The generic name of transmissible spongiform encephalopathies (TSEs) encompasses a series of nervous system diseases in man and animals, of a degenerative and progressive nature, having a very prolonged latency period, of fatal evolution and the clinical course of which is characterised by 2 successive phases: a psychic phase and an organic phase.
In relation to the remaining contagious diseases, they present a series of characteristics that gives them a specific nature:
Included in these diseases are: feline TE, mink TE, scrapie that affects ovine and caprine animals, BSE (bovine spongiform encephalopathy), CWD (chronic wasting disease of the deer), Kuru disease (associated with practices of cannibalism among humans) and vCJD (variant of the Creutzfeldt Jakob disease). All these processes are characterised by causing neural degeneration and accumulation in the cell’s cytoplasm of abnormal fibres made up of an amyloid protein.
As regards the aetiology of TSEs, several hypotheses have been considered. According to the most acceptable hypothesis, known as the "protein only" hypothesis, put forward by Stanley B. Prusiner, TSEs are caused by prions, the latter being the modified form of an intraneural protein and other tissues (cells from the immunological system), which has lost its normal function but has acquired the ability to convert the normal form into a pathological form. In other words, prions convert normal proteins into hazardous molecules by modifying their form.
The PrP (prion protein) exists primarily in two forms: the non-pathological or cellular form, denoted PrPC, and a pathogenic or inductor form, denoted PrPSc. Both forms have the same sequence of amino acids although their biochemical characteristics differ: PrPC is soluble in non-denaturant detergents and completely disintegrated by the proteases while PrPSc is not soluble in non-denaturant detergents and is relatively resistant to proteases.
Structural studies indicate differences: PrPC is predominantly helical while PrPSc has a folded sheet structure (at least 40%). The conversion into this folded sheet structure seems to be the fundamental event in diseases caused by prions. The mechanism of how cells die coinciding with the generation of prions is not clear since the simple accumulation of pathogenic protein does not appear to be sufficient to explain the disease.
The first clinical observation of BSE was made in 1985 in the United Kingdom, and in 1987 it was discovered that the process was associated with the meat and bones of sheep infected by scrapie. The most accepted hypothesis is that the transmission was caused by feed deriving from sheep infected by scrapie due to a change in the process of manufacturing these feeds, in which the infectious agent was found. In 1989, the United Kingdom banned the consumption of offal from cattle and in 1992, the epidemic spread with enormous magnitude while over 37,000 cases of the disease were diagnosed in that year, affecting almost 200,000 head of cattle.
Subsequently in the second half of the 90s, the process was diagnosed for the first time in Portugal, Switzerland, Germany, France, and in November 2000, in Spain. To date, the presence of BSE has been diagnosed in numerous countries the world over.
On 20th March 1996, the government of the United Kingdom announced in parliament the appearance of a pattern consistent with the Creutzfeldt-Jakob disease, new, and hitherto non-existent, known since then as a variant of the Creutzfeldt-Jakob disease (hereinafter vCJD), while at the same time it considered that there could be a relationship between it and bovine spongiform encephalopathy. This makes the implications of the disease in the health, social and economic sphere clear with an incalculable economic repercussion in the beef sector as well as a transformation in the focus given on the part of government departments on food safety.
In the light of such a situation, the institutions of the European Union have adopted different measures of protection against transmissible spongiform encephalopathies, which commenced with the restriction or prohibition of placing beef products from the United kingdom on the market and subsequently beef products from Portugal, but which culminated in the publication of Regulation (EC) 999/2001 laying down rules for the prevention, control and eradication of certain transmissible spongiform encephalopathies (and its subsequent amendments).
In 1994 Spain began to implement measures to prevent, control and eradicate TSEs by controlling the meat and bone meal contained in feed. Since 1997 control and monitoring of TSE programmes have been implemented based on passive monitoring that follow the criteria of the World Organisation for Animal Health (OIE) and the implementation of the Community rules.
In the light of the detection in Spain of the first case of BSE, on 22nd September 2000, Royal Decree 3454/2000 was published, establishing an Integrated Coordinated Programme of Monitoring and Control of TSEs in animals (but in any event it is complemented with the provisions of Regulation (EC) 999/2001 and its amendments). The publication of this rule was justified by the need to take concrete actions, emphasising the active monitoring programme (research in risk groups), to control substances for use in animal feed, to inspect establishments that convert by-products and dead animals and to control specific risk materials.
The strict measures of eradication at sources of scrapie were adopted on the theoretic possibility that these animals could have BSE. At the present time, the situation has changed since discriminatory diagnostic tests have been developed that differentiate BSE from scrapie. With the publication of Regulation (EC) 36/2005, the strategy of epidemiological monitoring of TSE in small ruminants includes conducting routine discriminatory tests between BSE and scrapie, which have to be carried out in all confirmed cases of scrapie.