What caused the 1918–30 Epidemic of Encephalitis Lethargica?

Encephalitis lethargica, often called epidemic encephalitis at the time of the epidemic, was prevalent worldwide during the years 1918-1930. The acute phase, characterized by somnolence and a mask-like facial appearance, was associated with a 20-40% mortality. Later in the epidemic, almost all those who had had an acute episode of encephalitis lethargica developed sequelae to a greater or lesser degree. In some cases, the symptoms persisted without respite to the chronic state; in others they developed weeks, months or years after the patient was thought to have recovered. The outstanding motor manifestation was the parkinsonian syndrome, present in almost every case. This resembled the picture of Parkinson's disease (paralysis agitans), except that the 'pill-rolling' movement typical of Parkinson's disease was often absent; the tremor in postencephalitic parkinsonism was usually coarse. The common general features of the latter were rigidity of all the muscles, loss of automatic or synergistic movements, loss of equilibrium, and a running or shuffling gait. Oculogyric crises were an important feature. There were mental changes, especially in children, and respiratory tics were often noted2. Sometimes signs of pyramidal tract damage were found3.