Transmissible spongiform encephalopathies (TSEs) are a family of neurodegenerative disorders of the central nervous system that include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle and Creutzfeldt-Jakob disease (CJD) and variant CJD (vCJD) in humans (WHO, 2002). These diseases display common characteristics including long incubation periods, a range of clinical symptoms and a similar pathology. TSEs result from the misfolding of a particular protein called a prion, which is normally found on the nerve membranes in the brain. Changes in conformation of this prion disrupts neighbouring prion proteins and associated nerve cells. The first member of this family of diseases identified was scrapie which has been endemic in sheep (and goats) in certain parts of Europe for over two centuries.
CJD was first described in 1921, and has been divided into four main groups, sporadic, hereditary, iatrogenic and variant. Sporadic CJD accounts for approximately 80% of all CJD cases, is found worldwide and mainly affects middle aged to older people. Inherited CJD accounts for 15% of all cases. Several mutations in the prion gene have been reported and the gene is inherited in an autosomal-dominant fashion (i.e. a child has a 50% chance of inheriting the abnormal gene). Iatrogenic CJD accounts for a small number of cases and is caused by exposure to infected human central nervous system or allied tissues. Examples include CJD in people that have had corneal transplants, and the use of human pituitary extracts for growth hormone or infertility treatment. Variant CJD (vCJD), the most recently identified form of the disease, was first described in 1996 in the UK and tends to affect younger people. It is widely accepted that vCJD in humans is the result of transmission of BSE from cattle via affected food.