Photo: GSK Marburg
The what and how of Transmissable Spongiform Encephalopathies
What are TSEs?
The transmissable Spongiform Encephalopathies are a unique set of naturally or experimentally transmissable neurodegenerative conditions. They are characterised by vacuolar changes in the grey matter of the brain and spinal cord. In addition, there is a response from astrocytes (support cells found in the brain), accompanied by an accumulation of a pathological, protease-resistant form of the naturally occurring PrP-protein. The nature of the agent that causes the disease is still unknown, but many believe that the faulty PrP-protein (also called prion) is a strong candidate. But whatever the nature of the infective agent, it is extremely resistant to many physical and chemical agents that are normally used to inactivate viruses.
Animal and human TSEs
The best known animal TSEs are mad cow disease (bovine spongiform encephalopathy or BSE, occurring in cattle) and scrapie (occurring in sheep and known since the 18th century). But TSE-like diseases have also been reported in goats, mink (transmissable mink encephalopathy, found in 1965), cats (feline spongiform encephalopathy, FES), antilopes, deer (chronic wasting disease) and exotic animals kept in zoos.
In humans, various TSEs are known. They are the following:
Creutzfeldt-Jakob disease (can be caused by an infection, by a genetic PrP mutation (approximately 10% of all known CJD cases) or by an unknown mechanism)
Gerstmann-Straussler-Scheinker syndrome (caused by a genetic PrP mutation)
Fatal Familial Insomnia (caused by a genetic PrP mutation)
Kuru (the disease is now extinct but was caused by an infection)
Variant CJD (the ‘new’ type of Creutzfeldt-Jakob disease, believed to be caused by consuming products from BSE-infected cattle)
Under natural conditions, the only TSEs that are transmissable appear to be scrapie of sheep, chronic wasting disease of deer and, to a limited extent, maternal transmission of BSE. The suspicion is that new variant CJD is associated with ingestion of food contaminated with the agent causing bovine spongiform encephalopathy (BSE).
The spread of BSE
In domestic cattle, BSE is a new disease, first recorded in 1986 in the UK. The origin of the disease is not known and it is now thought that the occurrence of BSE represents the amplification through the feed cycle of a spontaneous case of spongiform encephalopathy in cattle. The shortest incubation period reported in experimentally infected cattle has been 3 to 3.5 years. The United Kingdom is the only country that has reported a high incidence of the disease (1% of all cattle at the peak of the epidemic). Other countries with reports of BSE are Ireland, Switzerland and, to lesser extents, Portugal, Canada, Denmark, Italy, the Falkland Islands and the Sultanate of Oman. In the latter countries, BSE cases seem to be related to import of animals from the UK. The United States is completely free from BSE. In those countries in which cases of BSE have been reported, strict safety measures have been taken to prevent the further spread of the disease.