Mapping Spatio-Temporal Distribution of Encephalitis Lethargica Incidence in Sweden

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Mapping Spatio-Temporal Distribution of Encephalitis Lethargica Incidence in Sweden – with special attention to the 1921´s outbreak in Northern Sweden

Abstract
Encephalitis lethargica (EL), also known as the “European Sleeping Sickness,” emerged in Europe around 1916 and became a notifiable disease in Sweden in 1920. This study explores regional variations in EL incidence between 1920 and 1930, with a particular focus on the major outbreak in 1921.

Using data from Public Health in Sweden, EL incidence rates were calculated across Swedish regions, revealing a disproportionate concentration of cases in Västerbotten, Västernorrland, and Gävleborg. Västerbotten reported 243 cases in 1921, with 187 concentrated in Vilhelmina, making it the national hotspot. Monthly disease data from the Annual Reports of the Chief Provincial Doctor in Västerbotten were then analyzed, enabling a comparison of the local disease panorama.

While the cause of EL remains undetermined, our findings support the dual-pathogen hypothesis, suggesting that an interaction between two infectious agents may have contributed to its emergence in hotspots like Vilhelmina.