Bovine spongiform encephalopathy (BSE) is a relatively new disease foundprimarily in cattle. This disease of the bovine breed was first seen in theUnited Kingdom in November 1986 by histopathological examination of affectedbrains (Kimberlin, 1993) .
From the first discovery in 1986 to 1990 thisdisease 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 fourand five (Blowey, 1991). Due to the fact that BSE is a neurological disease, itis characterized by many distinct symptoms: changes in mental state ‘mad-cow’,abnormalities of posture, movement, and sensation (Hunter, 1993).
The durationof the clinical disease varies with each case, but most commonly lasts forseveral 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 bilaterallysymmetrical, non-inflammatory vacuolation of neuronal perikarya and grey-matterneuropil was the scientists’ overall conclusion (Stadthalle, 1993). Theselesions are consistent with the diseases of the more common scrapie family. Without further investigation, the conclusion was made that BSE was a new memberof 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 wasfound possible by the injection of brain homogenates from clinical cases(Swanson, 1990). This only confirmed that BSE is caused by a scrapie-likeinfectious agent. How does the transmission become so readily available among the entireUnited Kingdom feedlot population? Studies showed that the mode of infectionwas meat and bone meal that had been incorporated into concentrated feedstuffsas a protein-rich supplement (Glausiusz, 1996). It is thought that the outbreakwas started by a scrapie infection of cattle, but the subsequent course of theepidemic was driven by the recycling of infected cattle material within thecattle population (Lyall, 1996). Although the average rate of infection is verylow, the reason why this led to such a large number of BSE cases is that much ofthe United Kingdom dairy cattle population was exposed for many, continuousyears (Kimberlin, 1993).
To help control the outbreak, the British government in 1988 introduceda ban on the feeding of ruminant protein to other ruminant animals (Lacey, 1995). Such knowledge for the pathogenesis of the BSE disease shows precisely theactions that must be taken in order to control and minimize the risk ofinfection in healthy cattle around the world (Darnton, 1996). The appearance of BSE has made a sizable impact throughout much of theworld even though few countries, other than the United Kingdom, have experiencedpositive cases (Burton, 1996). The scare of an outbreak in other countries hasled to a great disruption in the trade economy, as well as other factorsconcerning each of the country’s general welfare.
However, a rapid increase inthe understanding of the disease over the last four years leaves few unansweredquestions of major importance (Masood, 1996). BSE has been prevented,controlled and eradicated. As mentioned, BSE was first recognized in the United Kingdom and it isonly there that a large-scale epidemic has occurred (Burton, 1996). By the endof 1990 well over 20,000 cases of BSE had been has been confirmed in England,Scotland, and Wales (Filders, 1990).
The deadly epidemic started simultaneouslyin several parts of the country and cases have been distributed over a wide areaever since (Cowell, 1996). Besides the United Kingdom, cases of BSE have occurred in the Republicof Ireland. Some of these cases were associated with the importation of liveanimals, 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 pregnantheifers imported from England in 1985. Various cases have also been confirmedin Europe, Switzerland, and France (Patel, 1996). The economic consequences of BSE in the United Kingdom have beenconsiderable. At the beginning, the only losses due to BSE were those directlyassociated with the death or slaughter of BSE infected animals (Cowell, 1996).
In August 1988, a slaughter policy with part compensation was introduced to helplessen the burden on individual farmers. As the number of BSE cases increased ,and more farmers were experiencing a second case, full compensation wasintroduced in February 1990 (Moore, 1996). In 1989 alone over 8,000 suspectedand confirmed cases of BSE were slaughtered. The compensation costs for theyear were well over 2. 8 million pounds and the slaughter costs amounted to 1.
6million pounds (Cockburn, 1996). Once studies had identified meat and bone meal as the vehicle ofinfection, the United Kingdom Government banned the feeding of all ruminant-derived protein to ruminants (Glausiusz, 1996). This had an immediate impact onthe cattle industry in terms of reduced exports and domestic sales of meat andbone meal (Hager, 1996). In 1990, the Commission of the European Communitiesbanned the importation, from the United Kingdom, of all live cattle born beforeJuly 1988. Panic throughout the world caused many countries to entirely ban theimportation of all live cattle from the United Kingdom. Some even went as faras to ban the importation of milk and milk products (Hunter, 1993).
BSE has also had economic consequences in the human food industries. Inthe winter of 1989/1990, the United Kingdom Government banned the use for humanfood of certain specified bovine meats which contained suspicious amounts of BSE(Cockburn, 1996). This ban was introduced as a precautionary measure to helpensure the risks to public health from BSE were kept to a minimum. Most of the information concerning BSE has come from extensive studiesof the scrapie agent. The agent is small enough to pass through bacteriologicalfilters, thus demonstrating that it is virus-like or subviral in size (Kimberlin,1993). Unfortunately, the agent has other properties which are atypical ofviruses.
The first contradiction is that infectivity is highly resistant tomany physicochemical treatments, such as heat, and exposure to ionizing or ultraviolet radiation (Swanson, 1990). Second, the disease does not induce an immuneresponse from the host (Stadthalle, 1993). These two controversies along withthe long incubation period explain why the scrapie group of agents have longbeen known as the “unconventional slow virus” (Westgarth, 1994). BSE is clearly not a disease of genetic origin.
It has occurred in themajority of United Kingdom dairy breeds and their crosses, in the proportionexpected from their representation in the national herd (Kimberlin, 1993). Analysis of available pedigrees excludes a simple Mendelian pattern ofinheritance as the sole cause of the disease. Studies further showed that theoccurrence of BSE was not associated with the importation of cattle, the use ofsemen, or the movement of breeding animals between herds (Hunter, 1993). By examining the epidemic curve, one can deduce that the disease ischaracteristic to that of an extended-source epidemic.
By a simple process ofelimination, the only common factor to be identified was the feeding ofproprietary feedstuffs (Darnton, 1996). Commercial calf pellets, cow cakes, orprotein supplements to home mixed rations have been fed to all cases where theinfectious BSE disease is subsequently present. The balance of evidence showsthat meat and bone meal is the primary vehicle of infection (Lacey, 1995). As previously explained, it is now assumed that scrapie was the originalcause of the BSE epidemic. It is very likely that the epidemic was started byone scrapie strain that is common in different breeds of sheep, or possibly, afew strains that behave in a similar manner when crossing the sheep-to-cattlespecies barrier (Hunter, 1993).
However, the continued exposure of cattle tosheep scrapie was not the ultimate driving force of the epidemic. On thecontrary, the epidemic would inevitably have been amplified into a severeoutbreak by the subsequent recycling, through meat and bone meal, of infectedcattle material within the cattle population (Westgarth, 1994). Because of thelength of BSE incubation periods, recycling would have already been establishedas the pattern for the epidemic long before BSE was even recognized (Cherfas,1990). After a comprehensive study of nearly 200 cases of BSE, scientists wereable to conclude that three significant clinical signs were present: -Changes in mental state were observed, most commonly seen as apprehension, frenzy and nervousness when confronted by doorways and other entrances.
-Abnormalities of posture and movement occurred in 93 percent of the cases. The most common manifestations were hind-limb ataxia, tremors, and falling. -Changes in sensation were a feature of 95 percent of all cases. The most striking evidence was continuous hyperaesthesia, to both touch andsound. These three most common clinical signs are consistent with a diffusecentral nervous system disorder (Stadthalle, 1993).
Other common signs wereloss of body condition (78 percent), live weight loss (73 percent), and areduced milk yield (70 percent). At some stage in the clinical course, about79 percent of all cases showed one of the above general signs along with signsin each of the three neurological categories previously listed (Swanson, 1990). Unfortunately, the slaughter of the great majority of affected animalsbecomes necessary at an early stage because of unmanageable behavior and injuryfrom repeated falling and uncontrollable behavior (Cowell, 1996). The durationof the clinical disease, from the earliest signs to death or slaughter, canrange from under two weeks to as long as a year. The average period is aboutone to two months (Lyall, 1996).
BSE resembles other members of the scrapie family in not having anygross pathological lesions associated with disease. Characteristichistopathological changes are found in the nervous system (Kimberlin, 1993). Incommon with the other diseases in the scrapie family, BSE has a distinctive non-inflammatory pathology with three main features: -The most important diagnostic lesion is the presence of bilaterally symmetrical neuronal vacuolation, in processes and in soma. -Hypertrophy of astrocytes often accompanies vacuolation. -Cerebral amyloidosis is an inconstant histopathological feature of the scrapie family of diseases.
At times, only one of the above will occur in an infected animal, while moreoften a combination of the three will occur (Swanson, 1990). Unfortunately, there are no routine laboratory diagnostic tests toidentify infected cattle before the onset of clinical disease. The diagnosis ofBSE therefore depends on the recognition of clinical signs and confirmation byhistological examination of the central nervous system (Westgarth, 1994). Aclinical diagnosis can also be confirmed by simple electron microscopeobservations, biochemical detection of SAF, or the constituent protein PrP(Hunter, 1996). At present, vaccination is not an appropriate way of preventing any ofthe diseases in the scrapie family.
There is no known protective immuneresponse to infection for a vaccine to enhance (Blowey, 1991). However, BSE isobviously not a highly contagious disease and it can be prevented by othersimple means because the epidemiology is also relatively simple: -Restrictions on trade in live cattle -Restrictions on trade in meat and bone meal -Sterilization of meat and bone meal -Restricted use of meat and bone meal -Minimizing exposure of the human population -Minimizing the exposure of other species (Moore, 1996)A great deal of concern, much of it avoidable, has been expressed overthe possible public health consequences of BSE. This is understanding giventhat the scrapie family of diseases include some that affect human beings (Patel,1996). As a result of research, the circumstances in which BSE might pose arisk to public health can be defined quite precisely, and simple measures havebeen devised to prevent this risk (Kimberlin, 1993).
It is important toemphasize that any primary human exposure would still be across a speciesbarrier and there would be no recycling of food-born infection in the humanpopulation, as happened with kuru and with BSE in cattle (Patel, 1996). Thelogical way to address this risk is to make sure that exposure to BSE is kept toa bare minimum. There are two scenarios for the future course of BSE. The first is thatBSE, like TME and kuru, is a dead-end disease.
If this is true and meat andbone meal was the sole source of the infection, then removing this source wouldbe sufficient for the eventual eradification of BSE from the United Kingdom(Hager, 1996). The alternative scenario is that there are natural routes oftransmission of BSE and that the outbreak could turn into an endemic infectionof cattle the way scrapie is in sheep (Burton, 1996). To sustain BSE infectionin the cattle population requires that each breeding cow is replaced by at leastone infected female calf, which then transmits infection to at least one of heroffspring. For BSE to become an endemic, the number of infected cattle wouldneed to increase by horizontal spread as seen in scrapie (Masood, 1996). Theessential prerequisite for controlling such a deadly disease is through goodbreeding and movement records which are currently being compiled in the UnitedKingdom following recent legislation (Stadthalle, 1993).
Meanwhile theprecautionary measures to safeguard other species, including human beings, arealready in place and refined to meet today’s needs.Category: Science