Download “Mad Cow” Disease: Bovine Spongiform Encephalopathy

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“Mad Cow” Disease:
Bovine Spongiform Encephalopathy
Bovine Spongiform Encephalopathy (BSE), otherwise
known as “Mad Cow”, is a fatal disease that affects the
central nervous system of cattle. The name comes from the
microscopic pores seen in the brain of an infected specimen
that cause it to resemble a sponge (Epidemiological
Bulletin, March 2001). This relatively new disease has had
tremendous impact on the political and economic status of
countries that rely heavily on the market of raising and
exporting cattle. Since the discovery of BSE in 1986, much
research has been done to determine the particular
epidemiology and pathology of the disease.
It is believed that the causative agent of Bovine
Spongiform Encephalopathy (BSE) is a proteinaceous
infectious particle, or prion.
Prions are proteins that
contain no DNA or RNA, yet are able to replicate themselves
without the use of genes.
The prion protein is found at
the surface of cells, mainly those of the spinal cord and
brain. There are two types of prion protein: normal (PrPC);
abnormal or infectious (PrPSc). The normal strain of the
prion is important to cell maintenance. It is soluble and
protease sensitive. It is the abnormal form of the protein
that causes BSE. (Epidemiological Bulletin, March 2001).
The abnormal PrPSc prion proteins are insoluble and
partially protease resistant so the body has difficulty
expelling them (American Association for the Advancement of
Science, May 9 2003).
The infectious proteins replicate
themselves by forcing normal, healthy prions into
conforming to the shape of the abnormal ones. The normal
form of the protein has a helical shape which the abnormal
protein changes into a sheet-like shape Current Science,
March 5 2004).
It is the infectious prion’s ability to
transform normal, useful prions into exact duplicates that
makes it difficult to estimate gestation times and track
the progress of the disease.
While the first case of BSE was identified in Great
Britain in 1986, it is believed that the first cattle
infections were as early as the 1970’s (Centers for Disease
Control, July 7 2005). It is believed that the infectious
prions entered the cattle by way of being feed meat and
bone meal made from sheep that were contaminated by
Scrapie, a relative of BSE that infects the sheep (Centers
for Disease Control, July 7 2005).
The beginning of the BSE epidemic appears to be
related to the changes made in the way that livestock
carcasses were processed into animal feed in the late 1970s
(American Scientist, Jul/Aug 2004).
The process of
liquefying separate batches of animal carcasses was
replaced by a process in which the carcasses moved
continuously through the rendering machines, which could
have prevented an even exposure to heat. The practice of
extracting the material with hydrocarbon solvents under
steam was also eliminated (American Scientist, Jul/Aug
2004).
These changes could have allowed the infectious
proteins to survive in the meat and bone meal, which was
subsequently fed to the herds of cattle in Great Britain.
Following the ban of meat and bone dietary supplements for
cattle in 1987, there was a significant decrese in the
number of BSE infected cows (American Scientist, Jul/Aug
2004).
BSE became of heightened interest to the medical
community after the disease was discovered in some domestic
cats and and wild cats, such as lions and tigers, kept in
captivity. It is believed that the domestic cats ingested
the infectious proteins in meat and bone meal based foods,
while the wild cats contracted the disease by eating raw
tissues that included those from the central nervous system
of contaminated cattle (Epidemiological Bulletin, March
2001).
The finding of BSE in animals other than cattle
caused the scientific community to worry the BSE may be
able to infect humans.
A decade after the first documented cases of BSE, the
scientific community saw its fears become reality. In 1996
a variant of Creutzfeldt-Jakob Disease (CJD), another
relative of the BSE infection, was found in a human in the
Great Britain.
While CJD is most commonly a hereditary
disease, the variant CJD prion most resembles the prion
that causes BSE. The way the prion proteins were folded in
the variant CJD infections greatly resembled those in the
BSE infected cattle(American Scientist, Jul/Aug 2004).
Variant CJD is also found predominately in victims under
thirty years of age, while the normal CJD infection
manifests itself in the fifty to seventy age range
(American Scientist, Jul/Aug 2004).
The time frame in
which the variant CJD incubates is another link to BSE.
The ten year span between the exposure to contaminated meat
products and the onset of variant CJD is consistant with
the ten year incubation period of the BSE prion (Centers
for Disease Control (June 29 2005). As of yet there is no
sure way to determine if a person has been infected with
the variant CJD. The only way to confirm the presence of
the abnormal prion is through autopsy, and like BSE, the
variant CJD is always fatal (Centers for Disease Control,
June 29 2005).
It is believed that the BSE epidemic began in the
United Kingdom due to the large ratio of sheep to cattle
farming.
Seeing as how BSE is thought to be an altered
form of the similar sheep disease Scrapie, which has been
recorded to have been around since the 1800s, it is
reasonable to assume that the disease would originate in a
region high in the number of sheep herds (Epidemiological
Bulletin March 2001). The following table illustrates the
trend in the number of cattle infected with BSE from its
discovery in 1986 up until the year 2000.
Number of cases of BSE reported in the United
Kingdom
Year
Up to
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
Great
Britan
Northern
Ireland
Isle
of Man
Jersey
Total
Guernsey United
Kingdom
442
0
0
0
4
2469
4
6
1
34
7137
29
6
4
52
14181
113
22
8
83
25032
170
67
15
75
36682
374
109
23
92
34370
459
111
35
115
23945
345
55
22
69
14302
173
33
10
44
8016
74
11
12
36
4312
23
9
5
44
3179
18
5
8
25
2274
7
3
6
11
1076
14
0
0
11
(Epidemiological Bulletin, March 2001)
446
2514
7228
14407
25359
37280
35090
24436
14562
8149
4393
3235
2301
1101
The data shows that the epidemic hit the largest
proportions from 1991 to 1994.
It is believed that this
crest in the number of infected animals was due to the use
of undiagnosed sick animals in the production of cattle
feed (Epidemiological Bulletin, March 2001). The beginning
in the downtrend can possibly be explained by the meat and
bone meal ban in the late 1980s.
The BSE prion proteins have been found in areas such
as the brain, spinal cord, dorsal root ganglia and the bone
marrow of cattle that were orally infected with BSE. This
points out that these are the areas most susceptible to the
transmission of the disease (Centers for Disease Control,
June 27 2005).
When experimental rodents were inoculated
with the abnormal protein it first began to transform the
cells of the spleen and lymph nodes and progresses to the
splanchnic nerves, where the sympathetic nerves leave the
abdomen.
From there the pathogen travels through the
spinal cord and stops in the brain. If the contaminant is
taken orally, as is the case with the meat and bone meal
feed, the pathogen can travel directly from the stomach and
into the brain stem via the vagus nerve (American
Scientist, Jul/Aug 2004). The abnormal proteins then
transform the normal proteins in the brain into the
pathogenic form and this process continues to the next
protein, and so on, until there is a large build-up of the
pathogen in the cells of these tissues (Medical
Microbiology And immunology, Aug. 2003). The accumulation
of the proteins in the brain kill off healthy neurons
leaving the “spongy” holes that the disease is named for
(Current Science, March 5 2004).
The direct mechanical
damage is hard to detect until after death because the
prions emit no toxins, nor do they illicit an immune
response.
Since prion proteins are already present in the body,
the similar pathogenic proteins are not detected by the
body’s immune system, making it impossible to detect
antibodies that may leave some clue to the disease’s
presence (Epidemiolgical Bulletin, March 2001). Since the
disease is only positively diagnosed after death by a
biopsy of brain tissue during autopsy, there is no way to
treat the disease.
As the brain is the organ ultimately
affected by the prion build-up symptoms include a wide
range of psychiatric and sensory symptoms when it first
begins to present itself.
These symptoms may include
ataxia in the early stages and dementia towrd the end of
the phase (Centers for Disease Control, June 29 2005).
Furthermore, the infected host may suffer from loss of
motor control, progressive dementia, some paralysis, and
wasting (Prusiner). As of yet there is no cure for BSE or
any of its relatives. BSE and the human form, variant CJD,
are always fatal. In fact, the best prognosis is that the
victim lives two years after the disease presents itself
(5% of cases). Most (90%) live only half that long after
the onset of the disease (Prusiner).