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BSE: Assessment of the risk to public health resulting from exposure to the Bovine Spongiform Encephalopathy (BSE) agent through consumption of beef and beef-products
June 2002
Note:
In response to the outbreak of bovine spongiform encephalopathy (BSE) in the United Kingdom, and its link with the fatal variant Creutzfeldt-Jakob disease (vCJD), Australia suspended the import of British beef in 1996. In January 2001, this suspension of beef and beef products was extended to thirty European countries when BSE was found in Europe.
The Food Standards Code was amended in July 2001 to require all beef and beef products sold in Australia to be BSE free.
The Australia New Zealand Food Authority (ANZFA) established a BSE Expert Group of domestic and international experts to systematically examine the available scientific data and information to estimate the risks to human health from consumption of beef and beef products containing the BSE agent. ANZFA conducted this risk assessment in consultation with national and international experts.
The BSE Expert Group's assessment report was finalised in June 2002. The Authority gratefully acknowledges the contribution from the members of the BSE Expert Group. [ Full details provided in Appendix 7 of Full Report ].
1.0 Executive Summary
Variant Creutzfeldt-Jakob Disease (vCJD), a progressive and fatal neuro-degenerative disorder in humans, was first diagnosed in the United Kingdom in 1996. It is now accepted that vCJD can develop as a result of ingestion of food containing a transmissible agent that causes Bovine Spongiform Encephalopathy (BSE). BSE, commonly known as ' mad cow disease', is a chronic degenerative disease affecting the central nervous system of cattle. This risk assessment analyses the available data to estimate the risk of Australians developing vCJD from exposure to food containing the BSE agent.
There have been approximately 131 cases of vCJD diagnosed worldwide to date, a very small number relative to the large number of people likely to have been exposed to the BSE agent in food. Although the number of cases is relatively small, the uncertainties about the incubation period between exposure to the BSE agent and the subsequent development of vCJD, as well as significant gaps in knowledge about the BSE agent, its characteristics and the transmission factors, dictate a conservative approach to the assessment of the potential health risk.
Transmission of the BSE agent to humans appears to be dependent on a number of factors. They include the likelihood of an animal with BSE entering the food chain, the part and amount of the animal carcase consumed and the infectivity of the tissues. The central nervous system of clinically affected cattle contains most of the infectious agent.
Experimental studies indicate that a single exposure to contaminated feed is sufficient to cause BSE in cattle; however it is not known the extent or frequency of exposure to the BSE agent in food that is required to result in vCJD in humans. Current scientific evidence indicates the BSE agent is extremely resistant to physical and chemical treatments that are normally used to inactivate proteins and micro-organisms. The thermal conditions used in processing of beef products such as canned meat and smallgoods will not inactivate the BSE agent. However, at least one manufacturing process, the sodium hydroxide treatment in gelatine production, can inactivate BSE agent infectivity.
Contamination of uninfected meat by an infected carcase is possible in abattoirs and must be considered in estimating risk. Contamination can occur even when extensive control measures have been implemented to reduce cross-contamination.
Not all beef products pose the same risk of transmitting the BSE agent to humans, because some products appear to contain more of the agent than others. Some products, such as the brain, can contain the agent in very high concentrations. Animal products that were identified in the risk assessment as containing significant amounts of the BSE agent include so-called specified risk material (SRM) such as the brain tissue and spinal cord, and mechanically recovered meat. The risk assessment also examined the potential for the BSE agent to be transmitted to consumers via edible collagen, edible tallow, gelatine, and milk and dairy products.
SRM and mechanically recovered meat may be used in the preparation of canned beef products and smallgoods in some countries and therefore are considered to pose a potential hazard to Australian consumers. If sourced from an animal infected with BSE, the potential for these products to contain the BSE agent is high, because the raw materials are often sourced from older cattle with potentially high levels of the BSE agent.
Food-grade gelatine that has been produced from porcine skins and bovine hides is considered to pose a negligible risk to consumers, because skins and hides from these animals have been shown to contain negligible levels of the BSE agent. Where gelatine is derived from bones using a process that includes a sodium hydroxide step, the risk has also been shown to be negligible. However gelatine manufactured from bones where there is not a sodium hydroxide step results in a higher level of risk.
Edible collagens are also produced from food-grade bovine hides. Collagens which have not been contaminated with SRM during slaughter, are also assessed as posing a negligible risk. T he current use of edible tallows by the food industry as an ingredient in products such as cakes, biscuits and pastries is estimated to pose a negligible risk of transmission of the BSE agent.
Current evidence indicates that milk and dairy products do not contain the BSE agent.
1.1 Summary of key findings
1. Epidemiological evidence indicates that BSE and vCJD are caused by the same agent. Strain typing evidence (both biological and molecular ) strongly suggests that vCJD and BSE are caused by an agent with similar biological and molecular properties.
2. The oral route is the most likely route of exposure to the BSE agent due to consumption of beef-derived products contaminated with the BSE agent. The risk of exposure to the BSE agent is dependent on the likelihood of an infected animal entering the food chain, the type of tissue used in the product and the amount of BSE infectivity present.
3. While some of the tests and bioassays may not have high enough sensitivity to detect low levels of the agent, experimental data have shown that infectivity is not found in up to 50 tissues of cattle with BSE, including most tissues that are used in the human diet, such as muscle and milk.
4. BSE exposure risk is highest from specified risk material (SRM), and in these tissues the central nervous system tissues from clinically-affected cattle would present the highest risk. Approximately 95% of the total infectivity is estimated to be in the central nervous system of clinically affected animals. The removal of SRM from a clinical BSE case will reduce the infectious load by about the same amount.
5. The amount and distribution of the BSE agent in an animal changes over time and therefore the age of animal at slaughter will influence the level of BSE infectivity entering the food chain from an infected animal. Risk of exposure to the BSE agent is low early in the incubation period and rises rapidly later in the incubation period.
6. The incubation period is such that clinical BSE is rarely detected in animals younger than 30 months.
7. Experimental studies indicate that a single dose is sufficient to cause disease in cattle but the effect of cumulative doses on humans is unclear. Repeated exposure to infection could increase the absolute numbers of individuals that might receive an infecting dose and thus the likelihood that exposure would result in infection.
8. Heating in the presence of moisture can reduce BSE infectivity but cooking cannot be relied upon to completely inactivate the BSE agent.
9. Most traditional methods for inactivating microorganisms, including ionising, ultraviolet and microwave radiation, will not inactivate the BSE agent, but sodium hydroxide and buffered sodium hypochlorite, under particular laboratory conditions, will inactivate the agent. Some of these treatments are suitable only for use in by-product industries, such as the manufacture of gelatine and soap.
Fresh meat and processed meat products
10. The slaughter practices which have a greater risk of contamination of carcases with SRM occur during the stunning procedure through dissemination of brain tissue and during carcase splitting through dispersal of spinal cord material. There is also a risk of contamination during butchering where SRM have not been removed. Other steps in the slaughtering process that may involve a risk of cross-contamination of carcases with SRM are head processing and carcase trimming.
11. In non-European countries, SRM and mechanically recovered meat may be used in the preparation of smallgoods and canned beef products. If these products are sourced from an animal incubating BSE, the risk from these products is increased as raw materials may be sourced from older animals.
12. The conditions to which meat and meat products are exposed during processing do not inactivate BSE infectivity. The thermal conditions used in processing of beef products such as canned meat and smallgoods are insufficient to completely inactivate the BSE agent. The conventional unit operations, such as filtration and washing, used in the food-processing industry, do not eliminate the BSE agent.
Milk and dairy products
13. Experimental studies to date have shown no detectable infectivity in the mammary gland of:- BSE-affected cows or goats;
- Suffolk sheep with natural scrapie,
- Colostrum of Suffolk sheep in the pre-clinical phase of natural scrapie, or
- Milk from cows with BSE or goats with natural scrapie.
14. The main epidemiological studies have suggested some low-level maternal transmission of BSE, enhanced in the late stages of maternal incubation, although no mechanism of transmission has been identified. Epidemiological studies examining the maternal transmission of BSE in a specific cohort study have been complicated however by the contamination of cattle feed after the implementation of the feed ban.
15. A study of BSE-infected cows in beef herds showed no evidence that colostrum or milk consumption during rearing was a risk factor for occurrence of BSE in the progeny.
16. The weight of available evidence does not support a conclusion that bovine milk is a risk factor in the occurrence of vCJD cases. Milk is the lowest risk animal product that is consumed widely by the population.
Gelatine
17. The BSE infective agent has been found to occur in the nervous tissue associated with some bovine bones. It has also been found in bovine bone marrow in the clinical phase of experimental BSE. Furthermore, there is some evidence that during slaughter the infective agent can contaminate parts of the hide that are later used in gelatine manufacture.
18. There has been no work to show the efficacy of the gelatine production process in its entirety in reducing the level of the TSE agent. However, studies on the majority of unit operations of gelatine manufacture have shown that the conventional acid bone and limed bone treatments reduced infectivity by more than 99.9%. Addition of a sodium hydroxide step, which is not in current commercial use, gives an infectivity reduction of more than 99.99999%.
19. On the basis of experimental evidence, the degree of inactivation achieved by the routine manufacturing procedures is considered to be sufficient to have inactivated as much BSE infectivity as might be present in bones under worst-case scenarios (i.e. bones from one complete BSE-infected carcase). The addition of a sodium hydroxide step, if undertaken, provides a further level of assurance.
20. Cross-contamination of hides during slaughtering is not likely to carry any more infectivity than bones into the start of the gelatine manufacturing process.
Tallow
21. Bovine tallow produced for edible purposes in Europe is routinely made from discrete adipose tissue. However in other parts of the world other animal tissues are used to make edible tallow. Adipose tissue in cattle has been shown to be free of BSE infectivity.
22. Tallow produced by rendering other animal tissues is usually subject to further refining involving neutralization of free fatty acid by sodium hydroxide, filtration and deodorisation before use in edible applications. This may further reduce any BSE infectivity.
23. On the basis of the available evidence, edible tallow used as an ingredient in a processed product present a negligible risk of carrying BSE infectivity. Bulk tallow may present a higher risk of exposure to BSE infectivity because, if produced by the rendering of animal tissues, further refining may not occur and the resultant tallow may be incorporated into a range of products.
Collagen
24. The parts of the hide used for making collagen do not include those which may be contaminated with SRM during slaughter. Collagen presents a negligible risk of carrying BSE infectivity.