Temperate Climate Malaria in 19th Century Denmark

Abstract Background A now-extinct strain of Plasmodium vivax was endemic in northern Europe until the early 20th century. In light of the emergence of other vector borne diseases in Europe under climate change, temperate climate malaria warrants a re-examination. The purpose of this article is to characterize malaria epidemiology in late-19th century Denmark. Methods Malaria, listed as “koldfeber”, morbidity and mortality data were digitized from Danish medical records during the period 1862-1914. The seasonality and spatial patterns of malaria, and the relationship of the disease with environmental factors such as soil types, clay content and altitude, were studied. In addition, the age, sex, mortality and seasonal patterns of malaria were studied in the high-morbidity period of 1862-1880. Results The highest incidence occurred in coastal eastern Denmark, and malaria was most frequent in the region of Lolland-Falster. Eastern Denmark is characterized with high soil clay contents, and Lolland-Falster has a low altitude. Incidence typically peaked in May and the case fatality ratio was 0.17%. The disease affected both sexes and all age groups except for infants. In 1873, a large spring epidemic occurred following flooding during a storm surge in November 1872.Conclusion The Plasmodium parasite likely survived the temperate-climate winters through dormancy in the human host, followed by hypnozoites reactivation in the spring. This possibly explains why malaria epidemics typically occurred around May. The age-structure of malaria morbidity with high adult prevalence, indicate that malaria was not highly endemic in the late 19th century Denmark. The investigation of the relationship between malaria and environmental factors suggests a close connection with local settings, such as soil types and altitude, and especially in combination with extreme weather events like floods, these factors may have led to larger malaria epidemics.