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Bovine Spongiform Encephalopathy

BOVINE SPONGIFORM ENCEPHALOPATHY

Few more challenging food safety issues exist today than that of bovine spongiform encephalopathy (BSE) and the human form of the disease, variant Creutzfeldt-Jakob disease (vCJD). While vCJD remains rare at this time, the lack of a screening diagnostic test, uncertainty about the extent of exposure to the agent among humans and animals, the long incubation period, and the resulting devastating and inevitably fatal disease combine to create a situation of extreme difficulty for those responsible for public-policy development. The true extent of the unfolding epidemic is unknown, and since the largest exposures to BSE occurred before the relationship to a human disease was recognized, the full impact on human health may not be known for many years to come. This makes it difficult to determine appropriate actions to take to protect the consumer.

THE EPIDEMIC OF BSE

BSE appeared as a completely novel disease of cattle in the United Kingdom (UK), the first known case being diagnosed retrospectively in 1985. By 1986, BSE was recognized as a transmissible spongiform encephalopathy (TSE) of cattle and considered to be analogous to scrapie, a disease of sheep that is not known to cause human disease. Regardless, investigations were conducted to determine how it was being transmitted among cattle. These studies determined that protein supplement feeds made with meat and bone meal (MBM) were the most likely source of the disease. MBM is extracted from cattle and sheep carcasses through a long cooking process called rendering. Rendering is an established practice, used since the turn of the twentieth century. Through rendering, any tissue remaining on animal carcasses after removal of principle tissues is converted into a cake-like material that is used in multiple industries, including the production of protein supplements for animal feeding. Recycling of the agent through rendering led to the rapid and diffuse spreading of BSE throughout most of the United Kingdom (see Figure 1).

Cattle feed sold in the European market spread the disease further. The first cases outside of the UK appeared in 1989 in the Republic of Ireland. By 1990, two other countries (Portugal and Switzerland) were affected, and France reported its first case in 1992. As of December 2000, eleven European countries outside of the UK had reports of BSE in their national herds (Republic of Ireland, Switzerland, Portugal, France, Belgium, Luxembourg, Netherlands, Liechtenstein; and in 2000, Denmark, Spain, and Germany). Three other countries (Canada, Oman, and Maldives Islands) have reported cases, but only in imported cattle. The UK reported approximately 180,000 cases (as of December 2000), with just over 1,300 BSE cases reported elsewhere.

BSE IN HUMANS

In April 1996, Dr. Robert Will of the United Kingdom Edinburgh CJD Surveillance Unit announced that ten persons had been identified with a novel form of Creutzfeldt-Jakob disease (CJD), and that these cases were sufficiently alike and sufficiently distinguishable from sporadic CJD that they could be considered a new variant of CJD

Figure 1

(vCJD). Surveillance for vCJD is conducted throughout the European Union (EU), and in some non-EU countries including the United States, Canada, and Australia. As of April 4, 2001, the number of vCJD cases reported in the UK was ninety-seven, with three cases reported in France and once in the Republic of Ireland.

Clinically, the illness begins with behavioral changes, but its progression is inevitable and unrelenting. The course is relatively prolonged (two years or more) with both mental and physical deterioration. The patient is eventually left in a vegetative state, unable to speak or move, and death is inevitable, as there is no treatment apart from supportive nursing. The average age of onset is 29 years (cases have ranged from 14 to 74 years of age). Diagnosis is made by magnetic resonance imaging, tonsillar biopsy, and cerebro-spinal fluid testing, and must be confirmed by neuropathology. EEG, used for the diagnosis of sporadic CJD, is not helpful. Brain biopsy is not recommended unless a treatable differential diagnosis is sought.

It is now widely considered that the same agent causes both BSE and vCJD. A number of causes have been proposedincluding organophosphates, vaccines, and other novel agents however, scientific support for these theories has been weak. The route of exposure for humans in unclear, but it seems reasonable to suspect cattle-based food.

Discussion of the potential health impact for humans began early, and by 2001 a wide variety of measures had been implemented, including the removal of animals known to have BSE from the human food chain and the removal during slaughter of known high-risk tissues from animal carcasses regardless of their BSE status (certain tissues of sheep and goats are also removed because of fears raised that sheep and goats may be harboring BSE without developing disease). These tissues included initially the brain, eyes, spinal cord and terminal ileum, but the list was later expanded to include the whole head (excluding tongue), spinal column, thymus, intestine, and dorsal root ganglion. In addition, reflecting current knowledge about the spread of infectivity, animals over certain ages (from 6 to 30 months of age depending on the level of safety required) are not consumed. Also to be considered is the possible contamination of skeletal muscle meat due to certain slaughter techniques that embolize brain tissue throughout the body, or which cause surface contamination of edible parts of the carcass.

CONTROL OF BSE

To stop the continued spread of BSE, three principle strategies are used: surveillance, preventing the exposure of ruminants (cattle, sheep, and goats) to feed made from ruminant protein, and slaughter of diseased animals. Because of the long incubation period for BSE (four to five years), cases have continued to be reported after the implementation of these measures. The peak of the epidemic was seen in 1992, at which point the UK was reporting over 30,000 cases of BSE per year.

Cases of BSE reported after the feed ban was implemented are known as "born-after-ban" (BAB) cases, and investigation of them revealed important holes in safety measures. BAB cases appear to be the result of an incomplete application of the feed ban and an incomplete understanding of the importance of even a small amount of contaminationa piece of brain tissue the size of a peppercorn is sufficient to infect a bovine animal. In the UK, feed bans had to be extended (in 1994) to include all mammalian protein in ruminant feeds, with a further extension in 1996 to prevent the use of mammalian MBM in all animal feed. Cross-contamination of tissues in slaughterhouses, feed mills, and other sites also required management. Furthermore, inspection, animal tracing systems, financial incentives, and fines were implemented to ensure that all risk materials were removed from carcasses and that animals with disease were reported and destroyed.

The European Union did not introduce its feed ban legislation (a simple ruminant-to-ruminant feed ban) until 1994, by which point it was too late to prevent the introduction of the disease. When the first human cases of vCJD occurred in 1996, the UK was prohibited from exporting bovine meat, cattle, and bovine-based products. As recently as November 2000, with the recognition of more than expected numbers of cases of BSE, it became clear to the EU that their interventions had been both inadequate and inadequately enforced. Measures to prohibit all animal protein feed to farm animals were to be introduced in the EU within 2001. However, the extent of spread of the epidemic among national herds will not be visible for a number of years.

At the international level, a question exists regarding non-European countries who also imported implicated MBM from the UK and Europe. Few non-EU countries have surveillance system for BSE, yet many nations imported and used implicated MBM, and many also imported live cattle. The risk that the disease has been imported into these countries without the financial capacity to enact the required interventions could result in a new foci of distribution of this devastating disease. International guidelines, such as those promulgated by the Office International des Epizooties, the Food and Agriculture Organization, and the World Health Organization are only partial solutions. It is clear that risk analysis must be conducted in each country in order to assess the risk that BSE was imported and whether the conditions exist for its further propagation.

Maura N. Ricketts

(see also: Transmissible Spongiform Encephalopathy; Veterinary Public Health; Zoonoses )

Bibliography

Brown, P.; Will, R. G.; Bradley, R.; Asher, D. M.; and Detwiler, L. (2000). "Bovine Spongiform Encephalopathy and Variant Creutzfeldt-Jakob Disease: Background, Evolution, and Current Concerns." Emerging Infections Diseases (1).

Knight, R. (1994). "The Relationship between New Variant Creutzfeldt-Jakob Disease and Bovine Spongiform Encephalopathy." Vox Sanguinus 76:203208.

Scott, M. R.; Will, R.; Ironside, J.; Nguyen, H-O. B.; Tremblay, P.; DeArmond, S. J.; and Prusiner, S. B. "Compelling Transgenetic Evidence for Transmission of Bovine Spongiform Encephalopathy Prions to Humans." Proceedings of the National Academy of Science 96(26):15,13715,142.

Ward, H. J. T. (2000). "Surveillance of Variant Creutzfeldt-Jakob Disease in the United Kingdom." EuroSurveillance 5(9):9094.

Wilesmith, J. W.; Wells, G. A. H.; Cranwell, M. P.; and Ryan, J. B. M. (1988). "Bovine Spongiform Encephalopathy: Epidemiological Studies." Veterinary Record 123:638644.

Will, R. G.; Zeidler, M.; Stewart, G. E.; Macleod, M. A.; Ironside, J. W.; Cousens, S. N. et al. (2000). "Diagnosis of New Variant Creutzfeldt-Jakob Disease." Annals of Neurology 47:575582.

Web Sites of Interest

For information about BSE reports: http://www.OIE.int.

For WHO consultations and opinions: http://www.who.int/emc/diseases/bse/index.html.

For information concerning EU decisions: http://europa.eu.int/comm/food/index_en.html.

Web site of the CJD Surveillance Unit (with connections to UK ministries and international surveillance systems sites): http://www.cjd.ed.ac.uk/index.htm.

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bovine spongiform encephalopathy

bovine spongiform encephalopathy (BSE) A degenerative disease of the brain that affects cattle and is caused by an abnormal form of a cellular protein (see prion). Known colloquially as `mad cow disease', it results in a build-up of amyloid tissue in the brain. The infective agent can be transmitted to other cattle via feed containing offal derived from infected animals. It can also, under certain circumstances, be transmitted to other species. Humans infected by eating contaminated beef or beef products have developed a variant form of Creutzfeldt–Jakob disease.

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bovine spongiform encephalopathy

bovine spongiform encephalopathy (BSE) In cattle, degeneration of the brain caused by infectious particles or prions, which may be transmitted by feeding infected meat. It is also known as ‘mad cow disease’. See also Creutzfeldt-Jakob disease (CJD)

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bovine spongiform encephalopathy

bovine spongiform encephalopathy: see prion.

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bovine spongiform encephalopathy

bovine spongiform encephalopathy See BSE.

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