord and intestines) from cattle at slaughter
(1989 and 1995)
A ban on cattle over 30 months
old from being used for food (1996)
institution of these measures, Great Britain has seen
a decrease in the number of cattle with BSE from a
peak incidence of 36,680 confirmed cases in 1992 to
2,254 confirmed in 1999 (information on the BSE
epidemic in Great Britain is available at www.defra.gov.uk).
Where can I get more information?
Access the Centers for Disease Control and
Prevention website at www.cdc.gov, the USDA
website at www.usda.gov or contact the Southern Nevada Health District, Office of Epidemiology at