Animal TSEs

Examples of TSEs in animals include scrapie in sheep and goats, transmissible mink encephalopathy (TME),   feline spongiform encephalopathy   (FSE),   and    bovine spongiform encephalopathy (BSE, labeled ‘mad cow disease’ by the media). Scrapie has been recognized as a distinct infection in sheep for over 250 years.  A major investigation into its etiology followed the vaccination of sheep for louping-ill virus with formalintreated extracts of ovine lymphoid tissue unknowingly contaminated with scrapie prions. Later Two years more than 1500 sheep developed scrapie from this vaccine. The scrapie agent has been extensively studied and experimentally transmitted to a range of laboratory animals, e.g. mice and hamsters. Infected sheep show severe and progressive neuro logical symptoms, such as abnormal gait. The name derives from the fact that sheep with the disorder repeatedly scrape themselves against fences and posts. The natural mode of transmission between sheep is unclear although it is readily communicable and the placenta has been implicated as a source of prions, which could account for horizontal spread within flocks.  In Iceland scrapie-infected flocks of sheep were destroyed and the pastures left vacant for several years.  However, reintroduction of sheep from flocks known to be free of scrapie for many years eventually resulted in scrapie, suggesting that it was able to retain infectivity in the soil for several years. Sheep have also been infected by feedstuff contaminated with BSE, to which they are susceptible.

Bovine spongiform encephalopathy (BSE) appeared in Great Britain in 1986 as a previously unknown disease. Affected cattle showed altered behavior and a staggering gait and post-mortem investigation revealed protease-resistant PrPsc in the brains of the cattle and the typical spongiform pathology. The origins of BSE suggest that cattle were infected with meat and bone meal (MBM) given as a nutritional supplement, contaminated with an unknown bovine TSE (research has shown that the BSE and scrapie prions are different, hence scrapie is not the ancestor of BSE). MBM was initially prepared by rendering the offal of sheep and cattle using a process that involved steam treatment and hydrocarbon solvent extraction but in the late 1970s the solvent procedure was eliminated from the process, resulting in high concentrations of fat in the MBM. It is postulated that this high fat content protected prions in the sheep or beef offal from being completely inac- tivated by the steam. Since 1988 the practice of using dietary protein supplements for domestic animals derived from rendered sheep or cattle offal has been forbidden in the UK. Statistics argue that this food ban has been effective in controlling the epidemic and British beef has been reported as being free of BSE (June 2006).