Because life expectancy is shorter than this in cattle destined for slaughter, many clinically undetected but infected animals entered the human food chain prior to 1989. MRM can include portions of spinal chord tissue from the vertebrae. All are rare, the overall incidence being approximately 1 per million. The average age of onset is 64 and average survival is 5 months but there is considerable clinical variability. This is the only type reported in Ireland.
Familial cases have an earlier onset and longer duration than sporadic.
Inheritance is autosomal dominant. Some familial cases have cerebellar degeneration as the outstanding feature with relatively mild dementia. The incubation period for intracranial grafts is approximately 18 months, whereas the pituitary cases are as long as 14 years. Fourteen cases of vCJD have now been confirmed in the UK, and one in France. The spectrum has now widened and the oldest patient is 51.