Comparative evaluation of the performance of two commercial kits for the detection of central nervous system tissue in meat

Detection of central nervous system tissue as bovine spongiform encephalopathy specified risk material in traditional Turkish meat products

BACKGROUND

This study used enzyme-linked immunosorbent assay kits to investigate the presence of central nervous system (CNS) tissue in commercial raw and processed traditional Turkish meat products offered for consumption in various markets.

RESULTS

Ninety-six raw traditional Turkish meat products (32 fresh raw beef patties, 32 cig kofta, 32 pastirma) and 64 processed traditional Turkish meat products (32 doner kebabs and 32 fresh processed beef patties) were analysed. CNS tissue was not found in pastirma, doner kebab, or fresh processed beef patty samples. The levels of CNS contamination in fresh raw beef patties were low (0.1% absorbance standard; 3.1%) and moderate (0.2% absorbance standard; 6.2%). The level of contamination in the cig kofta was low (0.1% absorbance standard; 18.8%).

CONCLUSION

CNS tissue was present in all raw traditional Turkish meat products except for pastirma.

Immunochemical detection of tissue from the central nervous system via myelin proteolipid protein: adaptation for food inspection and development of recombinant bivalent Fab mini-antibodies

Bovine spongiform encephalopathy (BSE) is transmitted by the ingestion of central nervous system (CNS) tissue of infected animals. Food inspection must, therefore, test for the presence of CNS tissue in meat and meat products. A Western blot assay for the specific CNS tissue marker myelin proteolipid protein (PLP) was optimized with considerably reduced analysis time, solvent consumption, and detection limit (0.001% CNS tissue in minced beef). Further, a PLP-specific recombinant bivalent fragment antigen binding mini-antibody (anti-PLP Fab) was obtained from a commercial phage display library. Western blot analysis with the anti-PLP Fab selectively detected CNS tissue in minced beef with a detection limit of 0.025%. Model experiments for meat processing revealed that assay sensitivity decreased with increasing temperature and prolonged heating time. A market survey with 687 sausage samples was performed using PLP-Western blot and enzyme-linked immunosorbent assay (ELISA) for glial fibrillary acidic protein (GFAP). Five samples were tested clearly positive by both assay systems, whereas in an additional six samples, CNS tissue was detected only by GFAP ELISA and in two samples only by PLP-Western blot.

A bioinformatics approach to the development of immunoassays for specified risk material in canned meat products

A bioinformatics approach to developing antibodies to specific proteins has been evaluated for the production of antibodies to heat-processed specified risk tissues from ruminants (brain and eye tissue). The approach involved the identification of proteins specific to ruminant tissues by interrogation of the annotation fields within the Swissprot database. These protein sequences were then interrogated for peptide sequences that were unique to the protein. Peptides were selected that met these criteria as close as possible and that were also theoretically resistant to either pepsin or trypsin. The selected peptides were synthesised and used as immunogens to raise monoclonal antibodies. Antibodies specific for the synthetic peptides were raised to half of the selected peptides. These antibodies have each been incorporated into a competitive enzyme-linked immunosorbent assay (ELISA) and shown to be able to detect the heat-processed parent protein after digestion with either pepsin or trypsin. One antibody, specific for alpha crystallin peptide (from bovine eye tissue), was able to detect the peptide in canned meat products spiked with 10% eye tissue. These results, although preliminary in nature, show that bioinformatics in conjunction with enzyme digestion can be used to develop ELISA for proteins in high-temperature processed foods and demonstrate that the approach is worth further study.

Comparative studies of a real-time PCR method and three enzyme-linked immunosorbent assays for the detection of central nervous system tissues in meat products

The removal of certain central nervous system (CNS) tissues (part of the bovine spongiform encephalopathy risk material) from the food chain is one of the highest priority tasks associated with avoiding contamination of the human food chain with the agent of bovine spongiform encephalopathy. A recently developed real-time PCR assay and three commercially available enzyme-linked immunosorbent assays (ELISAs) for the detection of CNS tissues in minced meat and three types of heat-treated sausages were evaluated. Bovine brain was used for spiking of internal reference material, and its detectability was examined during storage times of 12 months (for frozen minced meat and liver sausage) and 24 months (for sausages treated with medium and high heat). The real-time PCR method and both ELISA kits detected 0.1% CNS tissue in frozen minced meat and 0.1 or 1% CNS tissue in heat-treated meat products. The detectability of the amplified mRNA target region with the PCR assay was similar to the detectability of antigen by the ELISAs. Because the real-time PCR method also can be used to distinguish cattle, ovine, and caprine CNS tissues from porcine CNS tissues, it seems to be suitable as a routine diagnostic test for the sensitive and specific detection of CNS tissues in meat and meat products.

Comparison of analytical methods for the detection of central nervous system tissue in ground beef

Bovine spongiform encephalopathy is most likely transmitted by the consumption of central nervous system tissue of infected cows. The objective of this study was to compare the sensitivity or limits of detection of two central nervous system tissue detection assays (glial fibrillary acidic protein enzyme-linked immunosorbent assay [ELISA] and neuron-specific enolase Western blotting assay) in by-products and ground beef. By-products including brain, spinal cord, and ileum were collected from the slaughterhouse and used for analyses with ELISA and Western blotting assays. Beef samples were prepared by mixing ground beef with different amounts of spinal cord tissue (0, 0.03, 0.06, and 0.1%) and were analyzed using the two central nervous system tissue detection methods. Both analytical assays were applicable in detecting central nervous system tissue in ground beef. However, the ELISA method was considered superior because of its ease of use, high sensitivity, and rapidity as compared with the Western blot method.

Development of a dot blot assay for the rapid detection of central nervous system tissue on meat and contact surfaces

As a potential transmitter of bovine spongiform encephalopathy (BSE), tissue from bovine central nervous system (CNS) is not accepted in meat and meat products. Western blot analysis of the CNS marker myelin proteolipid protein (PLP) detects CNS contamination selectively and sensitively. In this study, a rapid dot blot assay using an anti-PLP antibody was developed to screen CNS contamination of meat and contact surfaces. The detection limit was 0.01% bovine brain in minced bovine muscle. When applied to a swab test, down to 0.5 mg of CNS tissue on meat or other surfaces was detectable. Other offal tissues or peripheral nerves did not interfere with the assay. The test allows a differentiation between mammalian and avian CNS but not among mammalian species. The swab test was applied immediately after slaughtering at several areas of the bovine head. CNS was not detectable at any region which may enter the food chain.

Myelin proteolipid protein (PLP) as a marker antigen of central nervous system contaminations for routine food control

Spreading transmissible spongiform encephalopathies (TSE) have been widely attributed to transmission by ingestion of mammalian central nervous system (CNS) tissue. Reliable exclusion of this epidemiological important route of transmission relies on an effective surveillance of food contamination. Here, myelin proteolipid protein (PLP) is identified as a specific and largely heat-resistant marker for detection of food contaminations by CNS tissue. PLP is a component of oligodendritic glial sheaths of neuronal processes that is specifically expressed in the CNS. A highly selective polyclonal antibody was developed directed against an epitope present in the full-length PLP protein, but absent from the developmentally regulated splice variant DM-20. In combination with a hydrophobic extraction of PLP from tissue samples, the antibody reliably detected PLP from spinal cord, cerebellum, and cortex of different mammalian species. Consistent with earlier reports on PLP expression, no cross-reactivity was observed with peripheral nerve or extraneural tissue, except for a very faint signal obtained with heart. When applied to an artificial CNS contamination present in sausages, the antibody reliably detected a low concentration (1%) of the contaminant. Application of heat, as used during conventional sausage manufacturing, led to a predominant alteration of arginine residues in the PLP protein and a partial loss of immunoreactivity. In contrast, a stretch of hydrophilic amino acids(112-122) proved to be heat-resistant, preserving the immunogenicity of this PLP epitope during heating. Taken together, the excellent CNS specificity of PLP immunodetection and the presence of a heat-resistant epitope have permitted the development of a highly sensitive immunoassay for CNS contamination in routine food control.

Prion biology relevant to bovine spongiform encephalopathy

Bovine spongiform encephalopathy (BSE) and chronic wasting disease (CWD) of deer and elk are a threat to agriculture and natural resources, as well as a human health concern. Both diseases are transmissible spongiform encephalopathies (TSE), or prion diseases, caused by autocatalytic conversion of endogenously encoded prion protein (PrP) to an abnormal, neurotoxic conformation designated PrPsc. Most mammalian species are susceptible to TSE, which, despite a range of species-linked names, is caused by a single highly conserved protein, with no apparent normal function. In the simplest sense, TSE transmission can occur because PrPsc is resistant to both endogenous and environmental proteinases, although many details remain unclear. Questions about the transmission of TSE are central to practical issues such as livestock testing, access to international livestock markets, and wildlife management strategies, as well as intangible issues such as consumer confidence in the safety of the meat supply. The majority of BSE cases seem to have been transmitted by feed containing meat and bone meal from infected animals. In the United Kingdom, there was a dramatic decrease in BSE cases after neural tissue and, later, all ruminant tissues were banned from ruminant feed. However, probably because of heightened awareness and widespread testing, there is growing evidence that new variants of BSE are arising "spontaneously," suggesting ongoing surveillance will continue to find infected animals. Interspecies transmission is inefficient and depends on exposure, sequence homology, TSE donor strain, genetic polymorphism of the host, and architecture of the visceral nerves if exposure is by an oral route. Considering the low probability of interspecies transmission, the low efficiency of oral transmission, and the low prion levels in nonnervous tissues, consumption of conventional animal products represents minimal risk. However, detection of rare events is challenging, and TSE literature is characterized by subsequently unsupported claims of species barriers or absolute tissue safety. This review presents an overview of TSE and summarizes recent research on pathogenesis and transmission.

Novel molecular method for detection of bovine-specific central nervous system tissues as bovine spongiform encephalopathy risk material in meat and meat products

The emergence of a new variant of Creutzfeldt-Jacob disease during the bovine spongiform encephalopathy epidemic has focused attention on the use of tissues from the central nervous system (CNS) in food. For efficient consumer protection, European legislation prohibits several bovine tissues, encompassing mainly the central nervous system, from the food chain. A quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR) was designed to identify bovine spongiform encephalopathy risk material in meat and meat products. This was based on an mRNA assay that used bovine, ovine, and caprine glial fibrillary acidic protein (GFAP) encoding gene sequences as a marker. The real-time RT-PCR assay allowed the detection of bovine, ovine, or caprine CNS tissues in meat and meat products. Bovine brain at a concentration of 0.01% yielded a positive PCR reaction. The real-time RT-PCR assay included a housekeeping gene as an endogenous control. The detection was not affected by heat treatment of the meat products. The quantitative real-time RT-PCR detection of GFAP mRNA appeared to be useful as a routine diagnostic test for the detection of illegal use of CNS tissues in meat and meat products. The stability of the specific region of GFAP mRNA also allows the detection of CNS tissues after meat processing steps. The use of organ- and species-specific subunits of mRNA might be a promising approach for the detection of other banned tissues.

Detection of bovine central nervous system tissue in liver sausages using a reverse transcriptase PCR technique and a commercial enzyme-linked immunosorbent assay

The suitability of a reverse transcriptase (RT) PCR assay was evaluated for the detection of bovine central nervous system (CNS) tissue specifically in liver sausages. Because of its emulsifying effect, CNS tissue was frequently added to this kind of meat product in the past. On standard samples, the RT-PCR technique reliably detected a concentration of 0.25% bovine CNS tissue in liver sausages stored for up to 28 days. Following the successful application of RT-PCR for the detection of bovine CNS tissue in these specially prepared samples, a field study was performed with a total of 258 liver sausages purchased in retail markets. All sausages were tested with both an RT-PCR assay and a commercial enzyme-linked immunosorbent assay (ELISA) kit. Nine (3.5%) of the retail liver sausage samples were positive for CNS tissue in the ELISA, but none were positive for this tissue in the RT-PCR assay. All positive ELISA results indicated the presence of 0.23 to 0.30% CNS tissue. Recent studies have indicated that the RT-PCR assay is not as sensitive for porcine CNS tissue as for bovine CNS tissue, which this assay can detect at 0.25%. Although the ELISA is not species specific, the CNS tissue detected by the ELISA is assumed to stem from a nonbovine species. The RT-PCR technique is a sensitive tool for the detection of bovine CNS tissues in a problematic matrix such as liver sausage. ELISA screening followed by a species-specific RT-PCR assay for bovine CNS tissue is a practical approach for monitoring meat products for compliance with European food regulations.

Transmissible spongiform encephalopathies: Assessment of exposure risk in the histological working environment using GC-MS detection of fatty acids as marker for central nervous tissues

To elucidate the risk of occupational exposure to the agent of transmissible spongiform encephalopathies (TSE) in the histological working environment, we assessed the principal suitability of three analytical methods for the detection of tissues of the central nervous system (CNT). We tested a neuron-specific enolase (NSE) Western blot, a glial fibrillary acidic protein (GFAP) ELISA and the GC-MS detection of some CNT typical fatty acids (FAs): omega9-tetracosenic acid, omega7-tetracosenic acid, lignoceric acid, and cerebronic acid. Histological sample processing (formalin fixation, dehydration, paraffin embedding) affected both of the immunochemical approaches considerably. The NSE Western blot produced negative results without exception. The results for the GFAP ELISA were better but still far too insensitive. Thus, both methods were judged to be unsuitable in their present form without major analytical adjustment. GC-MS sensitivity remained unaffected by the formalin fixation process. Sensitivity was reduced in the course of the final dehydration step using xylene in the histological sample processing. However, this reduction was found to be rather moderate (range 42-59%) when compared to the immunochemical methods. Overall, we judged GC-MS to be a promising analytical approach for the assessment of a potential TSE exposure risk via airborne CNT particles in the histological working environment. All the FAs we tested showed very low but detectable baseline contents. Thus, cut-off values must be used in the present GC-MS approach. The most suitable FA turned out to be omega9-tetracosenic acid due to the greatest difference between its content in histological CNT samples and the respective cut-off value (689:1). Preliminary results by GC-MS monitoring of CNT via omega9-tetracosenic acid (and other FAs) on filters of routinely used vacuum cleaners and on filters after air sampling indicate that the airborne CNT/TSE exposure risk in the histological laboratory is minor if existing at all. However, further in depth studies will have to validate our preliminary findings and assess these results in the light of possible future data on human oral and/or pulmonary TSE susceptibility.

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.