Transmissible spongiform encephalopathies: The threat of BSE to man

Estimated compliance for removal of specified risk materials from 18 U.S. beef packing plants

The removal of 18,345 specified risk materials was observed during audits of 18 U.S. beef processing facilities that, in total, account for over 90% of total U.S. beef slaughtered. Audited plants varied in capacity (280 to 6,000 head per day) and processed both "fed (young cattle)" and "nonfed (mature cows/bulls)" cattle. When all observations for removal of specified risk materials were combined from plants and adjusted for type of cattle processed, overall compliance with specified risk material removal regulations was 98.08%. A 100% compliance rate for removal of brains and distal ileums was recorded based on a total of 600 observations for removal of brains and a total of 2,400 observations for removal of distal ileums. Observations for removal of dorsal root ganglia were collected from 16 of the 18 plants, and overall compliance for dorsal root ganglia removal was 99.6% (4,783 of 4,800). Fifteen of the 16 plants were 100% compliant. For tonsils, data from 18 plants were collected, and tonsils were correctly removed from 92.8% (4,777 of 5,145) of tongues and heads. Data for spinal cord removal were collected from 18 plants, and the spinal cord was removed completely in line with U.S. Department of Agriculture-Food Safety and Inspection Service regulations for 99.43% of the observations. Based on the results of this study, packing plants have demonstrated that they are complying with regulations for removal of specified risk materials from beef meat products intended for human consumption greater than 98% of the time. To continue to assure food safety and consumer confidence, continued vigilance and provision of training programs for plant workers are essential.

BSE safety standards: An evaluation of public health policies of Japan, Europe, and USA

Since the advent of bovine spongiform encephalopathy (BSE) in the United Kingdom in 1986, new BSE cases have recently become rare. However, in Japan and the United States, positive cases have started to be seen recently. The rise in BSE cases paved the way for the human form of this disease, the variant Creutzfeldt-Jakob disease (vCJD). The observed trends in the UK may be attributed to effective implementation of public health policies coupled with increased vigilance through advancement in science and technology, or they may well be a reflection of the natural disease progression. We aim to discuss the BSE chronology of events, and compare examination methods, costs and cost-efficiency, management, and public policies of Japan, Europe, and the USA.

Assessment and modification of a Western blot assay for detection of central nervous system tissue in meat products in the United States

Health hazards associated with meat contaminated by the bovine spongiform encephalopathy agent have led to the development of tests for the presence of this agent. The objective of this study was to optimize a neuron-specific enolase Western blot assay for use in the United States. We compared the original test with a modified protocol to evaluate the detection limit for the presence of central nervous system (CNS) tissue in experimentally inoculated samples and compared and evaluated the utility of these tests for detecting CNS tissue in retail sausages. Sensitivity and specificity of the original and modified protocols were evaluated using the kappa statistic to assess agreement between the results of the two protocols. The original protocol resulted in 100% specificity and 92% sensitivity for raw samples and 92% specificity and 72% sensitivity for cooked samples. The modified protocol resulted in 92% specificity and 89% sensitivity for raw samples and 83% specificity and 75% sensitivity for cooked samples. The kappa statistic for protocol comparison was 0.94 for raw samples and 0.74 for cooked samples. Both protocols correctly identified CNS tissue in positive controls for each replicate. Although the Western blot technique should be considered for screening for the presence of bovine CNS tissue in meat samples, the techniques should be further optimized to address problems of low sensitivity. A test with higher sensitivity is needed to protect consumers from food safety threats associated with bovine CNS tissue.

Use of a marker organism to model the spread of central nervous system tissue in cattle and the abattoir environment during commercial stunning and carcass dressing

Due to concerns about a link between variant Creutzfeldt-Jakob disease in humans and similar prion protein-induced disease in cattle, i.e., bovine spongiform encephalopathy (BSE), strict controls are in place to exclude BSE-positive animals and/or specified risk materials including bovine central nervous system (CNS) tissue from the human food chain. However, current slaughter practice, using captive bolt guns, may induce disruption of brain tissues and mobilize CNS tissues into the bovine circulatory system, leading to the dispersion of CNS tissues (including prion proteins) throughout the derived carcass. This project used a marker (antibiotic-resistant) strain of Pseudomonas fluorescens to model the effects of commercial captive bolt stunning procedures on the movement of mobilized CNS material within slaughtered animals and the abattoir environment. The marker organism, introduced by injection through the bolt entry aperture or directly using a cartridge-fired captive bolt, was detected in the slaughter environment immediately after stunning and in the abattoir environment at each subsequent stage of the slaughter-dressing process. The marker organism was also detected on the hands of operatives; on slaughter equipment; and in samples of blood, organs, and musculature of inoculated animals. There were no significant differences between the results obtained by the two inoculation methods (P < 0.05). This study demonstrates that material present in, or introduced into, the CNS of cattle during commercial captive bolt stunning may become widely dispersed across the many animate and inanimate elements of the slaughter-dressing environment and within derived carcasses including meat entering the human food chain.

Identification of central nervous system tissue in retail meat products

A procedure to detect tissues from the central nervous system that involved quantification of cholesterol and immunochemical detection of neuron-specific enolase and glial fibrillary acidic protein was used to analyze 402 samples of heat-treated meat products from various food outlets in Germany. The cholesterol content of 16 samples (4.0%) indicated the possible presence of central nervous system tissue because the levels exceeded the normal maximum cholesterol content of cooked sausages. In 7 of these 16 heat-treated meat products, immunoblotting of both neuron-specific enolase and glial fibrillary acidic protein confirmed the presence of CNS tissue. Repeated sampling by veterinary officials and analysis by both cholesterol quantification and immunoblotting confirmed these findings. Whereas all of the control samples (with and without added central nervous system tissue) were correctly classified by both cholesterol quantification and immunoblotting, negative results of immunoblotting must be carefully interpreted in the case of intensively heat-treated meat products. Thus, studies have yet to establish an increase in sensitivity of immunoblotting of neuron-specific enolase and glial fibrillary acidic protein. However, the detection of illegal use of central nervous system tissue in heat-treated retail meat products demonstrates the need for suitable analytical methods to control transmissible encephalopathies and to enforce labeling laws.

BSE: a decade on--Part I

Bovine spongiform encephalopathy (BSE), popularly known as "mad cow disease", was discovered in 1986 and has accounted for the deaths of over 165,000 cattle in the UK (by the end of January, 1997) with about 34,000 (mainly dairy) herds involved. The syndrome in the cow includes changes in posture and temperament, apprehension, and loss of coordination. There are many parallels with scraple in sheep, with similar neuropathological changes in the hindbrain that give it a spongiform appearance under the microscope. The facts have been broadly reviewed in The Lancet in 1990 and 1993, and in much more detail elsewhere. In a two-part article, the first of which appears here, we now summarise recent developments.

Species barrier prevents an abnormal isoform of prion protein from accumulating in follicular dendritic cells of mice with Creutzfeldt-Jakob disease

The accumulation of abnormal prion protein in follicular dendritic cells did not occur in mice inoculated with materials from human Creutzfeldt-Jakob disease, whereas it always occurred in mice inoculated with mouse-adapted agents, suggesting an intense expression of the species barrier in the lymphoreticular system.

Transgenetic investigations of prion diseases of humans and animals

Prions cause transmissible and genetic neurodegenerative diseases. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein (PrPSc), which is encoded by a chromosomal gene. Although the PrP gene is single copy, transgenic mice with both alleles of the PrP gene ablated develop normally. A post-translational process, as yet unidentified, converts the cellular prion protein (PrPC) into PrPSc. Scrapie incubation times, neuropathology and prion synthesis in transgenic mice are controlled by the PrP gene. Mutations in the PrP gene are genetically linked to development of neurodegeneration. Transgenic mice expressing mutant PrP spontaneously develop neurological dysfunction and spongiform neuropathology. Investigations of prion diseases using transgenesis promise to yield much new information about these once enigmatic disorders.

From slow virus to prion: a review of transmissible spongiform encephalopathies

Spongiform encephalopathies include seven neurodegenerative diseases: three in man (Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease and kuru) and four in animals (scrapie, mink encephalopathy, bovine spongiform encephalopathy and chronic wasting disease in deer and elks). They are all transmissible to a variety of species, and man-to-man propagation of the diseases in the form of iatrogenic transmission has been well-documented. The infectious agent is highly unusual and the pathogenesis of infection remains controversial. The term prion was introduced to describe the proteinaceous infectious agent. Purification of this agent yielded a unique sialoglycoprotein, associated with the neuronal cell membrane, which is all or part of the infectious agent. Molecular genetics revealed variations in the prion protein; these are linked to or associated with the inherited forms of spongiform encephalopathies: familial Creutzfeldt-Jakob disease and Gerstmann-Sträussler-Scheinker disease. The histological triad of spongiform change, neuronal loss and astrocytosis dominate the histological picture of spongiform encephalopathies. A recent case which did not develop any of the histological hallmarks of disease, but did have genetic abnormalities typical of the disease, indicates that the true incidence of Creutzfeldt-Jakob disease may be considerably higher than previously accepted, and a combination of molecular screening and immunohistochemistry for prion protein should complement traditional neuropathology to establish the diagnosis. The descriptive term of spongiform encephalopathy may now have to be abandoned in favour of prion disease.

Molecular biology of prion diseases

Prions cause transmissible and genetic neurodegenerative diseases, including scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Sträussler-Scheinker diseases of humans. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein, which is encoded by a chromosomal gene. A posttranslational process, as yet unidentified, converts the cellular prion protein into an abnormal isoform. Scrapie incubation times, neuropathology, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neuro-degeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Understanding prion diseases may advance investigations of other neurodegenerative disorders and of the processes by which neurons differentiate, function for decades, and then grow senescent.