Unlike the other kinds of infectious disease which are spread by microbes, the infectious agent in BSE is a specific type of protein. Misshapen ("misfolded") prion proteins carry the disease between individuals and cause deterioration of the brain. BSE is a type of transmissible spongiform encephalopathy (TSE). TSEs can arise in animals that carry a rare mutant prion allele, which expresses prions that contort by themselves into the disease-causing shape. Most TSEs, however, occur sporadically in animals that do not have a prion protein mutation. Transmission can occur when healthy animals consume 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. Some TSE's are resistant to extreme temperatures and are not affected by household disinfectants.
The BSE epizootic in British cattle
The British BSE epizootic in cattle was recognized in 1986. It was first believed to have originated in sheep, in which the related prion disease scrapie is common. However, sheep and cattle TSEs are quite different and it is now thought more likely that BSE could have originated with a case of sporadic BSE in a single bovine. Prior to the BSE epizootic, cattle were fed with meat and bone meal, a high-protein substance obtained from the remnants of butchered animals, including cows and sheep. This practice allowed the accumulation of prions over many generations. As more animals became ill, more infectious tissue got into the feed, and so the number of cases reached epizootic proportions. The tissues that contain most of the pathogenic molecules are those of the brain and the nervous system, although infectious amounts have been shown experimentally to be present elsewhere, such as in blood.
The use of meat and bone meal as a protein supplement in cattle feed was widespread in Europe prior to about 1987. Worldwide, Soybean meal is the primary plant-based protein supplement fed to cattle. However, soybeans do not grow well in Europe, so cattle raisers throughout Europe turned to the less expensive animal byproduct 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 seems 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.
Following an epizootic of BSE in Britain, 157 people (up until 2004) 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. Of the 157 cases of vCJD in humans so far, 148 occurred in the United Kingdom, 6 in France, and one in Italy. 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 seems to be dropping). 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.
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 suggested 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. This study is a conjecture without any proof, however, the authors cast doubt by speculation and suggest further investigation. It is interesting to note though that the Indian sub-continent has had no occurrences of BSE.