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Transmissible spongiform encephalopathies, hypotheses and food safety: An overview
The Science of the Total Environment 217 Ž1998. 71]82
Transmissible spongiform encephalopathies, hypotheses and food safety: An overview L. Fishbein 4320 Ashford Lane, Fairfax, VA 22032, USA Received 10 March 1997; accepted 20 March 1997
Abstract The transmissible spongiform encephalopathies ŽTSEs. Žin both humans and animals. have been reviewed with the principal focus on bovine spongiform encephalopathy ŽBSE., its recent outbreak amongst cattle in the UK Ž‘mad cow disease’. and its sequelae. The possible reasons for this outbreak are discussed as well as a number of hypotheses reviewed for TSEs Že.g. prions, organophosphates, etc.. and current measures attendant with food safety and surveillance taken in the European Union. It is generally accepted that the combined weight of all the evidence to date supports the conclusion that the new rare but lethal variant Creutzfeldt-Jacob Disease ŽvCJD. is the human counterpart of the aetiological agent BSE and that eating meat from the infected animals is probably to blame for 24 deaths Ž23 in the UK and one in France. to date from vCJD. Considerable evidence from biochemical, immunologic, pathologic and genetic studies strongly suggests that PrP sc is the major component of the transmissible prion particle responsible for the rare fatal brain TSE diseases. Q 1998 Elsevier Science B.V. All rights reserved. Keywords: Bovine spongiform encephalopathy; Creutzfeldt-Jacob disease; Prions; Food safety
1. Introduction Few diseases have captured both scientific and public awareness and anxiety this decade, as has bovine spongiform encephalopathy ŽBSE. or as more popularly known in the lay press as ‘mad cow disease’ ŽPowell and Leiss, 1997.. Widespread anxiety was generated when officials in the United Kingdom made public on March 20, 1996, a possible association between BSE and an apparently new but lethal human variant Creutzfeldt-Jacob
disease ŽvCJD. had occurred in 10 people Žnine of whom were under 35 years of age. in the UK over the previous 14 months ŽWilesmith, 1996; Will et al., 1996a,b.. A similar case of this variant ŽvCJD. was described in France ŽChazot et al., 1996. and 10 additional cases in the UK by October, 1997 ŽWilesmith et al., 1996.. So far a total of 24 cases of vCJD are known Ž23 in the UK and one in France. ŽPocchiari, 1998.. Many in the UK are especially fearful about the link between BSE and vCJD since the incubation period for CJD ranges
0048-9697r98r$19.00 Q 1998 Elsevier Science B.V. All rights reserved. PII S0048-9697Ž98.00164-8
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from 10 to 40 years. Although many individuals who are not yet exhibiting symptoms of vCJD may nevertheless already have been infected. It is impossible to predict how many people are now incubating the new variant form of the disease ŽPocchiari, 1998.. The emergence of vCJD following the BSE epidemic in the UK has engendered an intense public debate Žnow scarcely abated. in the European Union and elsewhere on issues including: food safety risk assessment, regulation and communication; agricultural practices, consumer confidence in beef and beef products and on food markets in general; and in relations between member States of the European Union. A broad question has emerged of how to build public trust in a manner in which governments Žand their advisors and expert committees. handle scientific issues in a modern, media-based society. The general issue of generating rational policy in the face of public fears has been brought into sharper focus as a result of the BSE epidemic in the UK. 2. Bovine spongiform encephalopathies (BSEs) The epidemic of BSE was first identified in November 1986 as a new scrapie-like disease with the presence of characteristic spongiform lesions in the brains of clinically affected cattle and the appearance of a newly recognized form of neurological disease in cattle in the UK. Clinical signs of BSE in cattle involve behavioral gait and postural abnormalities and after a period of 1]6 months, convulsions and death. Extracts of affected cattle brains contain abnormal fibrils associated with scrapie and are believed by many to be derived from an abnormal isoform of the cellular protein PrP ŽWells et al., 1987, 1991; Hope et al., 1988; Kimberlin and Wilesmith, 1994.. During the period from 1988 when BSE was made a notifiable disease, and by the end of 1997, over 170 000 cases had been reported in over 34 000 farms in the UK. Others have suggested that more than a million cattle infected with BSE may have entered the food chain ŽAnderson et al., 1996.. Current evidence would suggest that the epidemic in the UK principally resulted from the
feeding of recycled sheep carcases and cattle meat in the form of meat and bone-meal ŽMBM. protein nutritional supplements contaminated with scrapie and BSE ŽWilesmith et al., 1991, 1996; Wilesmith, 1994; Kimberlin and Wilesmith, 1994; Collee and Bradley, 1997.. Although this practice in the UK had been carried out for decades, the sudden emergence of BSE is now presumed to have resulted from a number of changes introduced in 1981]1982 for economic reasons in the rendering practices from batch production to continuous flow systems. These changes included the removal of the use of hydrocarbon extraction solvents and by an increase in the proportion of meat and bone meal in animal feeds from 1% to 12%, all occurring in the early 1980s ŽWilesmith et al., 1991; Butler, 1996.. The effect of curtailing the use of solvent extraction in the production of meat and bone meal was to remove two partial disinfection steps for the scrapierBSE agent: the prolonged exposure to organic solvents at high temperatures, and the subsequent removal of the last traces of solvent by treating meat and bone-meal with superheated Žlive. steam ŽWilesmith et al., 1991.. Live steam was replaced by a heat extraction method which apparently allowed the survival of BSE in UK cattle ŽMAFF, 1989.. The consequence was to allow enough infectivity to survive in most meat and bone-meal to cause an extended common source epidemic ŽKimberlin and Wilesmith, 1994; Wilesmith, 1994.. The epidemic has mostly involved cattle that were born in dairy herds where it is common to raise calves on concentrated feeds from a very early age ŽKimberlin and Wilesmith, 1994.. Wilesmith et al. Ž1992. suggested from modeling and case control studies that a high proportion of all BSE cattle were infected as calves during the first 6 months of life. The age-specific incidences of BSE reflect a minimum incubation period of 2 years and a median of 4]6 years. Following a ban on the use of animal protein in ruminant feed, compulsory notification of the disease and slaughter of affected animals in 1988 and a ban on specified bovine offals ŽSBOs. Žhighly infective tissues such as brain, spinal cord, testes, spleen, intestine, eyes and distal ileum. from all meat and bone-meal in 1989, animal cases of BSE
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peaked in 1993 with almost 1000 cases per week. These measures did not prevent 30 000 head of cattle born after the ban from contracting BSE. By December 1996, over 1 million cattle had been culled as part of an ongoing effort to remove from the food chain those most likely to have contracted BSE ŽDigiulio et al., 1997.. It should be noted that other countries ŽFrance, Portugal, Switzerland, Netherlands and Ireland. had at most several hundred cases of BSE in native cattle which are believed to be related to the use of imported cattle feed from the UK. Other countries ŽDenmark, Germany, Italy, Canada, Falkland Islands and Oman. reported BSE only in imported cattle from the UK ŽMeslin et al., 1997.. BSE has yet to be identified in domestic cattle in North America. It should be noted that under-reporting is believed to have occurred ŽSchreuder and Straub, 1996.. For example, around 1700 contaminated cattle could be expected to have occurred among 57 900 pure bred cattle exported from the UK to other European countries between 1985 and 1990, although only a much smaller number had been reported from these countries. The BSE epidemic is expected to disappear in the UK by the year 2001 ŽAnderson et al., 1996.. Although it is conceded that vertical transmission from cow to calf may occur at low levels Žvia mechanisms which are not yet clear., these are probably not enough to maintain the BSE epidemic ŽAnderson et al., 1996.. Although the BSE epidemic is declining in the UK, it is estimated that around half a million BSE-infected animals have entered the human food chain in the UK before the 1989 ban on the use of bovine tissues likely to contain the BSE agent in food for human consumption and another 300 000 between the ban and the end of 1995 ŽAnderson et al., 1996; Dora, 1998.. 3. Transmissible spongiform encephalopathies (TSEs) Transmissible spongiform encephalopathies ŽTSEs. are a group of rare, progressively non-inflammatory neurodegenerative diseases affecting humans and animals. They are characterized by relatively long incubation periods, no detectable
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antibody response and are diagnosed on clinical manifestations and on histopathology which demonstrate a varying degree of spongiform degeneration of neurones, astrocytic gliosis and amyloid plaque formation ŽAllen, 1993; Kimberlin and Wilesmith, 1994; Brown, 1997.. TSEs can be genetic, sporadic or transmissible; all three forms occur in humans and appear to be invariably fatal ŽLantos, 1992.. The pathogenic agentŽs. has not been unequivocally identified and pathogenesis remains poorly understood. 3.1. Human TSEs Human TSEs Žalso identified as prion diseases. can present as sporadic, genetic or acquired infectious disorders ŽPrusiner, 1991, 1994, 1997; Collinge and Palmer, 1994; Brown, 1997.. About 15% of human TSEs are inherited and are associated with coding mutations in the prion-protein gene ŽPRNP. ŽWindl et al., 1996.. At least 18 pathogenic mutations are now recognized within the PrP gene leading to inherited prion disease ŽCollinge and Palmer, 1994.. Human TSEs include: kuru, which was transmitted through ritual cannibalism of dead elders amongst the Fore speaking population of Papua New Guinea, and described around 1900 ŽAlpers, 1987.; familial forms such as Gertsmann-Straussler-Scheinker ¨ ŽGSS. syndrome Žvertically transmitted in an apparently autosomal dominant fashion; the incidence estimated to be between 1 and 10 per hundred million. ŽGertsmann et al., 1976; Hsiao et al., 1989.; fatal familial insomnia ŽFFI. ŽMedori et al., 1992a,b.; sporadic CJD, iatrogenic CJD Žtransmitted via neurosurgical procedures . ŽWeller, 1989; Brown et al., 1994., and the new variant ŽvCJD. ŽWill et al., 1996a,b; Wilesmith et al., 1996; Chazot et al., 1996; Almond and Pattison, 1997.. It is not yet clear whether all of the 18 types of inherited prion diseases will be transmitted to experimental animals ŽCollinge and Palmer, 1994.. 3.2. Animal TSEs Scrapie, which was first identified in sheep in the 17th century occurs as a natural and fairly common affliction in sheep and goats. Sheep
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scrapie is enzootic in Great Britain affecting 0.5]1.0% of the sheep population per year. Scrapie also exists in domestic sheep in North America. It is characterized by many variants and can be transmitted vertically and horizontally. Environments which have housed scrapie-infected sheep seem to harbor the infectious agent for prolonged periods, despite disinfection and eradication measures ŽBruce, 1993.. Scrapie has never been documented to have been transmitted to humans ŽAllen, 1993; Digiulio et al., 1997; Aguzzi and Weissmann, 1997.. Raymond et al. Ž1997. in their molecular assessment Žutilizing an in-vitro system. of the potential transmission of scrapie and BSE to humans suggested that BSE would be no more transmissible to humans than is sheep scrapie. Spongiform encephalopathy has occurred as infrequent outbreaks in commercially bred mink and as sporadic cases in wild or semi-captive ungulates. Chronic wasting disease ŽCWD. first identified in 1967, naturally occurs in 3% of the populations of mule deer, black-tailed deer and Rocky Mountain elk in Colorado and Wyoming in the US and in the Canadian provinces of Ontario and Saskatchewan. Cases have occurred primarily in captive animals, but a few free-ranging animals have been identified. The minimum incubation period for CWD is 18 months, and the time between onset and death is 3 weeks]8 months. The origin of this disease is not known. There was no obvious connection between this disease and rendered animal protein, nor with scrapieinfected sheep ŽWilliams and Young, 1992; Digiulio et al., 1997.. CWD is not the result of food-borne exposure to the infectious agent. CWD appears to be maintained within captive populations by lateral and possibly maternal transmission ŽWilliams and Young, 1992.. BSE is known to have caused ‘prion disease’ in a number of other species including domestic cats Žfeline spongiform encephalopathy. and captive ungulates Žnyala and kudu. presumably as a result of ingestion of BSE-contaminated feed ŽFraser et al., 1992; Allen, 1993; Bruce et al., 1994; Foster et al., 1996.. Bruce et al. Ž1997. reported that the strain of agent from cattle affected by BSE produces a characteristic pattern of disease in mice
that is retained after experimental passage through a variety of intermediate species thus reproducing what is believed to be the route of infection of cattle. BSE was found to be experimentally transmitted by injection of affected brain homogenates to cattle ŽDawson et al., 1991.; mice ŽFraser et al., 1992. and marmosets ŽBaker et al., 1993.. CJD and vCJD have also been experimentally transmitted through inoculation of brain tissue into the brain of mice ŽBruce et al., 1997.. Wild cats developed BSE after being fed affected meat, and mice and primates also contacted TSEs under experimental conditions. The incubation period is dose- and route-dependent and the oral route is the least efficient at transmitting the agent. The oral route requires sufficient challenge as evidenced by experimental transmission from sheep to goat ŽPattison and Millson, 1961; Pattison et al., 1972.. While there is no evidence that infectivity can be transmitted intravenously from human to human, plasma fractions from CJD patients were found to transmit the disease when injected into the brains of mice ŽMorris, 1997; WHO, 1997.. 4. Comparison of Creutzfeldt-Jacob disease (CJD) and variant-Creutzfeldt-Jacob disease (vCJD) 4.1. Creuzfeldt-Jacob disease (CJD) CJD was first described in the 1920s and although the most common human spongiform encephalopathy, it is a rare progressive neurogenerative disorder responsible for about 1 death per million people annually worldwide ŽBrown et al., 1985; Collinge et al., 1996; Brown, 1997.. Until recently CJD was considered of limited public relevance. CJD may be sporadic, acquired from environmental sources, especially iatrogenic sources or familial due to mutation in a gene or chromosome 20 ŽGoldfarb et al., 1994.. Recognized iatrogenic routes of transmission of CJD are treatment andror contamination with human cadaveric pituitary growth hormone or gonadotropins, dura matter or corneal grafting and the use of inadequately sterilized neurosurgical instruments and contaminated stereostatic instruments ŽBrown et al., 1985; Weller, 1989..
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The experimental evidence, although limited, identifies a theoretical risk that prion disease might be transmitted by transfusion ŽDealler, 1996., there is no clear evidence that this has occurred ŽFlanagan and Barbara, 1996.. Esmonde et al. Ž1993. reviewed 202 cases of CJD reported to the CJD surveillance unit in Edinburgh. There was no excess of cases with a history of blood transfusion or indeed donation of blood. Transfusion was calculated to be a potential cause of a maximum of only 0.2% of clinical cases of CJD in the UK population if the BSE epidemic were to spread to humans ŽDealler, 1996.. It should also be noted that Dodelet and Cashman Ž1998. recently reported that cellular prion protein ŽPrP c . is expressed in CD34-positive stem cells, very early in haemopoiesis. Since PrP c expression is essential for prion disease pathogenesis, it was suggested that bone marrow should perhaps be considered a high-risk tissue. It should also be noted that a new precautionary measure was recently announced in the UK to prevent infection of blood-product recipients with new-variant vCJD ŽMitchell, 1998.. The UK will, in the future import blood plasma, although there is no evidence that vCJD can be contracted from blood products. Whole blood for transfusions will continue to be from UK sources. The majority of sporadic CJD cases are homozygous at polymorphic residue 129, a common protein polymorphism in human PrP that is known to be important in genetic susceptibility to human prion diseases ŽBaker et al., 1991; Collinge et al., 1991; Palmer et al., 1991; Prusiner, 1991, 1994, 1997; Brown, 1997.. The classic sporadic variety of CJD usually occurs in the late middle age Žaverage age 60 years.; however CJD occasionally affects younger people including adolescents. Death usually occurs within 12 months of onset. Typically signs of CJD begin with loss of memory or confusion, however behavioral aberrations or gait instability are the presenting symptoms in about 1r3 of the patients. Later clinical features include a wide range of neurologic signs referable to the visual and motor cortex, basal ganglia and cerebellum. At autopsy microscopic sections of the brain exhibit widespread spongiform change accompanied
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by gliosis and neuronal loss while immunochemical staining reveals the presence of ‘prion’ protein positive amyloid deposits ŽBrown, 1997.. Five to 10% of CJD cases occur in family clusters and in these cases display an autosomal dominant pattern of inheritance ŽPrusiner, 1993.. Affected individuals show characteristic mutations in the prion protein gene coding ŽCollinge et al., 1991; Palmer et al., 1991.. 4.2. Variant Creutzfeldt-Jacob disease (¨ CJD ) The new variant form of CJD ŽvCJD. began to appear about 10 years after BSE was first identified in cattle in the UK and was described in March 1996 in a series of 10 young patients in the UK ŽWill et al., 1996a,b; Brown, 1997.. A similar case was described in France ŽChazot et al., 1996. and 10 others in the UK by October, 1997 ŽWilesmith et al., 1996.. To date 24 cases of vCJD are known. All 24 patients were potentially exposed to contaminated bovine products during the 1980s, before the magnitude of the BSE problem was recognized and measures were taken to prevent human exposure. The new variant vCJD is part of the CJD spectrum of diseases and shares with sporadic cases of CJD the features of progressive dementia, ataxia and myoclonus ŽWill et al., 1996a,b.. The neuropathology of vCJD is characteristic and includes large Kuru-like PrP positive amyloid plaques surrounded by a halo of spongiform Žflorid plaques.; deposition is in grey matter and there is prominent involvement of the cerebellum. However the recent cases of vCJD are as a group remarkable for their comparative youth at onset of symptoms Ž19]39 years. than classical sporadic types of CJD Ž55]70 years.. The new variant types of CJD exhibit an onset of the disease with psychiatric andror sensory symptoms, the absence of characteristic electro-encephalographic changes and an unusually long duration Žaverage, 14 months.. The prion protein genotype ŽPRPN. at codon 120 is homozygotic methioninermethionine in all cases of the new variant and predominantly but not always homozygotic in sporadic CJD ŽCollinge et al., 1991, 1996; Collinge and Palmer, 1994..
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Currently definitive diagnosis of either CJD or its new variant vCJD is possible only after death by histological examination of the brain. The only procedures capable of distinguishing between the variant and the sporadic forms of the diseases are evaluation of neuropathological lesions of PrP glycotyping or Western blot. Two newly described procedures have been suggested to offer earlier identification of vCJD. Neuroimaging techniques such as single photon emission computed tomography ŽSPECT. Žde Silva et al., 1998. and cranial magnetic resonance imaging ŽWill and Zeider, 1996; Zeidler et al., 1997. have shown promise in the identification of a small number of vCJD cases. The recent diagnosis of CJD by measurements of serum S-100 protein in a prospective case-control study employing 224 patients ŽOtto et al., 1998. also suggested to be promising and requiring further validation ŽPocchiari, 1998.. The new variant ŽvCJD. has strain characteristics distinct from other types of CJD and which resemble BSE transmitted to mice, domestic cat and macaque, consistent with BSE being the source of this new disease ŽScott et al., 1993; Bruce et al., 1994; Collinge et al., 1996.. 5. Hypotheses for transmissible spongiform encephalopathy (TSE) diseases Various hypotheses concerning the nature of the pathogen in BSE, CJD and the new variant vCJD have been advanced including speculations about parasites, viruses, self-replicating protein prions and organophosphates and whether BSE is an autoimmune disease due to bacteria showing molecular mimicry with brain antigens ŽPrusiner, 1997; Ebringer et al., 1997; Chesebro, 1998.. In regard to viruses, it is noted that although to date no direct evidence for a TSE virus has been obtained, it should not deter future search efforts despite the difficulties ŽChesebro, 1998.. 5.1. Prions CJD and BSE are both transmissible spongiform encephalopathies, fatal neurogenerative diseases that can effect both humans and animals. They are characterized by deposits of protein aggregates in the brain. The principal hypothesis
proposed to date is that BSE, CJD and vCJD are prion diseases which are transmitted by infectious proteins rather than by traditional infectious agents such as viruses or bacteria ŽPrusiner, 1991, 1994, 1997; Prusiner et al., 1990; Hill et al., 1997; Hegde et al., 1998.. These infectious proteins, unlike viruses and bacteria, are resistant to heat, ultra violet and ionizing radiation as well as disinfectants such as formaldehyde and gluteraldehyde. The hallmark of all prion diseases, whether sporadic, dominantly inherited or acquired by infection, is that they involve the aberrant metabolism and resulting accumulation of the prion protein ŽPrusiner, 1991, 1994, 1997.. Prions are defined as proteinaceous infectious particles that lack nucleic acid. PrP c is the cellular protein and PrP sc is the pathologic isoform. This conversion of normal cellular protein into PrP sc abnormal disease-causing isoform involves a conformation change whereby the alpha-helical content diminishes and the amount of beta-sheet increases ŽPrusiner, 1997. resulting in profound changes in the properties of the protein. For example, PrP c is readily digested by proteases whereas PrP sc is partially resistant ŽOesch et al., 1985.. The normal prion protein ŽPrP c . which is found in neuronal tissue is encoded by a single copy host gene and has unknown physiological functions. PrP c is believed to be transformed upon infection into an insoluble and proteinase resistant isoform PrP sc which forms the central component of the transmissible agent Žpossibly via changes in form as both proteins are chemically identical.. This information would be transmitted without the involvement of nucleic acids. In a recent molecular analysis of prion strain variation and the aetiology of ‘new variant’ CJD, Collinge et al. Ž1996. reported that strains of transmissible spongiform encephalopathies are distinguished by differing physico-chemical properties of PrP sc , the disease-related isoform of prion protein which can be maintained on transmission to transgenic mice ŽScott et al., 1993; Collinge et al., 1996.. The new variant vCJD has strain characteristics distinct from other types of CJD and which resemble those of BSE transmit-
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ted to mice, domestic cat and macaque consistent with BSE being the source of this new disease strain. The clinical, molecular and pathological features Ž‘signature’. of the BSE agent in cynomologus macaques is identical to that of vCJD in humans ŽLasmezas et al., 1996.. Collinge et al. Ž1996. suggested that the prion protein may itself encode disease phenotype. The new variant vCJD is associated with a unique and highly consistent appearance of protease-resistant PrP on Western blots, involving a characteristic pattern of glycosylation. It was noted that whereas transmission of CJD to inbred mice produces a pattern characteristic of the inoculated CJD, transmission of BSE produces a glycoform ratio pattern closely similar to new variant CJD. As noted earlier, experimental BSE in macaques and naturally acquired BSE in domestic cat show an indistinguishable glycoform pattern to experimental murine BSE and vCJD. Additionally, transmission of types 1, 2 and 3 CJD to transgenic mice expressing human PrP reveals persistence on conversion of strain type dependent on PRNP codon-129 genotype, hence providing supportive evidence for the ‘protein only’ hypothesis of infectiousness and indicating that strain variation might be encoded by a combination of PrP conformation and glycosylation. Hence considerable evidence from biochemical, immunologic, pathologic and genetic studies strongly suggest that PrP sc is the major, if not the only component of the transmissible prion particle ŽHill et al., 1997; Scott et al., 1993; Prusiner, 1991, 1994, 1997.. Korth et al. Ž1997. recently described a monoclonal antibody, 15B3, that could discriminate between the normal and disease specific forms of PrP. This long sought antibody could be of great value in characterizing the infectious particle as well as for the diagnosis of TSEs such as BSE or CJD in humans. The monoclonal antibody 15B3 specifically precipitates bovine, murine or human PrP sc but not PrP c suggesting that it recognizes an epitope common to prions from different species, and may represent a starting point for the production of further diagnostic or therapeutic tools for TSEs. A number of arguments have been marshalled against the protein-only hypothesis. It should be
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noted that not only can ‘prion disease’ appear in the absence of detectable levels of PrP sc ŽAguzzi and Weissmann, 1997., but protease-resistant PrP with no infectivity can be generated ŽRiesner et al., 1996.. It was suggested that there may be several structures of PrP that differ from that of PrP c , the infectious form of which PrP sc may or may not coincide with a protease-resistant structure ŽAguzzi and Weissmann, 1997.. A number of other difficulties with the prion hypothesis have been cited. For example, the hypothesis is not compatible with the different strains of TSEs which can be differentiated based on incubation period, histopathological pattern of brain lesions and infectivity to other species ŽCarlson, 1991.. It still has not been unambiguously resolved what triggers the normal cell protein to transform into the disease-causing isoform of the protein. Mutations in PrP were found to be associated with the Gertsmann-Straussler¨ Scheinker ŽGSS. syndrome and CJD and could lead to changes in the glycoprotein resulting in amyloid formation. It is still not known whether the mutant protein would lead to the generation of the infectious agent or make individuals more susceptible, allowing the replication of a relatively prevalent, usually subclinical, infectious particle ŽPrusiner, 1993..
5.2. Autoimmune disease
It has been suggested that bovine spongiform encephalopathy ŽBSE. could be an autoimmune disease produced following exposure of cattle to feedstuffs containing bacteria exhibiting molecular mimicry between bacterial components and bovine tissue. For example, three bacteria Ž Acinetobacter calcoaceticus, Ruminococcus albus and Agrobacter tumefaciens. share sequences with the encephalitogenic peptide of bovine myelin, while three molecules in E. coli show molecular mimicry with host-encoded prion protein. It was further proposed that immune responses against these bacteria at both T and B cell levels may cause neurological tissue injury resembling BSE ŽEbringer et al., 1997..
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5.3. Chronic pesticide-initiated modification of prions It has recently been proposed by Purdy Ž1996a,b. that BSE may have originated via exposure of bovine embryo to various specific highdose lipophilic formulations of organophosphates. Specifically, high-dose formulations of organophosphate phosmet containing phthalimide which were applied compulsorily and exclusively in the UK during the 1980s and early 1990s as the primary trigger that initiated the deformation of prion protein and the onset of the BSE epidemic. It was suggested that the multi-site binding metabolites of these organophosphates could penetrate the fetus covalently phosphorylating various active sites on fetal prion protein. Further, the extra charged phosphate groups left on aged prion protein blocks both proteases and chaperones from accessing their catalyticrbonding sites leading to the creation of undegradable, misfolded isoform of prion protein PrP sc . It was noted by Purdy Ž1996a,b. that high dose phosmet Žand its phthalimide ingredient. formulations were employed in the UK for the control of warble fly and control of other ectoparasites such as lice by direct application on the backs of cattle in the 1980s. Additionally organophosphates were used on stored grain for prevention of weevil-mite infestation. Treatment alternated between the use of pirimphosmethyl and malathion. Maximum residue limits of organophosphates on stored grain had to be raised from 2 to 10 ppm during the 1980s and early 1990s, with much of the wheat from these stores going as an ingredient for cow concentrate feed during the 1980s. It was suggested that the timing, distribution and dynamics of usage of these specific organophosphates correlates with the epidemiology of BSE as well as accounting for the 23 000 cattle that have developed the disease in the UK, yet were born after the 1988 ban on scrapie-contaminated cattle feed. 6. Food safety and surveillance issues related to BSE and vCJD The recent crisis in the UK resulting from the
outbreak of BSE in cattle and the resultant deaths of 23 individuals Žand one in France. due to a new human variant of Creutzfeldt-Jacob disease ŽvCJD. linked to BSE and the consumption of beef has led to a wide clamour and insistence from a broad panoply of consumers, scientists and government officials for greater assurance of safety and surveillance of the food supply. Following its BSE epidemic in cattle and in response to concern about possible transmission to humans, the United Kingdom implemented an active surveillance system for CJD in 1990. By 1993, France, Germany, The Netherlands and Italy joined the UK and initiated surveillance systems employing standard methods and exchanging expertise ŽWientjens et al., 1994.. A protocol for a case control study examining risk factors for CJD is being implemented linked to the active surveillance in these countries. Additionally, methods to support wide implementation of CJD surveillance are being developed and tested by WHO and should be available in early 1998 ŽWHO, 1997., and will include information and training modules for clinical and pathological diagnosis, implementation and evaluation of the surveillance system, measures to protect public health information and communication on BSErCJD ŽDora, 1998.. A number of basic measures for the protection of public health from the potential risk of acquiring BSE have been issued by the World Health Organization ŽWHO., Organization International des Epizootes ŽOIE., the Food and Agricultural Organization ŽFAO. as well as by national and regional governments and organizations Že.g. European Union.. Basically these measures fall into three groups: Ža. to avoid BSE-contaminated animals and meat products entering the food chain or are to be used in the production of medicinal products; Žb. to provide monitoring and surveillance of BSE and CJD; and Žc. to identify research needs and support their implementation. It is noted that these measures are being revised and updated as required regularly ŽDora, 1998.. In the above regard, WHO Ž1996. has issued recommendations on the measures to protect public health agreed to by WHO, FAO and OIE.
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These recommendations included: Ža. no part or product of any animal which has shown signs of transmissible spongiform encephalopathy ŽTSE., nor tissues that are likely to contain BSE agent should enter the food chain Žhuman or animal.; Žb. all countries should establish surveillance and compulsory notification of BSE; Žc. all countries should ban the use of ruminant tissues in ruminant feed; Žd. milk and milk products are considered safe; Že. gelatin and tallow are only considered safe if effective rendering procedures are used; and Žf. medicinal products and medical devices should be obtained from countries with no sporadic cases of BSE and measures recommended to minimize the risk. The International Animal Health Code, produced by the Organization International des Epizootes ŽOIE. was provisionally amended in May, 1996 to state the following: Ža. meat and bone meal containing ruminant protein from countries with high incidence of BSE should not be traded; Žb. those Žmeat and bone meal products as above. from countries with low incidence of BSE should not be traded for use in ruminant feed; and Žc. putting restrictions on trade of specified offals Žbrain, eyes, spinal cord, thymus, spleen and distal ileum.. The OIE has also started to differentiate between countries with high and low incidence of BSE and has issued guidelines for the surveillance and monitoring of BSE ŽDora, 1998.. FAO Ž1998. is in the process of publishing a report on animal feeding and food supply that is already available on the Internet. The Codex Alimentarius Committee that implements the Joint FAOrWHO food standards programme has also made recommendations regarding carcasses and that all parts of affected cattle suspected of BSE be destroyed. The European Commission DG24 responsible for consumer safety, maintains a world wide web page with information on measures to protect public health taken by the UK and by the EC ŽDora, 1998.. The European Union’s Scientific Steering Committee on January 23, 1998 approved a plan that could exempt a number of countries, including the US from the EU’s April 1, 1997 ban on specified risk material ŽSRM. linked to BSE ŽAnon, 1998a.. The plan would allow the EU to
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assess the risk on a regional basis. The Steering Committee would require countries claiming BSE-free status to provide it all available data. The assessment would judge risk at three levels: Ža. incident risk } the probability that an infectious animal Žor material thereof. enters the food andror animal feed chain; Žb. propagation risk } the probability that an initial infection is propagated within the system of a given region and within a given time period; and Žc. human exposure risk } the probability that a human being is exposed to an infective dose of the BSE agent, within a given time period. The incident risk and propagation risk taken together will be used to assess the geographical risk. It should be noted that the first risk assessments will not consider the human exposure risk. Since the consumption of infectious meat-andbone meal ŽMBM. is considered one of the major sources of BSE, EU’s Steering Committee said that it is essential to have a clear understanding of the consumption and source of MBM in a given geographical area in order to assess the probability that BSE might occur in its animal populations Žincident risk.. Additionally the suppression of specified bovine offals and specified risk material ŽSRM. Že.g. cattle brains, spinal cords, spleens. from entering the food and feed chains will reduce the risk of BSE infection. The Committee categorizes some tissues at a higher level than warranted by their intrinsic infectivity, for practical reasons relating to slaughterhouse contamination. Tissues considered as possessing high infectivity include: bovine brain, eyes, spinal cord, dorsal root ganglia, dura matter, pituitary, skull vertebral column and lungs. The EU recently established a system to tag and track cattle in an attempt to contain further outbreaks of BSE ŽAnon, 1998b.. The series of three regulations also requires cattle ‘passports’ and national registers to follow cattle movements. Each member state is to use inspection results to compile an annual report to the Commission listing the number of herds in the member state, the number of inspections and animals checked, any breach found and any sanctions imposed. The British government in December, 1997, in an attempt to further minimize the risk of BSE
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from the consumption of British meat, recently banned the sale of beef and lamb on the bone. It closely followed the advisory committee’s advice that ‘no beef with bone from cattle over 6 months old’ be sold and that ‘cattle slaughtered between 24 and 30 months of age for human consumption be deboned in licensed plants’ ŽAnon, 1998c.. The British government announced in December 1997 that it will conduct an inquiry into the outbreak of BSE which will focus on the government’s handling of the crisis following the discovery of BSE in the 1980s. It will cover a period of time between that discovery and March 1996 when the government announced that BSE and a human brain disease CJD were linked ŽAnon, 1998d.. It is also important to note specific bovinerelated products that have come under scrutiny. For example, since tests on milk from BSE-infected animals have not shown any BSE infectivity and there is evidence from other animal and human spongiform encephalopathies to suggest that milk will not transmit these diseases, milk and milk products are considered safe even in countries such as the UK with a high incidence of BSE. Gelatin is considered safe for human consumption since its preparation involves a chemical extraction process that destroys BSE infectivity. Similarly tallow is considered safe if effective rendering procedures are used. In the US, the source of raw materials for gelatin manufacture is exclusively from pork skins, beef hides or domestic beef bones. The food safety risk from all foods made from gelatin is considered extremely low ŽAnon, 1998e.. Note in proof The proceedings of the British Board of Inquiry are accessible on the web site: http:rrwww.bse.org.uk ŽButler D. British BSE reckoning tells a dism al tale. N ature 1998;392:532]533.. References Aguzzi A, Weissmann C. Prion research: the next frontiers. Nature 1997;389:795]798.
Allen IV, editor. Spongiform encephalopathie. Br Med Bull 1993;49Ž4.:725]1012. Almond J, Pattison J. Human BSE. Nature 1997;389:437]438. Alpers MP. Epidemiology and clinical aspects of kuru. In: Prusiner SP, McKinley MP, editors. Prions: novel infectious pathogens causing scrapie and Creutzfeldt-Jakob disease. San Diego: Academic Press, 1987:451]465. Anderson RM, Donnely CA, Ferguson NM. Transmission dynamics and epidemiology of BSE in British cattle. Nature 1996;382:779]788. Anon. EU considers exemptions to ban on animal material linked to mad cow disease. World Food Chem News 1998a;4Ž21.:3, 17]19. Anon. EU establishes system to track cattle as part of BSE prevention plan. World Food Chem News 1998b;4Ž19.: 18]19. Anon. More evidence needed to confirm infection of bone marrow, study finds. World Food Chem News 1998c;4Ž20.:4. Anon. Britain announces year-long inquiry into BSE crisis. World Food Chem News 1998d;4Ž19.:11]12. Anon. Food gelatin industries protest spine removal in FDA BSE guide. Food Chem News 1998e;39Ž48.:4]5. Baker HF, Poulter M, Crow TJ, Frith CD, Lofthouse R, Ridley RM. Amino acid polymorphism in human prion protein and age of death in inherited prion disease. Lancet 1991;337:1286. Baker HF, Ridley RM, Wells GAH. Experimental transmission of BSE and scrapie to the common marmoset. Vet Rec 1993;132:403]406. Brown P. The risk of bovine spongiform encephalopathy Ž‘mad cow disease’. to human health. J Am Med Assoc 1997;278:1008]1011. Brown P, Cervenakova L, Goldfarb LG, et al. Iatrogenic Creutzfeldt-Jacob disease: an example of the interplay between ancient genes and modern medicine. Neurology 1994;44:291]293. Brown P, Gajdusek DL, Gibbs CJ, Asher DM. Potential epidemic of Creutzfeldt-Jacob disease from human growth hormone therapy. N Engl J Med 1985;313:728]730. Bruce ME. Scrapie strain variation and mutation. Brit Med Bull 1993;49:822]838. Bruce ME, Chree A, McConnell I, Foster J, Pearson G, Fraser H. Transmission of bovine spongiform encephalopathy and scrapie in mice: strain variation and the species barrier. Phil Trans R Soc Lond B 1994;343:405]411. Bruce ME, Will RG, Ironside JW. Transmissions to mice indicate that ‘new variant’ CJD is caused by BSE agent. Nature 1997;389:498]501. Butler D. Did UK ‘dump’ contaminated feed after ban? Nature 1996;381:544. Carlson GA. Genetics of prion diversity and host susceptibility. Curr Top Microbiol Immunol 1991;172:153]164. Chazot G, Broussole E, Lapras CI, et al. New variant of Creutzfeldt-Jacob disease in a 26 year-old French man. Lancet 1996;347:1181. Chesebro B. BSE and prions: uncertainties about the agent. Science 1998;279:42]44.
L. Fishbein r The Science of the Total En¨ ironment 217 (1998) 71]82 Collee JG, Bradley R. BSE: a decade on. Lancet 1997;349:636]641, 715]721. Collinge J, Palmer MS, Dryden AJ. Genetic predisposition to iatrogenic Creutzfeldt-Jacob disease. Lancet 1991;337: 1441]1442. Collinge J, Palmer MS. Molecular genetics of human prion diseases. Phil Trans R Soc Lond B 1994;343:371]378. Collinge J, Sidle KCL, Meads J, Ironside J, Hill AF. Molecular analysis of prion strain variation and the aetiology of new variant ‘CJD’. Nature 1996;383:685]690. Dawson M, Wells GAH, Parker BNJ, Scott AC. Transmission studies of BSE in cattle, hamsters, pigs and domestic fowl. In: Bradley R, Savey M, Marchant BA, editors. Sub-Acute spongiform encephalopathies. Current topics in veterinary medicine and animal sciences. Dordrecht: Kluwer Academic, 1991;55:25]32. Dealler S. A matter of debate: the risk of bovine spongiform encephalopathy to humans posed by blood transfusion in the UK. Transfusion Med 1996;6:217. de Silva R, Hadley D, Russell A, Turner M, Zeidler M. Single emission computed tomography in the identification of new variant Creutzfeldt-Jakob disease: case reports. Br Med J 1998;316:593]594. Digiulio K, Crawford L, Woo R, Gaull G. International symposium on spongiform encephalopathies: generating rational policy in the face of public fears. Trends Food Sci Technol 1997;8:204]206. Dodelet VC, Cashman NR. Prion protein expression in human leukocyte differentiation. Blood 1998;91:1556]1561. Dora C. Bovine spongiform encephalopathy and the new variants Creutzfeldt-Jacob disease. In: van der Heijden K, Younes M, Fishbein L, Miller S, editors. Food safety } science, regulation and control. Marcel Dekker, New York, 1998, in press. Ebringer A, Pirt J, Wilson C, Cunningham P, Thorpe C, Ettelaie C. Bovine spongiform encephalopathy: Is it an autoimmune disease due to bacteria showing molecular mimicry with brain antigens. Environ Health Perspect 1997;105:1172]1174. Esmonde TFG, Will RG, Slattery JM, et al. Creutzfeldt-Jakob disease and blood transfusion. Lancet 1993;341:205]207. FAO. Reports on animal feeding and food safety 1998; Internet Web site:- http:rrwww.fao.orgrwaicentrfaoinfor economicresnranimalrfeedrhttoc.htm ) . Flanagan P, Barbara JAJ. Prion disease and blood transfusion. Transfusion Med 1996;6:213]215. Foster JD, Brule M, McConnell I, Chree A, Fraser H. Detection of BSE infectivity in brain and spleen of experimentally infected sheep. Vet Rec 1996;138:546]548. Fraser H, Bruce ME, Chree A, McConnell I, Wells GA. Transmission of bovine spongiform encephalopathy and scrapie to mice. J Gen Virol 1992;73:1891]1897. ¨ Gertsmann J, Straussler E, Scheinker I. Uber eine eigenartige ¨ hereditar-familiare ¨ ¨ erkrankung zentral nervensystems zugleich ein beitrage zur frage des vorzeitigen lokalen alterns. Z Neurol 1976;154:736]762.
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Goldfarb LG, Brown P, Cervenakova L, Gajdusek DC. Genetic analysis of Creutzfeldt-Jakob disease and related disorders. Phil Trans R Soc Lond B 1994;343:379]384. Hegde RS, Mastrianni JA, Scott MR, DeFea KA, et al. A transmembrane form of the prion protein in neurodegenerative disease. Science 1998;279:827]833. Hill AF, Desbruslais M, Joiner S, Sidle KCL, et al. The same prion strain causes vC JD and B SE . N ature 1997;389:448]450. Hope J, Reekie JD, Hunter N, et al. Fibrils from brains of cows with new cattle disease containing scrapie-associated protein. Nature 1988;336:390]392. Hsiao K, Baker HF, Crow TJ, Poulter M, Owen F. Linkage of a prion protein missense variant to Gertsmann-Straussler ¨ Syndrome. Nature 1989;338:342]345. Kimberlin RH, Wilesmith JW. Bovine spongiform encephalopathy-epidemiology, low dose exposure and risks. Ann NY Acad Sci 1994;724:210]220. Korth C, Stierli B, Streit P, Moser M, Schaller O, Fischer R. Prion ŽPrP sc .-specific epitope defined by a monoclonal antibody. Nature 1997;390:74]77. Lantos PL. From slow virus to protein: a review of transmissible spongiform encephalopathies. Histopathology 1992;20:1]11. Lasmezas CI, Deslys JP, Demaimay R, et al. BSE transmission to macaques. Nature 1996;381:743]744. MAFF. Report of the Working Party on Bovine Spongiform Encephalopathy. London: Ministry of Agriculture, Fisheries and Food, Department of Health, 1989. Medori R, Montagna P, Tritschler HJ, Le Blanc A. Fatal familial insomnia: a second kindred with mutation of prion protein gene at codon 179. Neurol 1992a;42:669]670. Medori R, Tritschler HJ, LeBlanc A. Fatal familial insomnia, a prion disease with a mutation at codon 178 of the prion protein gene. N Engl J Med 1992b;326:444]449. Meslin FX, Stohr K, Formenty P. Emerging zoonoses. World Health 1997;50:18]19. Mitchell P. UK to import blood plasma because of fears over nvCJD. Lancet 1998;351:720. Morris K. WHO reconsiders risks from Creutzfeldt-Jakob disease. Lancet 1997;349:1001. Oesch B, Westaway D, Walchl M, McKinley MP, Kent BH, Aebersold R. A cellular gene encodes scrapie PrP 27-30 protein. Cell 1985;40:735]776. Otto M, Wiltfang I, Schutz E, Zerr I. Diagnosis of Creutzfeldt-Jakob Disease by measurement of S-100 protein in serum: prospective case control study. Br Med J 1998;316:577]579. Palmer MS, Dryden AJ, Hughes FT, Collinge J. Homozygous prion protein genotype predisposes to sporadic Creutzfeldt-Jacob Disease. Nature 1991;352:340]342. Pattison IH, Millson GC. Experimental transmission of scrapie to goats and sheep via the oral route. J Comp Pathol 1961;71:171]172. Pattison IH, Hoare MN, Jebbett JN. Spread of scrapie to
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sheep and goats by oral dosing of fetal membranes from scrapie-infected sheep. Vet Rec 1972;9:465]468. Pocchiari M. Early identification of variant Creutzfeldt-Jakob Disease. Br Med J 1998;316:18]19. Powell D, Leiss W. Mad cows and mother’s milk } the perils of poor risk communication. McGill-Queen’s University Press, Montreal, 1997:3]25. Prusiner SB, Scott M, Foster D, Pan KM, et al. Transgenic studies implicate interactions between homologous PrP isoforms in scrapie prion replication. Cell 1990;63:673]686. Prusiner SB. Molecular biology of prion diseases. Science 1991;252:1515]1520. Prusiner SB. Prion disease of humans and animals. J R Coll Phys 1993;28:1]30. Prusiner SB. Molecular biology and genetics of prion diseases. Phil Trans R Soc Lond B 1994;343:447]463. Prusiner SB. Prion diseases and BSE crisis. Science 1997;278:245]251. Purdy M. The UK epidemic of BSE: slow virus or chronic pesticide-initiated modification of the prion protein? I. Mechanisms for a chemically induced pathogenesisrtransmissibility. Med Hypotheses 1996a;46:429]443. Purdy M. The UK epidemic of BSE: slow virus or chronic pesticide-initiated modification of the prion protein? II. An epidemic perspective. Med Hypotheses 1996b;46:445]454. Raymond GJ, Hope J, Kocisko DA, Proiola SA, Raymond LD. Molecular assessment of the potential transmissibilities of BSE and scrapie to humans. Nature 1997;388:285]288. Riesner D, Kelling SK, Post K, Wille H, Serban H, Growth D. Disruption of prion rods generates 10 nm spherical particles having high alpha-helical content and lacking scrapie infectivity. J Virol 1996;70:1714]1722. Schreuder BEC, Straub OC. BSE: a European problem. Vet Rec 1996;6:575]577. Scott M, Foster D, Torchia M, et al. Propagation of prions with artificial properties in transgenic mice expressing chimeric PrP genes. Cell 1993;73:979]988. Weller RO. Iatrogenic transmission of Creutzfeldt-Jacob disease. Psychol Med 1989;19:1]4. Wells GAH, Scott AC, Johnson CT, et al. A novel progressive spongiform encephalopathy in cattle. Vet Rec 1987;121:419]420. Wells GAH, Wilesmith JW, McGill IS. Bovine spongiform
encephalopathy: a neuropathological perspective. Brain Pathol 1991;1:69]78. WHO. Report of a WHO consultation on public health issues related to human and animal transmissible spongiform encephalopathies. With the participation of FAO and OIE. Geneva: World Health Organization, 2]3 April, 1996. WHO. EURO Regional Committee Meeting, Istanbul, September 1997. Report on activities regarding human TSEs and BSE. Geneva: World Health Organization, 1997. Wientjens DPWM, Will RG, Hofman A. Creutzfeldt-Jacob disease: a collaborative study in Europe. J Neurol Neuropathol 1994;57:1285]1299. Wilesmith JW, Ryan JBM, Atkinson MJ. Bovine epidemiological studies on the origin. Vet Rec 1991;128:199]203. Wilesmith JW, Ryan JMB, Hueston WD. Bovine spongiform encephalopathy: case-control studies of calf-feeding practices and meat and bone meal inclusion in proprietary concentrates. Res Vet Sci 1992;52:325]331. Wilesmith JW. An epidemiologist’s view of bovine spongiform encephalopathy. Phil Trans R Soc Lond B 1994;343: 357]361. Wilesmith JW. A new variant of Creutzfeldt-Jacob disease in the UK. Lancet 1996;347:921]925. Wilesmith JW, Ryan JBM, Atkinson MJ. Bovine spongiform encephalopathy; epidemiologic studies on the origin. Vet Rec 1996;128:199]203. Will R, Zeider M. Diagnosing Creutzfeldt-Jakob disease. Brit Med J 1996;313:833]834. Will RG, Ironside JW, Zeidler M, et al. A new variant of Creutzfeldt-Jacob disease in the UK. Lancet 1996a; 347:921]995. Will RG, Zeidler M, Brown P, et al. Cerebrospinal-fluid test for new-variant Creutzfeldt-Jacob disease. Lancet 1996b;349:955. Williams ES, Young S. Spongiform encephalopathies in cervidae. Rev Sci Tech Off Int Epiz 1992;11:551]567. Windl O, Dempster M, Estibeiro JP et al. Genetic basis of Creutzfeldt-Jacob disease in the United Kingdom: a systematic analysis of predisposing mutations and allelic variations in the PrNP gene. Hum Genet 1996;98:259]264. Zeidler M, Johnstone EC, Bamber RWK, Dickens CM, Fischer CJ. New variant Creutzfeldt-Jacob disease: psychiatric feature. Lancet 1997;350:908]910.
Transmissible spongiform encephalopathies, hypotheses and food safety: An overview L. Fishbein 4320 Ashford Lane, Fairfax, VA 22032, USA Received 10 March 1997; accepted 20 March 1997
Abstract The transmissible spongiform encephalopathies ŽTSEs. Žin both humans and animals. have been reviewed with the principal focus on bovine spongiform encephalopathy ŽBSE., its recent outbreak amongst cattle in the UK Ž‘mad cow disease’. and its sequelae. The possible reasons for this outbreak are discussed as well as a number of hypotheses reviewed for TSEs Že.g. prions, organophosphates, etc.. and current measures attendant with food safety and surveillance taken in the European Union. It is generally accepted that the combined weight of all the evidence to date supports the conclusion that the new rare but lethal variant Creutzfeldt-Jacob Disease ŽvCJD. is the human counterpart of the aetiological agent BSE and that eating meat from the infected animals is probably to blame for 24 deaths Ž23 in the UK and one in France. to date from vCJD. Considerable evidence from biochemical, immunologic, pathologic and genetic studies strongly suggests that PrP sc is the major component of the transmissible prion particle responsible for the rare fatal brain TSE diseases. Q 1998 Elsevier Science B.V. All rights reserved. Keywords: Bovine spongiform encephalopathy; Creutzfeldt-Jacob disease; Prions; Food safety
1. Introduction Few diseases have captured both scientific and public awareness and anxiety this decade, as has bovine spongiform encephalopathy ŽBSE. or as more popularly known in the lay press as ‘mad cow disease’ ŽPowell and Leiss, 1997.. Widespread anxiety was generated when officials in the United Kingdom made public on March 20, 1996, a possible association between BSE and an apparently new but lethal human variant Creutzfeldt-Jacob
disease ŽvCJD. had occurred in 10 people Žnine of whom were under 35 years of age. in the UK over the previous 14 months ŽWilesmith, 1996; Will et al., 1996a,b.. A similar case of this variant ŽvCJD. was described in France ŽChazot et al., 1996. and 10 additional cases in the UK by October, 1997 ŽWilesmith et al., 1996.. So far a total of 24 cases of vCJD are known Ž23 in the UK and one in France. ŽPocchiari, 1998.. Many in the UK are especially fearful about the link between BSE and vCJD since the incubation period for CJD ranges
0048-9697r98r$19.00 Q 1998 Elsevier Science B.V. All rights reserved. PII S0048-9697Ž98.00164-8
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from 10 to 40 years. Although many individuals who are not yet exhibiting symptoms of vCJD may nevertheless already have been infected. It is impossible to predict how many people are now incubating the new variant form of the disease ŽPocchiari, 1998.. The emergence of vCJD following the BSE epidemic in the UK has engendered an intense public debate Žnow scarcely abated. in the European Union and elsewhere on issues including: food safety risk assessment, regulation and communication; agricultural practices, consumer confidence in beef and beef products and on food markets in general; and in relations between member States of the European Union. A broad question has emerged of how to build public trust in a manner in which governments Žand their advisors and expert committees. handle scientific issues in a modern, media-based society. The general issue of generating rational policy in the face of public fears has been brought into sharper focus as a result of the BSE epidemic in the UK. 2. Bovine spongiform encephalopathies (BSEs) The epidemic of BSE was first identified in November 1986 as a new scrapie-like disease with the presence of characteristic spongiform lesions in the brains of clinically affected cattle and the appearance of a newly recognized form of neurological disease in cattle in the UK. Clinical signs of BSE in cattle involve behavioral gait and postural abnormalities and after a period of 1]6 months, convulsions and death. Extracts of affected cattle brains contain abnormal fibrils associated with scrapie and are believed by many to be derived from an abnormal isoform of the cellular protein PrP ŽWells et al., 1987, 1991; Hope et al., 1988; Kimberlin and Wilesmith, 1994.. During the period from 1988 when BSE was made a notifiable disease, and by the end of 1997, over 170 000 cases had been reported in over 34 000 farms in the UK. Others have suggested that more than a million cattle infected with BSE may have entered the food chain ŽAnderson et al., 1996.. Current evidence would suggest that the epidemic in the UK principally resulted from the
feeding of recycled sheep carcases and cattle meat in the form of meat and bone-meal ŽMBM. protein nutritional supplements contaminated with scrapie and BSE ŽWilesmith et al., 1991, 1996; Wilesmith, 1994; Kimberlin and Wilesmith, 1994; Collee and Bradley, 1997.. Although this practice in the UK had been carried out for decades, the sudden emergence of BSE is now presumed to have resulted from a number of changes introduced in 1981]1982 for economic reasons in the rendering practices from batch production to continuous flow systems. These changes included the removal of the use of hydrocarbon extraction solvents and by an increase in the proportion of meat and bone meal in animal feeds from 1% to 12%, all occurring in the early 1980s ŽWilesmith et al., 1991; Butler, 1996.. The effect of curtailing the use of solvent extraction in the production of meat and bone meal was to remove two partial disinfection steps for the scrapierBSE agent: the prolonged exposure to organic solvents at high temperatures, and the subsequent removal of the last traces of solvent by treating meat and bone-meal with superheated Žlive. steam ŽWilesmith et al., 1991.. Live steam was replaced by a heat extraction method which apparently allowed the survival of BSE in UK cattle ŽMAFF, 1989.. The consequence was to allow enough infectivity to survive in most meat and bone-meal to cause an extended common source epidemic ŽKimberlin and Wilesmith, 1994; Wilesmith, 1994.. The epidemic has mostly involved cattle that were born in dairy herds where it is common to raise calves on concentrated feeds from a very early age ŽKimberlin and Wilesmith, 1994.. Wilesmith et al. Ž1992. suggested from modeling and case control studies that a high proportion of all BSE cattle were infected as calves during the first 6 months of life. The age-specific incidences of BSE reflect a minimum incubation period of 2 years and a median of 4]6 years. Following a ban on the use of animal protein in ruminant feed, compulsory notification of the disease and slaughter of affected animals in 1988 and a ban on specified bovine offals ŽSBOs. Žhighly infective tissues such as brain, spinal cord, testes, spleen, intestine, eyes and distal ileum. from all meat and bone-meal in 1989, animal cases of BSE
L. Fishbein r The Science of the Total En¨ ironment 217 (1998) 71]82
peaked in 1993 with almost 1000 cases per week. These measures did not prevent 30 000 head of cattle born after the ban from contracting BSE. By December 1996, over 1 million cattle had been culled as part of an ongoing effort to remove from the food chain those most likely to have contracted BSE ŽDigiulio et al., 1997.. It should be noted that other countries ŽFrance, Portugal, Switzerland, Netherlands and Ireland. had at most several hundred cases of BSE in native cattle which are believed to be related to the use of imported cattle feed from the UK. Other countries ŽDenmark, Germany, Italy, Canada, Falkland Islands and Oman. reported BSE only in imported cattle from the UK ŽMeslin et al., 1997.. BSE has yet to be identified in domestic cattle in North America. It should be noted that under-reporting is believed to have occurred ŽSchreuder and Straub, 1996.. For example, around 1700 contaminated cattle could be expected to have occurred among 57 900 pure bred cattle exported from the UK to other European countries between 1985 and 1990, although only a much smaller number had been reported from these countries. The BSE epidemic is expected to disappear in the UK by the year 2001 ŽAnderson et al., 1996.. Although it is conceded that vertical transmission from cow to calf may occur at low levels Žvia mechanisms which are not yet clear., these are probably not enough to maintain the BSE epidemic ŽAnderson et al., 1996.. Although the BSE epidemic is declining in the UK, it is estimated that around half a million BSE-infected animals have entered the human food chain in the UK before the 1989 ban on the use of bovine tissues likely to contain the BSE agent in food for human consumption and another 300 000 between the ban and the end of 1995 ŽAnderson et al., 1996; Dora, 1998.. 3. Transmissible spongiform encephalopathies (TSEs) Transmissible spongiform encephalopathies ŽTSEs. are a group of rare, progressively non-inflammatory neurodegenerative diseases affecting humans and animals. They are characterized by relatively long incubation periods, no detectable
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antibody response and are diagnosed on clinical manifestations and on histopathology which demonstrate a varying degree of spongiform degeneration of neurones, astrocytic gliosis and amyloid plaque formation ŽAllen, 1993; Kimberlin and Wilesmith, 1994; Brown, 1997.. TSEs can be genetic, sporadic or transmissible; all three forms occur in humans and appear to be invariably fatal ŽLantos, 1992.. The pathogenic agentŽs. has not been unequivocally identified and pathogenesis remains poorly understood. 3.1. Human TSEs Human TSEs Žalso identified as prion diseases. can present as sporadic, genetic or acquired infectious disorders ŽPrusiner, 1991, 1994, 1997; Collinge and Palmer, 1994; Brown, 1997.. About 15% of human TSEs are inherited and are associated with coding mutations in the prion-protein gene ŽPRNP. ŽWindl et al., 1996.. At least 18 pathogenic mutations are now recognized within the PrP gene leading to inherited prion disease ŽCollinge and Palmer, 1994.. Human TSEs include: kuru, which was transmitted through ritual cannibalism of dead elders amongst the Fore speaking population of Papua New Guinea, and described around 1900 ŽAlpers, 1987.; familial forms such as Gertsmann-Straussler-Scheinker ¨ ŽGSS. syndrome Žvertically transmitted in an apparently autosomal dominant fashion; the incidence estimated to be between 1 and 10 per hundred million. ŽGertsmann et al., 1976; Hsiao et al., 1989.; fatal familial insomnia ŽFFI. ŽMedori et al., 1992a,b.; sporadic CJD, iatrogenic CJD Žtransmitted via neurosurgical procedures . ŽWeller, 1989; Brown et al., 1994., and the new variant ŽvCJD. ŽWill et al., 1996a,b; Wilesmith et al., 1996; Chazot et al., 1996; Almond and Pattison, 1997.. It is not yet clear whether all of the 18 types of inherited prion diseases will be transmitted to experimental animals ŽCollinge and Palmer, 1994.. 3.2. Animal TSEs Scrapie, which was first identified in sheep in the 17th century occurs as a natural and fairly common affliction in sheep and goats. Sheep
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scrapie is enzootic in Great Britain affecting 0.5]1.0% of the sheep population per year. Scrapie also exists in domestic sheep in North America. It is characterized by many variants and can be transmitted vertically and horizontally. Environments which have housed scrapie-infected sheep seem to harbor the infectious agent for prolonged periods, despite disinfection and eradication measures ŽBruce, 1993.. Scrapie has never been documented to have been transmitted to humans ŽAllen, 1993; Digiulio et al., 1997; Aguzzi and Weissmann, 1997.. Raymond et al. Ž1997. in their molecular assessment Žutilizing an in-vitro system. of the potential transmission of scrapie and BSE to humans suggested that BSE would be no more transmissible to humans than is sheep scrapie. Spongiform encephalopathy has occurred as infrequent outbreaks in commercially bred mink and as sporadic cases in wild or semi-captive ungulates. Chronic wasting disease ŽCWD. first identified in 1967, naturally occurs in 3% of the populations of mule deer, black-tailed deer and Rocky Mountain elk in Colorado and Wyoming in the US and in the Canadian provinces of Ontario and Saskatchewan. Cases have occurred primarily in captive animals, but a few free-ranging animals have been identified. The minimum incubation period for CWD is 18 months, and the time between onset and death is 3 weeks]8 months. The origin of this disease is not known. There was no obvious connection between this disease and rendered animal protein, nor with scrapieinfected sheep ŽWilliams and Young, 1992; Digiulio et al., 1997.. CWD is not the result of food-borne exposure to the infectious agent. CWD appears to be maintained within captive populations by lateral and possibly maternal transmission ŽWilliams and Young, 1992.. BSE is known to have caused ‘prion disease’ in a number of other species including domestic cats Žfeline spongiform encephalopathy. and captive ungulates Žnyala and kudu. presumably as a result of ingestion of BSE-contaminated feed ŽFraser et al., 1992; Allen, 1993; Bruce et al., 1994; Foster et al., 1996.. Bruce et al. Ž1997. reported that the strain of agent from cattle affected by BSE produces a characteristic pattern of disease in mice
that is retained after experimental passage through a variety of intermediate species thus reproducing what is believed to be the route of infection of cattle. BSE was found to be experimentally transmitted by injection of affected brain homogenates to cattle ŽDawson et al., 1991.; mice ŽFraser et al., 1992. and marmosets ŽBaker et al., 1993.. CJD and vCJD have also been experimentally transmitted through inoculation of brain tissue into the brain of mice ŽBruce et al., 1997.. Wild cats developed BSE after being fed affected meat, and mice and primates also contacted TSEs under experimental conditions. The incubation period is dose- and route-dependent and the oral route is the least efficient at transmitting the agent. The oral route requires sufficient challenge as evidenced by experimental transmission from sheep to goat ŽPattison and Millson, 1961; Pattison et al., 1972.. While there is no evidence that infectivity can be transmitted intravenously from human to human, plasma fractions from CJD patients were found to transmit the disease when injected into the brains of mice ŽMorris, 1997; WHO, 1997.. 4. Comparison of Creutzfeldt-Jacob disease (CJD) and variant-Creutzfeldt-Jacob disease (vCJD) 4.1. Creuzfeldt-Jacob disease (CJD) CJD was first described in the 1920s and although the most common human spongiform encephalopathy, it is a rare progressive neurogenerative disorder responsible for about 1 death per million people annually worldwide ŽBrown et al., 1985; Collinge et al., 1996; Brown, 1997.. Until recently CJD was considered of limited public relevance. CJD may be sporadic, acquired from environmental sources, especially iatrogenic sources or familial due to mutation in a gene or chromosome 20 ŽGoldfarb et al., 1994.. Recognized iatrogenic routes of transmission of CJD are treatment andror contamination with human cadaveric pituitary growth hormone or gonadotropins, dura matter or corneal grafting and the use of inadequately sterilized neurosurgical instruments and contaminated stereostatic instruments ŽBrown et al., 1985; Weller, 1989..
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The experimental evidence, although limited, identifies a theoretical risk that prion disease might be transmitted by transfusion ŽDealler, 1996., there is no clear evidence that this has occurred ŽFlanagan and Barbara, 1996.. Esmonde et al. Ž1993. reviewed 202 cases of CJD reported to the CJD surveillance unit in Edinburgh. There was no excess of cases with a history of blood transfusion or indeed donation of blood. Transfusion was calculated to be a potential cause of a maximum of only 0.2% of clinical cases of CJD in the UK population if the BSE epidemic were to spread to humans ŽDealler, 1996.. It should also be noted that Dodelet and Cashman Ž1998. recently reported that cellular prion protein ŽPrP c . is expressed in CD34-positive stem cells, very early in haemopoiesis. Since PrP c expression is essential for prion disease pathogenesis, it was suggested that bone marrow should perhaps be considered a high-risk tissue. It should also be noted that a new precautionary measure was recently announced in the UK to prevent infection of blood-product recipients with new-variant vCJD ŽMitchell, 1998.. The UK will, in the future import blood plasma, although there is no evidence that vCJD can be contracted from blood products. Whole blood for transfusions will continue to be from UK sources. The majority of sporadic CJD cases are homozygous at polymorphic residue 129, a common protein polymorphism in human PrP that is known to be important in genetic susceptibility to human prion diseases ŽBaker et al., 1991; Collinge et al., 1991; Palmer et al., 1991; Prusiner, 1991, 1994, 1997; Brown, 1997.. The classic sporadic variety of CJD usually occurs in the late middle age Žaverage age 60 years.; however CJD occasionally affects younger people including adolescents. Death usually occurs within 12 months of onset. Typically signs of CJD begin with loss of memory or confusion, however behavioral aberrations or gait instability are the presenting symptoms in about 1r3 of the patients. Later clinical features include a wide range of neurologic signs referable to the visual and motor cortex, basal ganglia and cerebellum. At autopsy microscopic sections of the brain exhibit widespread spongiform change accompanied
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by gliosis and neuronal loss while immunochemical staining reveals the presence of ‘prion’ protein positive amyloid deposits ŽBrown, 1997.. Five to 10% of CJD cases occur in family clusters and in these cases display an autosomal dominant pattern of inheritance ŽPrusiner, 1993.. Affected individuals show characteristic mutations in the prion protein gene coding ŽCollinge et al., 1991; Palmer et al., 1991.. 4.2. Variant Creutzfeldt-Jacob disease (¨ CJD ) The new variant form of CJD ŽvCJD. began to appear about 10 years after BSE was first identified in cattle in the UK and was described in March 1996 in a series of 10 young patients in the UK ŽWill et al., 1996a,b; Brown, 1997.. A similar case was described in France ŽChazot et al., 1996. and 10 others in the UK by October, 1997 ŽWilesmith et al., 1996.. To date 24 cases of vCJD are known. All 24 patients were potentially exposed to contaminated bovine products during the 1980s, before the magnitude of the BSE problem was recognized and measures were taken to prevent human exposure. The new variant vCJD is part of the CJD spectrum of diseases and shares with sporadic cases of CJD the features of progressive dementia, ataxia and myoclonus ŽWill et al., 1996a,b.. The neuropathology of vCJD is characteristic and includes large Kuru-like PrP positive amyloid plaques surrounded by a halo of spongiform Žflorid plaques.; deposition is in grey matter and there is prominent involvement of the cerebellum. However the recent cases of vCJD are as a group remarkable for their comparative youth at onset of symptoms Ž19]39 years. than classical sporadic types of CJD Ž55]70 years.. The new variant types of CJD exhibit an onset of the disease with psychiatric andror sensory symptoms, the absence of characteristic electro-encephalographic changes and an unusually long duration Žaverage, 14 months.. The prion protein genotype ŽPRPN. at codon 120 is homozygotic methioninermethionine in all cases of the new variant and predominantly but not always homozygotic in sporadic CJD ŽCollinge et al., 1991, 1996; Collinge and Palmer, 1994..
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Currently definitive diagnosis of either CJD or its new variant vCJD is possible only after death by histological examination of the brain. The only procedures capable of distinguishing between the variant and the sporadic forms of the diseases are evaluation of neuropathological lesions of PrP glycotyping or Western blot. Two newly described procedures have been suggested to offer earlier identification of vCJD. Neuroimaging techniques such as single photon emission computed tomography ŽSPECT. Žde Silva et al., 1998. and cranial magnetic resonance imaging ŽWill and Zeider, 1996; Zeidler et al., 1997. have shown promise in the identification of a small number of vCJD cases. The recent diagnosis of CJD by measurements of serum S-100 protein in a prospective case-control study employing 224 patients ŽOtto et al., 1998. also suggested to be promising and requiring further validation ŽPocchiari, 1998.. The new variant ŽvCJD. has strain characteristics distinct from other types of CJD and which resemble BSE transmitted to mice, domestic cat and macaque, consistent with BSE being the source of this new disease ŽScott et al., 1993; Bruce et al., 1994; Collinge et al., 1996.. 5. Hypotheses for transmissible spongiform encephalopathy (TSE) diseases Various hypotheses concerning the nature of the pathogen in BSE, CJD and the new variant vCJD have been advanced including speculations about parasites, viruses, self-replicating protein prions and organophosphates and whether BSE is an autoimmune disease due to bacteria showing molecular mimicry with brain antigens ŽPrusiner, 1997; Ebringer et al., 1997; Chesebro, 1998.. In regard to viruses, it is noted that although to date no direct evidence for a TSE virus has been obtained, it should not deter future search efforts despite the difficulties ŽChesebro, 1998.. 5.1. Prions CJD and BSE are both transmissible spongiform encephalopathies, fatal neurogenerative diseases that can effect both humans and animals. They are characterized by deposits of protein aggregates in the brain. The principal hypothesis
proposed to date is that BSE, CJD and vCJD are prion diseases which are transmitted by infectious proteins rather than by traditional infectious agents such as viruses or bacteria ŽPrusiner, 1991, 1994, 1997; Prusiner et al., 1990; Hill et al., 1997; Hegde et al., 1998.. These infectious proteins, unlike viruses and bacteria, are resistant to heat, ultra violet and ionizing radiation as well as disinfectants such as formaldehyde and gluteraldehyde. The hallmark of all prion diseases, whether sporadic, dominantly inherited or acquired by infection, is that they involve the aberrant metabolism and resulting accumulation of the prion protein ŽPrusiner, 1991, 1994, 1997.. Prions are defined as proteinaceous infectious particles that lack nucleic acid. PrP c is the cellular protein and PrP sc is the pathologic isoform. This conversion of normal cellular protein into PrP sc abnormal disease-causing isoform involves a conformation change whereby the alpha-helical content diminishes and the amount of beta-sheet increases ŽPrusiner, 1997. resulting in profound changes in the properties of the protein. For example, PrP c is readily digested by proteases whereas PrP sc is partially resistant ŽOesch et al., 1985.. The normal prion protein ŽPrP c . which is found in neuronal tissue is encoded by a single copy host gene and has unknown physiological functions. PrP c is believed to be transformed upon infection into an insoluble and proteinase resistant isoform PrP sc which forms the central component of the transmissible agent Žpossibly via changes in form as both proteins are chemically identical.. This information would be transmitted without the involvement of nucleic acids. In a recent molecular analysis of prion strain variation and the aetiology of ‘new variant’ CJD, Collinge et al. Ž1996. reported that strains of transmissible spongiform encephalopathies are distinguished by differing physico-chemical properties of PrP sc , the disease-related isoform of prion protein which can be maintained on transmission to transgenic mice ŽScott et al., 1993; Collinge et al., 1996.. The new variant vCJD has strain characteristics distinct from other types of CJD and which resemble those of BSE transmit-
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ted to mice, domestic cat and macaque consistent with BSE being the source of this new disease strain. The clinical, molecular and pathological features Ž‘signature’. of the BSE agent in cynomologus macaques is identical to that of vCJD in humans ŽLasmezas et al., 1996.. Collinge et al. Ž1996. suggested that the prion protein may itself encode disease phenotype. The new variant vCJD is associated with a unique and highly consistent appearance of protease-resistant PrP on Western blots, involving a characteristic pattern of glycosylation. It was noted that whereas transmission of CJD to inbred mice produces a pattern characteristic of the inoculated CJD, transmission of BSE produces a glycoform ratio pattern closely similar to new variant CJD. As noted earlier, experimental BSE in macaques and naturally acquired BSE in domestic cat show an indistinguishable glycoform pattern to experimental murine BSE and vCJD. Additionally, transmission of types 1, 2 and 3 CJD to transgenic mice expressing human PrP reveals persistence on conversion of strain type dependent on PRNP codon-129 genotype, hence providing supportive evidence for the ‘protein only’ hypothesis of infectiousness and indicating that strain variation might be encoded by a combination of PrP conformation and glycosylation. Hence considerable evidence from biochemical, immunologic, pathologic and genetic studies strongly suggest that PrP sc is the major, if not the only component of the transmissible prion particle ŽHill et al., 1997; Scott et al., 1993; Prusiner, 1991, 1994, 1997.. Korth et al. Ž1997. recently described a monoclonal antibody, 15B3, that could discriminate between the normal and disease specific forms of PrP. This long sought antibody could be of great value in characterizing the infectious particle as well as for the diagnosis of TSEs such as BSE or CJD in humans. The monoclonal antibody 15B3 specifically precipitates bovine, murine or human PrP sc but not PrP c suggesting that it recognizes an epitope common to prions from different species, and may represent a starting point for the production of further diagnostic or therapeutic tools for TSEs. A number of arguments have been marshalled against the protein-only hypothesis. It should be
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noted that not only can ‘prion disease’ appear in the absence of detectable levels of PrP sc ŽAguzzi and Weissmann, 1997., but protease-resistant PrP with no infectivity can be generated ŽRiesner et al., 1996.. It was suggested that there may be several structures of PrP that differ from that of PrP c , the infectious form of which PrP sc may or may not coincide with a protease-resistant structure ŽAguzzi and Weissmann, 1997.. A number of other difficulties with the prion hypothesis have been cited. For example, the hypothesis is not compatible with the different strains of TSEs which can be differentiated based on incubation period, histopathological pattern of brain lesions and infectivity to other species ŽCarlson, 1991.. It still has not been unambiguously resolved what triggers the normal cell protein to transform into the disease-causing isoform of the protein. Mutations in PrP were found to be associated with the Gertsmann-Straussler¨ Scheinker ŽGSS. syndrome and CJD and could lead to changes in the glycoprotein resulting in amyloid formation. It is still not known whether the mutant protein would lead to the generation of the infectious agent or make individuals more susceptible, allowing the replication of a relatively prevalent, usually subclinical, infectious particle ŽPrusiner, 1993..
5.2. Autoimmune disease
It has been suggested that bovine spongiform encephalopathy ŽBSE. could be an autoimmune disease produced following exposure of cattle to feedstuffs containing bacteria exhibiting molecular mimicry between bacterial components and bovine tissue. For example, three bacteria Ž Acinetobacter calcoaceticus, Ruminococcus albus and Agrobacter tumefaciens. share sequences with the encephalitogenic peptide of bovine myelin, while three molecules in E. coli show molecular mimicry with host-encoded prion protein. It was further proposed that immune responses against these bacteria at both T and B cell levels may cause neurological tissue injury resembling BSE ŽEbringer et al., 1997..
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5.3. Chronic pesticide-initiated modification of prions It has recently been proposed by Purdy Ž1996a,b. that BSE may have originated via exposure of bovine embryo to various specific highdose lipophilic formulations of organophosphates. Specifically, high-dose formulations of organophosphate phosmet containing phthalimide which were applied compulsorily and exclusively in the UK during the 1980s and early 1990s as the primary trigger that initiated the deformation of prion protein and the onset of the BSE epidemic. It was suggested that the multi-site binding metabolites of these organophosphates could penetrate the fetus covalently phosphorylating various active sites on fetal prion protein. Further, the extra charged phosphate groups left on aged prion protein blocks both proteases and chaperones from accessing their catalyticrbonding sites leading to the creation of undegradable, misfolded isoform of prion protein PrP sc . It was noted by Purdy Ž1996a,b. that high dose phosmet Žand its phthalimide ingredient. formulations were employed in the UK for the control of warble fly and control of other ectoparasites such as lice by direct application on the backs of cattle in the 1980s. Additionally organophosphates were used on stored grain for prevention of weevil-mite infestation. Treatment alternated between the use of pirimphosmethyl and malathion. Maximum residue limits of organophosphates on stored grain had to be raised from 2 to 10 ppm during the 1980s and early 1990s, with much of the wheat from these stores going as an ingredient for cow concentrate feed during the 1980s. It was suggested that the timing, distribution and dynamics of usage of these specific organophosphates correlates with the epidemiology of BSE as well as accounting for the 23 000 cattle that have developed the disease in the UK, yet were born after the 1988 ban on scrapie-contaminated cattle feed. 6. Food safety and surveillance issues related to BSE and vCJD The recent crisis in the UK resulting from the
outbreak of BSE in cattle and the resultant deaths of 23 individuals Žand one in France. due to a new human variant of Creutzfeldt-Jacob disease ŽvCJD. linked to BSE and the consumption of beef has led to a wide clamour and insistence from a broad panoply of consumers, scientists and government officials for greater assurance of safety and surveillance of the food supply. Following its BSE epidemic in cattle and in response to concern about possible transmission to humans, the United Kingdom implemented an active surveillance system for CJD in 1990. By 1993, France, Germany, The Netherlands and Italy joined the UK and initiated surveillance systems employing standard methods and exchanging expertise ŽWientjens et al., 1994.. A protocol for a case control study examining risk factors for CJD is being implemented linked to the active surveillance in these countries. Additionally, methods to support wide implementation of CJD surveillance are being developed and tested by WHO and should be available in early 1998 ŽWHO, 1997., and will include information and training modules for clinical and pathological diagnosis, implementation and evaluation of the surveillance system, measures to protect public health information and communication on BSErCJD ŽDora, 1998.. A number of basic measures for the protection of public health from the potential risk of acquiring BSE have been issued by the World Health Organization ŽWHO., Organization International des Epizootes ŽOIE., the Food and Agricultural Organization ŽFAO. as well as by national and regional governments and organizations Že.g. European Union.. Basically these measures fall into three groups: Ža. to avoid BSE-contaminated animals and meat products entering the food chain or are to be used in the production of medicinal products; Žb. to provide monitoring and surveillance of BSE and CJD; and Žc. to identify research needs and support their implementation. It is noted that these measures are being revised and updated as required regularly ŽDora, 1998.. In the above regard, WHO Ž1996. has issued recommendations on the measures to protect public health agreed to by WHO, FAO and OIE.
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These recommendations included: Ža. no part or product of any animal which has shown signs of transmissible spongiform encephalopathy ŽTSE., nor tissues that are likely to contain BSE agent should enter the food chain Žhuman or animal.; Žb. all countries should establish surveillance and compulsory notification of BSE; Žc. all countries should ban the use of ruminant tissues in ruminant feed; Žd. milk and milk products are considered safe; Že. gelatin and tallow are only considered safe if effective rendering procedures are used; and Žf. medicinal products and medical devices should be obtained from countries with no sporadic cases of BSE and measures recommended to minimize the risk. The International Animal Health Code, produced by the Organization International des Epizootes ŽOIE. was provisionally amended in May, 1996 to state the following: Ža. meat and bone meal containing ruminant protein from countries with high incidence of BSE should not be traded; Žb. those Žmeat and bone meal products as above. from countries with low incidence of BSE should not be traded for use in ruminant feed; and Žc. putting restrictions on trade of specified offals Žbrain, eyes, spinal cord, thymus, spleen and distal ileum.. The OIE has also started to differentiate between countries with high and low incidence of BSE and has issued guidelines for the surveillance and monitoring of BSE ŽDora, 1998.. FAO Ž1998. is in the process of publishing a report on animal feeding and food supply that is already available on the Internet. The Codex Alimentarius Committee that implements the Joint FAOrWHO food standards programme has also made recommendations regarding carcasses and that all parts of affected cattle suspected of BSE be destroyed. The European Commission DG24 responsible for consumer safety, maintains a world wide web page with information on measures to protect public health taken by the UK and by the EC ŽDora, 1998.. The European Union’s Scientific Steering Committee on January 23, 1998 approved a plan that could exempt a number of countries, including the US from the EU’s April 1, 1997 ban on specified risk material ŽSRM. linked to BSE ŽAnon, 1998a.. The plan would allow the EU to
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assess the risk on a regional basis. The Steering Committee would require countries claiming BSE-free status to provide it all available data. The assessment would judge risk at three levels: Ža. incident risk } the probability that an infectious animal Žor material thereof. enters the food andror animal feed chain; Žb. propagation risk } the probability that an initial infection is propagated within the system of a given region and within a given time period; and Žc. human exposure risk } the probability that a human being is exposed to an infective dose of the BSE agent, within a given time period. The incident risk and propagation risk taken together will be used to assess the geographical risk. It should be noted that the first risk assessments will not consider the human exposure risk. Since the consumption of infectious meat-andbone meal ŽMBM. is considered one of the major sources of BSE, EU’s Steering Committee said that it is essential to have a clear understanding of the consumption and source of MBM in a given geographical area in order to assess the probability that BSE might occur in its animal populations Žincident risk.. Additionally the suppression of specified bovine offals and specified risk material ŽSRM. Že.g. cattle brains, spinal cords, spleens. from entering the food and feed chains will reduce the risk of BSE infection. The Committee categorizes some tissues at a higher level than warranted by their intrinsic infectivity, for practical reasons relating to slaughterhouse contamination. Tissues considered as possessing high infectivity include: bovine brain, eyes, spinal cord, dorsal root ganglia, dura matter, pituitary, skull vertebral column and lungs. The EU recently established a system to tag and track cattle in an attempt to contain further outbreaks of BSE ŽAnon, 1998b.. The series of three regulations also requires cattle ‘passports’ and national registers to follow cattle movements. Each member state is to use inspection results to compile an annual report to the Commission listing the number of herds in the member state, the number of inspections and animals checked, any breach found and any sanctions imposed. The British government in December, 1997, in an attempt to further minimize the risk of BSE
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from the consumption of British meat, recently banned the sale of beef and lamb on the bone. It closely followed the advisory committee’s advice that ‘no beef with bone from cattle over 6 months old’ be sold and that ‘cattle slaughtered between 24 and 30 months of age for human consumption be deboned in licensed plants’ ŽAnon, 1998c.. The British government announced in December 1997 that it will conduct an inquiry into the outbreak of BSE which will focus on the government’s handling of the crisis following the discovery of BSE in the 1980s. It will cover a period of time between that discovery and March 1996 when the government announced that BSE and a human brain disease CJD were linked ŽAnon, 1998d.. It is also important to note specific bovinerelated products that have come under scrutiny. For example, since tests on milk from BSE-infected animals have not shown any BSE infectivity and there is evidence from other animal and human spongiform encephalopathies to suggest that milk will not transmit these diseases, milk and milk products are considered safe even in countries such as the UK with a high incidence of BSE. Gelatin is considered safe for human consumption since its preparation involves a chemical extraction process that destroys BSE infectivity. Similarly tallow is considered safe if effective rendering procedures are used. In the US, the source of raw materials for gelatin manufacture is exclusively from pork skins, beef hides or domestic beef bones. The food safety risk from all foods made from gelatin is considered extremely low ŽAnon, 1998e.. Note in proof The proceedings of the British Board of Inquiry are accessible on the web site: http:rrwww.bse.org.uk ŽButler D. British BSE reckoning tells a dism al tale. N ature 1998;392:532]533.. References Aguzzi A, Weissmann C. Prion research: the next frontiers. Nature 1997;389:795]798.
Allen IV, editor. Spongiform encephalopathie. Br Med Bull 1993;49Ž4.:725]1012. Almond J, Pattison J. Human BSE. Nature 1997;389:437]438. Alpers MP. Epidemiology and clinical aspects of kuru. In: Prusiner SP, McKinley MP, editors. Prions: novel infectious pathogens causing scrapie and Creutzfeldt-Jakob disease. San Diego: Academic Press, 1987:451]465. Anderson RM, Donnely CA, Ferguson NM. Transmission dynamics and epidemiology of BSE in British cattle. Nature 1996;382:779]788. Anon. EU considers exemptions to ban on animal material linked to mad cow disease. World Food Chem News 1998a;4Ž21.:3, 17]19. Anon. EU establishes system to track cattle as part of BSE prevention plan. World Food Chem News 1998b;4Ž19.: 18]19. Anon. More evidence needed to confirm infection of bone marrow, study finds. World Food Chem News 1998c;4Ž20.:4. Anon. Britain announces year-long inquiry into BSE crisis. World Food Chem News 1998d;4Ž19.:11]12. Anon. Food gelatin industries protest spine removal in FDA BSE guide. Food Chem News 1998e;39Ž48.:4]5. Baker HF, Poulter M, Crow TJ, Frith CD, Lofthouse R, Ridley RM. Amino acid polymorphism in human prion protein and age of death in inherited prion disease. Lancet 1991;337:1286. Baker HF, Ridley RM, Wells GAH. Experimental transmission of BSE and scrapie to the common marmoset. Vet Rec 1993;132:403]406. Brown P. The risk of bovine spongiform encephalopathy Ž‘mad cow disease’. to human health. J Am Med Assoc 1997;278:1008]1011. Brown P, Cervenakova L, Goldfarb LG, et al. Iatrogenic Creutzfeldt-Jacob disease: an example of the interplay between ancient genes and modern medicine. Neurology 1994;44:291]293. Brown P, Gajdusek DL, Gibbs CJ, Asher DM. Potential epidemic of Creutzfeldt-Jacob disease from human growth hormone therapy. N Engl J Med 1985;313:728]730. Bruce ME. Scrapie strain variation and mutation. Brit Med Bull 1993;49:822]838. Bruce ME, Chree A, McConnell I, Foster J, Pearson G, Fraser H. Transmission of bovine spongiform encephalopathy and scrapie in mice: strain variation and the species barrier. Phil Trans R Soc Lond B 1994;343:405]411. Bruce ME, Will RG, Ironside JW. Transmissions to mice indicate that ‘new variant’ CJD is caused by BSE agent. Nature 1997;389:498]501. Butler D. Did UK ‘dump’ contaminated feed after ban? Nature 1996;381:544. Carlson GA. Genetics of prion diversity and host susceptibility. Curr Top Microbiol Immunol 1991;172:153]164. Chazot G, Broussole E, Lapras CI, et al. New variant of Creutzfeldt-Jacob disease in a 26 year-old French man. Lancet 1996;347:1181. Chesebro B. BSE and prions: uncertainties about the agent. Science 1998;279:42]44.
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