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Bovine Spongiform Encephalopathy

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1211 words
Science & Nature

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(BSE) is a relatively new disease found
primarily in cattle. This disease of the bovine breed was first seen in the
United Kingdom in November 1986 by histopathological examination of affected
brains (Kimberlin, 1993) . From the first discovery in 1986 to 1990 this
disease developed into a large-scale epidemic in most of the United Kingdom,
with very serious economic consequences (Moore, 1996).
BSE primarily occurs in adult cattle of both male and female genders.
The most common age at which cows may be affected is between the ages of four
and five (Blowey, 1991). Due to the fact that BSE is a neurological disease, it
is characterized by many distinct symptoms: changes in mental state 'mad-cow',
abnormalities of posture, movement, and sensation (Hunter, 1993). The duration
of the clinical disease varies with each case, but most commonly lasts for
several weeks. BSE continues to progress and is usually considered fatal
(Blowey, 1991).
After extensive research, the pathology of BSE was finally determined.
Microscopic lesions in the central nervous system that consist of a bilaterally
symmetrical, non-inflammatory vacuolation of neuronal perikarya and grey-matter
neuropil was the scientists' overall conclusion (Stadthalle, 1993). These
lesions are consistent with the diseases of the more common scrapie family.
Without further investigation, the conclusion was made that BSE was a new member
of the scrapie family (Westgarth, 1994).
Transmission of BSE is rather common throughout the cattle industry.
After the incubation period of one to two years, experimental transmission was
found possible by the injection of brain homogenates from clinical cases
(Swanson, 1990). This only confirmed that BSE is caused by a scrapie-like
infectious agent.
How does the transmission become so readily available among the entire
United Kingdom feedlot population? Studies showed that the mode of infection
was meat and bone meal that had been incorporated into concentrated feedstuffs
as a protein-rich supplement (Glausiusz, 1996). It is thought that the outbreak
was started by a scrapie infection of cattle, but the subsequent course of the
epidemic was driven by the recycling of infected cattle material within the
cattle population (Lyall, 1996). Although the average rate of infection is very
low, the reason why this led to such a large number of BSE cases is that much of
the United Kingdom dairy cattle population was exposed for many, continuous
years (Kimberlin, 1993).
To help control the outbreak, the British government in 1988 introduced
a ban on the feeding of ruminant protein to other ruminant animals (Lacey, 1995).
Such knowledge for the pathogenesis of the BSE disease shows precisely the
actions that must be taken in order to control and minimize the risk of
infection in healthy cattle around the world (Darnton, 1996).
The appearance of BSE has made a sizable impact throughout much of the
world even though few countries, other than the United Kingdom, have experienced
positive cases (Burton, 1996). The scare of an outbreak in other countries has
led to a great disruption in the trade economy, as well as other factors
concerning each of the country's general welfare. However, a rapid increase in
the understanding of the disease over the last four years leaves few unanswered
questions of major importance (Masood, 1996). BSE has been prevented,
controlled and eradicated.
As mentioned, BSE was first recognized in the United Kingdom and it is
only there that a large-scale epidemic has occurred (Burton, 1996). By the end
of 1990 well over 20,000 cases of BSE had been has been confirmed in England,
Scotland, and Wales (Filders, 1990). The deadly epidemic started simultaneously
in several parts of the country and cases have been distributed over a wide area
ever since (Cowell, 1996).
Besides the United Kingdom, cases of BSE have occurred in the Republic
of Ireland. Some of these cases were associated with the importation of live
animals, meat, and bone meal from the United Kingdom (Cherfas, 1990).
Two cases of BSE have also occurred in cattle from the country of Oman.
These animals were thought to be part of a consignment of fourteen pregnant
heifers imported from England in 1985. Various cases have also been confirmed
in Europe, Switzerland, and France (Patel, 1996).
The economic consequences of BSE in the United Kingdom have been
considerable. At the beginning, the only losses due to BSE were those directly
associated with the death or slaughter of BSE infected animals (Cowell, 1996).
In August 1988, a slaughter policy with part compensation was introduced to help
lessen the burden on individual farmers. As the number of BSE cases increased ,
and more farmers were experiencing a second case, full compensation was
introduced in February 1990 (Moore, 1996). In 1989 alone over 8,000 suspected
and confirmed cases of BSE were slaughtered. The compensation costs for the
year were well over 2.8 million pounds and the slaughter costs amounted to 1.6
million pounds (Cockburn, 1996).
Once studies had identified meat and bone meal as the vehicle of
infection, the United Kingdom Government banned the feeding of all ruminant-
derived protein to ruminants (Glausiusz, 1996). This had an immediate impact on
the cattle industry in terms of reduced exports and domestic sales of meat and
bone meal (Hager, 1996). In 1990, the Commission of the European Communities
banned the importation, from the United Kingdom, of all live cattle born before
July 1988. Panic throughout the world caused many countries to entirely ban the
importation of all live cattle from the United Kingdom. Some even went as far
as to ban the importation of milk and milk products (Hunter, 1993).
BSE has also had economic consequences in the human food industries. In
the winter of 1989/1990, the United Kingdom Government banned the use for human
food of certain specified bovine meats which contained suspicious amounts of BSE
(Cockburn, 1996). This ban was introduced as a precautionary measure to help
ensure the risks to public health from BSE were kept to a minimum.
Most of the information concerning BSE has come from extensive studies
of the scrapie agent. The agent is small enough to pass through bacteriological
filters, thus demonstrating that it is virus-like or subviral in size (Kimberlin,
1993). Unfortunately, the agent has other properties which are atypical of
viruses. The first contradiction is that infectivity is highly resistant to
many physicochemical treatments, such as heat, and exposure to ionizing or ultra
violet radiation (Swanson, 1990). Second, the disease does not induce an immune
response from the host (Stadthalle, 1993). These two controversies along with
the long incubation period explain why the scrapie group of agents have long
been known as the "unconventional slow virus" (Westgarth, 1994).
BSE is clearly not a disease of genetic origin. It has occurred in the
majority of United Kingdom dairy breeds and their crosses, in the proportion
expected from their representation in the national herd (Kimberlin, 1993).
Analysis of available pedigrees excludes a simple Mendelian pattern of
inheritance as the sole cause of the disease. Studies further showed that the
occurrence of BSE was not associated with ...

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