Bovine spongiform encephalopathy (BSE), commonly known as mad-cow disease, 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 30 months to 8 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 180,000 cattle have been infected and 4.4 million slaughtered during the eradication program.[2]
The disease may be most easily transmitted to human beings by eating food contaminated with the brain, spinal cord or digestive tract of infected carcasses.[3] However, it should also be noted that the infectious agent, although most highly concentrated in nervous tissue, can be found in virtually all tissues throughout the body, including blood.[4] In humans, it is known as new variant Creutzfeldt–Jakob disease (vCJD or nvCJD), and by October 2009, it had killed 166 people in the United Kingdom, and 44 elsewhere[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 epizootic 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 cause of BSE may be from the contamination of MBM from sheep with scrapie that were processed in the same slaughterhouse. The epidemic was probably accelerated by the recycling of infected bovine tissues prior to the recognition of BSE.[9] 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 the United Kingdom, 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][10]
This first reported case in North America was in December 1993 from Alberta, Canada.[11],[12] Another case reported later in May 2003. The first known U.S. occurrence came in December of the same year though it was later confirmed that it was a cow of Canadian origin and imported to the U.S.[13] Canada announced two additional cases of BSE from Alberta in early 2005.[14] In June 2005 Dr. John Clifford, chief veterinary officer for the United States Department of Agriculture animal health inspection service confirmed a fully domestic case of BSE in Texas. Dr. Clifford would not identify the ranch, calling that "privileged information."[15] The 12 year old animal was alive at the time when Oprah Winfrey raised concerns about cannibalistic feeding practices on her show[16] which aired April 16, 1996.
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The infectious agent in BSE is believed to be a specific type of misfolded protein called a prion. Prion proteins carry the disease between individuals and cause deterioration of the brain. BSE is a type of transmissible spongiform encephalopathy (TSE).[17] 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.[18] In the 5th century BCE, Hypocrates described a similar illness in cattle and sheep, which he believed also occurred in man. [19] Publius Flavius Vegetius Renatus records cases of a disease with similar characteristics in the 4th and 5th century AD.[20] 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.[21]
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.[22]
Cattle are normally herbivores, eating 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 required said animal by-products to undergo a high temperature steam boiling process, this requirement had been eased in the United Kingdom 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."[23]
The first confirmed animal to fall ill with the disease occurred in 1986 in the United Kingdom, lab tests the following year indicated the presence of BSE; it was only in November 1987 that the British Ministry of Agriculture accepted it had a new disease on its hands.[24] 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.[5] 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 the UK — one each in the Republic of Ireland, Canada and the United States of America. 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. 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.[25][26]
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 the United Kingdom was the importation from the Indian subcontinent of bone meal which contained CJD infected human remains.[27] 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.[28][29] 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.[30]
During the course of the investigation into the BSE epizootic, an enquiry was also made into the activities of the Department of Health and its Medicines Control Agency. On May 7, 1999, in his written statement to the BSE Inquiry,[31] David Osborne Hagger reported on behalf of the Medicines Control Agency that in a previous enquiry the Agency had been asked to:
"... identify relevant manufacturers and obtain information about the bovine material contained in children’s vaccines, the stocks of these vaccines and how long it would take to switch to other products." It was further reported that the: "... use of bovine insulin in a small group of mainly elderly patients was noted and it was recognised that alternative products for this group were not considered satisfactory." A medicines licensing committee report that same year recommended that: "... no licensing action is required at present in regard to products produced from bovine material or using prepared bovine brain in nutrient media and sourced from outside the United Kingdom, the Channel Isles and the Republic of Ireland provided that the country of origin is known to be free of BSE, has competent veterinary advisers and is known to practise good animal husbandry." In 1990 the British Diabetic Association became concerned regarding the safety of bovine insulin and the government licensing agency assured them that: "... there was no insulin sourced from cattle in the Britain or Ireland and that the situation in other countries was being monitored." In 1991 a European Community Commission: "... expressed concerns about the possible transmission of the BSE/scrapie agent to man through use of certain cosmetic treatments." Sources in France reported to the British Medicines Control Agency: "... that there were some licensed surgical sutures derived from French bovine material." Concerns were also raised: "... regarding a possible risk of transmission of the BSE agent in gelatin products."[32]
| Country | BSE cases | vCJD cases |
|---|---|---|
| Austria | 5 | 0 |
| Belgium | 133[33] | 0 |
| Canada | 17[34] | 1[5] |
| Czech Republic | 28[35] | 0 |
| Denmark | 14[36] | 0 |
| Falkland Islands | 1 | 0 |
| Finland | 1 | 0 |
| France[37] | 900 | 25[5] |
| Germany | 312 | 0 |
| Greece | 1[38] | 0 |
| Hong Kong | 2 | 0 |
| Republic of Ireland | 1,353 | 4[5] |
| Israel | 1[39] | 56 |
| Italy | 138[40] | 2[5] |
| Japan | 26 | 1[5] |
| Liechtenstein | 2 | 0 |
| Luxembourg | 2 | 1 |
| Netherlands | 85[41] | 3[5] |
| Oman | 2 | 0 |
| Poland | 21 | 0 |
| Portugal | 875 | 2[5] |
| Saudi Arabia | 0 | 1[5] |
| Slovakia | 15 | 0 |
| Slovenia | 7 | 0 |
| Spain | 412 | 5[5] |
| Sweden | 1 | 0 |
| Switzerland | 453 | 0 |
| Thailand | [42] | 2 |
| United Kingdom | 183,841 | 175[5] |
| United States | 3[34] | 3[5] |
| Total | 188,579 | 280 |
The table to the right summarizes reported cases of BSE and of vCJD by country. BSE is the disease in cattle, while vCJD is the disease in people.
The tests used for detecting BSE vary considerably as do the regulations in various jurisdictions for when, and which cattle, must be tested. For instance, in the E.U. the cattle tested are older (30 months+), while many cattle are slaughtered earlier than that. At the opposite end of the scale, Japan tests all cattle at the time of slaughter. Tests are also difficult as the altered prion protein has very low levels in blood or urine, and no other signal has been found. Newer tests are faster, more sensitive, and cheaper, so it is possible that future figures may be more comprehensive. Even so, currently the only reliable test is examination of tissues during an autopsy.
It is notable that there are no cases reported in Argentina, Australia, Brazil, New Zealand, Uruguay, and Vanuatu where cattle are mainly fed outside on grass pasture and, mostly in Australia, non-grass feeding is done only as a final finishing process before the animals are slaughtered for meat.
As for vCJD in humans, autopsy tests are not always done and so those figures too are likely to be too low, but probably by a lesser fraction. In the United Kingdom anyone with possible vCJD symptoms must be reported to the Creutzfeldt-Jakob Disease Surveillance Unit. In the United States, the CDC has refused to impose a national requirement that physicians and hospitals report cases of the disease. Instead, the agency relies on other methods, including death certificates and urging physicians to send suspicious cases to the National Prion Disease Pathology Surveillance Center (NPDPSC) at Case Western Reserve University in Cleveland, which is funded by the CDC.
Soybean meal is cheap and plentiful in the United States. The million and a half tons of cotton seed meal produced in the U.S. every year that is not suitable for humans or any other simple-stomach animal is even cheaper than soybean meal. It is just as nutritious and cattle eat it just as well as soybean meal. Historically, meat and bone meal, blood meal and meat scraps have almost always commanded a higher price as a feed additive than oil seed meals in the U.S. so there was not much incentive to use animal products to feed ruminants. As a result, the use of animal byproduct feeds was never common, as it was in Europe. However, U.S. regulations only partially prohibit the use of animal byproducts in feed. In 1997, regulations prohibited the feeding of mammalian byproducts to ruminants such as cows and goats. However, the byproducts of ruminants can still be legally fed to pets or other livestock including pigs and poultry, such as chickens. In addition, it is legal for ruminants to be fed byproducts from some of these animals.[43] A proposal to end the use of cow blood, restaurant scraps, and poultry litter (fecal matter, feathers)[44] in January 2004 has yet to be implemented.[45]
In February 2001, the USGAO reported that the FDA, which is responsible for regulating feed, had not adequately enforced the various bans.[46] Compliance with the regulations was shown to be extremely poor before the discovery of the Washington cow, but industry representatives report that compliance is now total. Even so, critics call the partial prohibitions insufficient. Indeed, U.S. meat producer Creekstone Farms alleges that the USDA is preventing BSE testing from being conducted.[47]
The USDA has issued recalls of beef supplies that involved introduction of downer cows into the food supply. Hallmark/Westland Meat Packing Company was found to have used electric shocks to prod downer cows into the slaughtering system in 2007.[48] Possibly due to pressure from large agribusiness, the United States has drastically cut back on the number of cows inspected for BSE.[49]
The BSE crisis led to the European Union banning exports of British beef with effect from March 1996; the ban would last for 10 years before it was finally lifted on 1 May 2006,[50] despite attempts in May 1996 by British prime minister John Major to get the ban lifted.
It was successfully negotiated that beef from Wales was allowed to be exported to the Dutch market, which had formerly been an important market for Northern Irish beef. Of two approved export establishments in the United Kingdom in 1999, one was in Scotland - an establishment to which live beef was supplied from Northern Ireland. As the incidence of BSE was very low in Northern Ireland (partly due to the early adoption of an advanced herd tagging and computerization system in the region) - only six cases of BSE by 1999 - calls were made to remove the E.U. ban on exports with regard to Northern Irish beef.[51]
Japan was the top importer of U.S. beef, buying 240,000 tons valued at $1.4 billion in 2003. After the discovery of the first case of BSE in the U.S. on December 23, 2003, Japan halted U.S. beef imports. In December 2005, Japan once again allowed imports of U.S. beef, but reinstated its ban in January 2006 after a technical violation of the U.S.-Japan beef import agreement: a vertebral column, which should have been removed prior to shipment, was included in a shipment of veal.
Tokyo yielded to U.S. pressure to resume imports, ignoring consumer worries about the safety of U.S. beef, said Japanese consumer groups. Michiko Kamiyama from Food Safety Citizen Watch and Yoko Tomiyama from Consumers Union of Japan[52] said about this: "The government has put priority on the political schedule between the two countries, not on food safety or human health."
65 nations implemented full or partial restrictions on importing U.S. beef products because of concerns that U.S. testing lacked sufficient rigor. As a result, exports of U.S. beef declined from 1,300,000 metric tons in 2003, (before the first mad cow was detected in the U.S.) to 322,000 metric tons in 2004. This has increased since then to 771,000 metric tons in 2007.[53]
On December 31, 2006, Hematech, Inc, a biotechnology company based in Sioux Falls, South Dakota, announced that it had used genetic engineering and cloning technology to produce cattle that lacked a necessary gene for prion production - thus theoretically making them immune to BSE.[54]
Cows affected by BSE are usually apart from the herd and will show progressively deteriorating behavioural and neurological signs. One notable sign is an increase in aggression. Cattle will react excessively to noise or touch and will slowly become ataxic.[55]
Systemic signs of disease such as a drop in milk production, anorexia and lethargy are present.[55]
There continues to be a very practical problem with diagnosis of BSE. It has an incubation period of months to years during which there are no symptoms, even though the pathway of converting the normal brain PrP protein into the toxic, disease-related PrPSc form has started. At present, there is virtually no way to detect PrPSc reliably except by examining postmortem brain tissue using neuropathological and immunohistochemical methods. Accumulation of the abnormally folded PrPSc form of the PrP protein is a characteristic of the disease, but it is present at very low levels in easily accessible body fluids like blood or urine. Researchers have tried to develop methods to measure PrPSc, but there are still no fully accepted methods for use in materials such as blood.
The traditional method of diagnosis relies on histopathological examination of the medulla oblongata of the brain, and other tissues, at post mortem. Immunohistochemistry can be used to demonstrate prion protein accumulation.[55]
In 2010, A team from New York described detection of PrPSc even when initially present at only one part in a hundred thousand million (10−11) in brain tissue. The method combines amplification with a novel technology called Surround Optical Fiber Immunoassay (SOFIA) and some specific antibodies against PrPSc. After amplifying and then concentrating any PrPSc, the samples are labelled with a fluorescent dye using an antibody for specificity and then finally loaded into a micro-capillary tube. This tube is placed in a specially constructed apparatus so that it is totally surrounded by optical fibres to capture all light emitted once the dye is excited using a laser. The technique allowed detection of PrPSc after many fewer cycles of conversion than others have achieved, substantially reducing the possibility of artefacts, as well as speeding up the assay. The researchers also tested their method on blood samples from apparently healthy sheep that went on to develop scrapie. The animals’ brains were analysed once any symptoms became apparent. The researchers could therefore compare results from brain tissue and blood taken once the animals exhibited symptoms of the diseases, with blood obtained earlier in the animals’ lives, and from uninfected animals. The results showed very clearly that PrPSc could be detected in the blood of animals long before the symptoms appeared. After further development and testing, this method could be of great value in surveillance as a blood or urine-based screening test for BSE.[56][57]
A ban on feeding cattle meat and bone meat to cattle has resulted in a reduction in cases in countries where the disease was present. In disease-free countries, control relies on import control, feeding regulations and surveillance measures. [55]
At the abattoir in the UK, the brain, spinal cord, trigeminal ganglia, intestines, eyes and tonsils from cows are classified as Specified Risk Material (SRM) and must be disposed of appropriately. [55]
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