The average age of onset of symptoms is sixty-five. Clinical signs include impairment of thought, sight, and movement due to the destruction of brain cells, and a dementia resembling that of Alzheimer's disease.
Muscle spasms occur, creating rigidity and jerkiness, and there is a loss of balance.
Death is inevitable and swift, usually within months. Between 1990 and 1994, the incidence of HSE in Britain has nearly doubled and continues to increase. A new form of HSE has been diagnosed in people who have had no genetic predisposition to it. The new HSE differs from CJD in that there is no genetic predisposition, it has a 10 year incubation period, the microscopic appearance of the diseased nerve cells is different, and victims die within one year of exhibiting symptoms. As with cattle, the primary symptom is dementia.