What causes spongiform encephalopathies?
For many years no-one knew exactly what was causing the spongiform encephalopathies. A virus was presumed to be the culprit, even though biologists couldn’t find anything that looked like a virus in sick animals. The agent was called a slow (or unconventional) virus because of the long incubation period between inoculation and disease – two years for kuru in chimpanzees – and the long clinical course before the inevitable fatal outcome. In the 1970s experiments were starting to show that the mystery microbe in sheep (the scrapie agent) was remarkably resistant to many forms of sterilisation. Healthy animals could be infected even after the extract from diseased animals had been zapped with radiation or treated to destroy the genes of any bacterium or virus. It was this ability to survive normal sterilisation that enabled BSE to be passed on to cattle by meat-and-bone meal. The microbe that causes scrapie seems to have no genes By 1982 an American scientist, Stanley Prusiner, following
