Prions cause a group of animal and human neurodegenerative diseases which are now classified together because of their etiology and pathogenesis. The infectious agent is not a virus, nor does it contain DNA sequences, it is a brain protein which has undergone modification. Prion proteins are thus thought to exist in two forms, the benign Prion protein (PrPc)and the infectious ‘scrapie form’ (PrPSc).83,84 The normal Prion protein consists of strands of amino acids twirled into helices whereas in the infectious form the amino acid strands are flattened into β-sheets which can cause transmissible dementias. Transmissible dementias are degenerative conditions associated with neuronal loss, and neuronal vacuolation or spongiform (spongy) changes. Furthermore, the changes are accompanied by the accumulation of the abnormal proteinase-resistant prion protein known as PrPSc which has undergone posttranscriptural changes, and the ensuing disease is thus termed a prion disease .
Prion disease is becoming a high-profile public health issue, particularly since the Bovine Spongiform Encephalitis (BSE) or “mad cow’s disease” epidemic rocked Britain. Beginning in 1986, this previously unknown prion disease decimated the beef industry in Britain and it has been proposed that some 160 000 cattle were affected. The infectious agent
was probably transferred to the animals by routinely feeding them meat and bone meal dietary supplements. Carcass meal is predominantly prepared from animals that have died of disease, or animals that have been condemned at the abattoir as unfit for human consumption. The carcass meal and excrement is heat-sterilized to kill the bacteria, but recently chemical sterilization has become the method of choice in many countries, as it is cheaper. Since 1988 the feeding of dietary protein supplements derived from sheep or cattle offal has been banned in Britain and it is argued, that this step has brought the epidemic under control. The incidence of reported cases has indeed declined since the peak was reached in 1992 (figure 4.15).
Figure 4.15. The number of bovine spongiform encephalopathy cases reported in Great Britain during the epidemic. (Adapted from references 83, 85).
Spongiform encephalopathies are transmissible dementias, and occur in man as Creutzfeldt-Jacob disease (CJD), Gerstman-Straussler syndrome, and kuru. In other animals it occurs as scrapie in sheep and transmissible mink encephalopathy in ranch mink. In the UK alone some 75 000 people per year die demented, of which 50% have Alzheimer’s disease and 2% have Creutzfeldt-Jacob disease.86 The latter disease is characterized by a sudden onset of disease, with rapid progression through dementia and death within a year.87 Economic pressures led to initial denials that consumption of BSE contaminated beef could lead to Creutzfeldt-Jacob disease, but in 1996, after a decade of ministerial denials the British Government reluctantly admitted the link between the two.88,89 Since this time, European governments have reacted strongly, banning British beef90 and even slaughtering thousands of head of cattle to restore consumer confidence. In Switzerland the government subsidized the slaughter of 230 000 cows born before 1 December 1990 to restore faith in Swiss beef.91
Prior to the admission that there was a link between BSE and CJD it was argued, that cross species contamination was not possible and that one could not contract the disease from eating contaminated food. However, instances of exotic animals in captivity with transmissible dementias have been linked to consumption of meat and bone meal, and instances in domestic cats in the UK are assumed to be due to BSE-infected offal in cat food.92 In early 1996 it was recognized that the new variant of CJD that was affecting 12 young persons in the UK had been transmitted via BSE contaminated beef. The experimental transmission of the disease by inoculating macaques (Old World monkeys) with BSE infected brain homogenate proved that cross species transmission was possible,93,94 and it is now even well established that transmission occurs between infected cows and their calves thus raising questions about the future of the epidemic.95,96,97
It has been argued, that what happened in Britain is but the tip of the iceberg, and that major epidemics could erupt worldwide. Thousands of Europeans could be unknowingly infected with the disease and could die, and even in the US some researchers feel that the conditions which led to the UK outbreak can lead to similar events in the USA.98. Statistics already suggest, that BSE is now Europe-wide and by May 1996 Switzerland had reported 211 cases of BSE, Eire 125, France 18, Portugal 30, whilst a total of 71 706 tonnes of British meat and bone meal and 33 424 breeding bovines have been exported to EU member states from 1985 to 1990.99 Even if infection of new animals ceases, it has been predicted that between 15 000 and 24 000 new cases of BSE would occur between 1996 and 1999.100