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導讀：台灣翻譯學學會執行長 蘇正隆

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導讀內容

Beef & Mad-cow Disease

Bovine spongiform encephalopathy (BSE), commonly known as mad-cow disease (MCD), is a fatal, neurodegenerative disease in cattle, that causes a spongy degeneration in the brain and spinal cord. BSE has a long incubation period, about 4 years, usually affecting adult cattle at a peak age onset of four to five years, all breeds being equally susceptible.^[1] In the United Kingdom, the country worst affected, more than 179,000 cattle have been infected and 4.4 million slaughtered during the eradication programme.^[2]

It is believed by most scientists that the disease may be transmitted to human beings who eat the brain or spinal cord of infected carcasses.^[3] In humans, it is known as new variant Creutzfeldt-Jakob disease (vCJD or nvCJD), and by October 2009, it had killed 165 people in Britain, and 44 elsewhere^[4] with the number expected to rise because of the disease's long incubation period.^[5] Between 460,000 and 482,000 BSE-infected animals had entered the human food chain before controls on high-risk offal were introduced in 1989.^[6]

A British inquiry into BSE concluded that the epidemic was caused by cattle, who are normally herbivores, being fed the remains of other cattle in the form of meat and bone meal (MBM), which caused the infectious agent to spread.^[7][8] The origin of the disease itself remains unknown. The infectious agent is distinctive for the high temperatures at which it remains viable; this contributed to the spread of the disease in Britain, which had reduced the temperatures used during its rendering process.^[7] Another contributory factor was the feeding of infected protein supplements to very young calves.^[7][9]

Infectious agent

Microscopic "holes" of tissue sections are examined in the lab. Source: APHIS

The infectious agent in BSE is believed to be a specific type of misfolded protein called a prion. Those prion proteins carry the disease between individuals and cause deterioration of the brain. BSE is a type of transmissible spongiform encephalopathy (TSE).^[10] TSEs can arise in animals that carry an allele which causes previously normal protein molecules to contort by themselves from an alpha helical arrangement to a beta pleated sheet, which is the disease-causing shape for the particular protein. Transmission can occur when healthy animals come in contact with tainted tissues from others with the disease. In the brain these proteins cause native cellular prion protein to deform into the infectious state, which then goes on to deform further prion protein in an exponential cascade. This results in protein aggregates, which then form dense plaque fibers, leading to the microscopic appearance of "holes" in the brain, degeneration of physical and mental abilities, and ultimately death.

Different hypotheses exist for the origin of prion proteins in cattle. Two leading hypotheses suggest that it may have jumped species from the disease scrapie in sheep, or that it evolved from a spontaneous form of "mad cow disease" that has been seen occasionally in cattle for many centuries.^[11] Publius Flavius Vegetius Renatus records cases of a disease with similar characteristics in the 4th and 5th Century AD.^[12] The British Government enquiry took the view the cause was not scrapie as had originally been postulated, and was some event in the 1970s that it was not possible to identify.^[13]

Findings published in PLoS Pathogens (September 12, 2008) suggest that mad cow disease also is caused by a genetic mutation within a gene called Prion Protein Gene. The research shows, for the first time, that a 10-year-old cow from Alabama with an atypical form of bovine spongiform encephalopathy had the same type of prion protein gene mutation as found in human patients with the genetic form of Creutzfeldt-Jakob disease, also called genetic CJD for short. Besides having a genetic origin, other human forms of prion diseases can be sporadic, as in sporadic CJD, as well as foodborne. That is, they are contracted when people eat products contaminated with mad cow disease. This form of Creutzfeldt-Jakob disease is called variant CJD.^[14]

Not all scientists agree that the danger of contracting the disease warrants taking extreme measures. They stress that human infection by mad cow disease has been statistically very small.^{[citation needed]}

The BSE epidemic in British cattle

Cattle are normally herbivores. In nature, cattle eat grass. In modern industrial cattle-farming, various commercial feeds are used, which may contain ingredients including antibiotics, hormones, pesticides, fertilizers, and protein supplements. The use of meat and bone meal, produced from the ground and cooked left-overs of the slaughtering process as well as from the cadavers of sick and injured animals such as cattle, sheep, or chickens, as a protein supplement in cattle feed was widespread in Europe prior to about 1987.^[3] Worldwide, soya bean meal is the primary plant-based protein supplement fed to cattle. However, soya beans do not grow well in Europe, so cattle raisers throughout Europe turned to the less expensive animal by-product feeds as an alternative. A change to the rendering process in the early 1980s may have resulted in a large increase of the infectious agents in the cattle feed. A contributing factor was suggested to have been a change in British laws that allowed a lower temperature sterilization of the protein meal. While other European countries like Germany required said animal byproducts to undergo a high temperature steam boiling process, this requirement had been eased in Britain as a measure to keep prices competitive. Later the British Inquiry dismissed this theory saying "changes in process could not have been solely responsible for the emergence of BSE, and changes in regulation were not a factor at all."^[15]

The first animal to fall ill with the disease occurred in 1984 in Britain, lab tests the following year indicated the presence of BSE, it was only in November 1986 that the UK Ministry of Agriculture accepted it had a new disease on its hands.^{[citation needed]} Subsequently, 165 people (up until October 2009) acquired and died of a disease with similar neurological symptoms subsequently called vCJD, or (new) variant Creutzfeldt-Jakob disease.^[4] This is a separate disease from 'classical' Creutzfeldt-Jakob disease, which is not related to BSE and has been known about since the early 1900s. Three cases of vCJD occurred in people who had lived in or visited Britain — one each in Ireland, Canada and the United States. There is also some concern about those who work with (and therefore inhale) cattle meat and bone meal, such as horticulturists, who use it as fertilizer. Up to date statistics on all types of CJD are published by the National Creutzfeldt-Jakob Disease Surveillance Unit (NCJDSU) in Edinburgh.

For many of the vCJD patients, direct evidence exists that they had consumed tainted beef, and this is assumed to be the mechanism by which all affected individuals contracted it. Disease incidence also appears to correlate with slaughtering practices that led to the mixture of nervous system tissue with hamburger and other beef. It is estimated that 400,000 cattle infected with BSE entered the human food chain in the 1980s.^{[citation needed]} Although the BSE epizootic was eventually brought under control by culling all suspect cattle populations, people are still being diagnosed with vCJD each year (though the number of new cases currently has dropped to fewer than 5 per year). This is attributed to the long incubation period for prion diseases, which are typically measured in years or decades. As a result the extent of the human vCJD outbreak is still not fully known.

The scientific consensus is that infectious BSE prion material is not destroyed through normal cooking procedures, meaning that contaminated beef foodstuffs prepared "well done" may remain infectious.^[16][17]

In 2004 researchers reported evidence of a second contorted shape of prions in a rare minority of diseased cattle. If valid, this would imply a second strain of BSE prion. Very little is known about the shape of disease-causing prions, because their insolubility and tendency to clump thwarts application of the detailed measurement techniques of structural biology. But cruder measures yield a "biochemical signature" by which the newly discovered cattle strain appears different from the familiar one, but similar to the clumped prions in humans with traditional CJD Creutzfeldt-Jakob Disease. The finding of a second strain of BSE prion raises the possibility that transmission of BSE to humans has been underestimated, because some of the individuals diagnosed with spontaneous or "sporadic" CJD may have actually contracted the disease from tainted beef. So far nothing is known about the relative transmissibility of the two disease strains of BSE prion.

Alan Colchester, a professor of neurology at the University of Kent, and Nancy Colchester, writing in the September 3, 2005 issue of the medical journal, The Lancet, proposed a theory that the most likely initial origin of BSE in Britain was the importation from the Indian subcontinent of bone meal which contained CJD infected human remains.^[18] The government of India vehemently responded to the research calling it "misleading, highly mischievous; a figment of imagination; absurd," further adding that India maintained constant surveillance and had not had a single case of either BSE or vCJD.^[19][20] The authors responded in the January 22, 2006 issue of The Lancet that their theory is unprovable only in the same sense as all other BSE origin theories are and that the theory warrants further investigation.^[21]