Approximately 55% of milking herds contain cases of BSE. There were 850 cases per week in 1994, whereas before 1986 it was unknown (although rare cases may have occurred).
Scrapie is endemic in sheep in many parts of the world, though not Australia and New Zealand where strict quarantine controls have been in force. Some breeds are more susceptible than others, leading to wide variations in prevalence.
As well as dementia (except in most cases of kuru), there may be movement disorders, neurological deficits, depression, insomnia and behavioural changes. The pattern of symptoms varies between the diseases.
Classical CJD cases have a characteristic EEG abnormality consisting of generalised repetitive triphasic periodic complexes.