Bovine spongiform encephalopathy epidemiology and demographics
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Adnan Ezici, M.D[2]
Overview
Following an epizootic of BSE in Britain, 165 people (up until 2007) acquired and died of a disease with similar neurological symptoms subsequently called vCJD, or (new) variant Creutzfeldt-Jakob disease. 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. It is notable that there are no cases reported in Australia and New Zealand where cattle are mainly fed outside on grass pasture and, mainly in Australia, non-grass feeding is done only as a final finishing process before the animals are processed for meat.
Epidemiology and Demographics
- Cattle, like most other food animals, are normally herbivores. In nature, cattle eat grass or grains. 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. 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."[1]
- Following an epizootic of BSE in Britain, 165 people (up until 2007) acquired and died of a disease with similar neurological symptoms subsequently called vCJD, or (new) variant Creutzfeldt-Jakob disease. 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 UK CJD Surveillance Centre 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 less 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 full 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.[2][3]
- In 2004 researchers reported evidence of a second contorted shape of prions in a rare minority of diseased cattle. In other words, this implies 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.
- In 2005 a controversial paper in The Lancet introduced a theory that BSE might have originated in British cattle when they ate imported animal feed that included infected human remains from Hindu funeral ceremonies in India. [3] This paper is merely a conjecture, however, and the authors suggest only that further investigation should occur.
- In the UK, the country worst affected, 179,000 cattle were infected and 4.4 million killed as a precaution.[4]
BSE statistics by country
| Country | BSE cases | vCJD cases | Country | BSE cases | vCJD cases |
|---|---|---|---|---|---|
| Austria | 5 | 0 | Belgium | 125 | 0 |
| Canada | 10 | 1 | Czech Rep | 9 | 0 |
| Denmark | 15 | 0 | Falkland Islands | 1 | 0 |
| Finland | 1 | 0 | France | 900+ | 11 |
| Germany | 312 | 0 | Greece | 1 | 0 |
| Hong Kong | 2 | 0 | Israel | 1 | 0 |
| Italy | 117 | 1 | Japan | 26 | 1 |
| Lichtenstein | 2 | 0 | Luxembourg | 2 | 1 |
| Netherlands | 75 | 1 | Oman | 2 | 0 |
| Poland | 21 | 0 | Portugal | 875 | 2 |
| Republic of Ireland | 1353 | 4 | Slovakia | 15 | 0 |
| Slovenia | 7 | 0 | Spain | 412 | 2 |
| Sweden | 1 | 0 | Switzerland | 453 | 0 |
| Thailand | n/a | 2 | UK | 183823 | 160 |
| US | 3 | 3 | Total | 188535 | 170 (+ 6 results pending) |
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 EU 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 small 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 Australia and New Zealand where cattle are mainly fed outside on grass pasture and, mainly in Australia, non-grass feeding is done only as a final finishing process before the animals are processed 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 UK anyone with possible vCJD symptoms must be reported to the UK Creutzfeldt-Jakob Disease Surveillance Unit and so it is unlikely that any cases would be missed. In the U.S., 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.
References
- ↑ http://www.bseinquiry.gov.uk/report/volume2/chaptea3.htm#820576
- ↑ http://www.mayoclinic.com/health/mad-cow-disease/ID00012
- ↑ http://www.fsis.usda.gov/Fact_Sheets/Bovine_Spongiform_Encephalopathy_Mad_Cow_Disease/index.asp
- ↑ Brown, David. "The 'recipe for disaster' that killed 80 and left a £5bn bill", The Daily Telegraph, June 19, 2001.