Abstract
BackgroundUnderstanding of the impacts of climatic variability on human health remains poor despite a possibly increasing burden of vector-borne diseases under global warming. Numerous socioeconomic variables make such studies challenging during the modern period while studies of climate–disease relationships in historical times are constrained by a lack of long datasets. Previous studies have identified the occurrence of malaria vectors, and their dependence on climate variables, during historical times in northern Europe. Yet, malaria in Sweden in relation to climate variables is understudied and relationships have never been rigorously statistically established. This study seeks to examine the relationship between malaria and climate fluctuations, and to characterise the spatio-temporal variations at parish level during severe malaria years in Sweden 1749–1859.MethodsSymptom-based annual malaria case/death data were obtained from nationwide parish records and military hospital records in Stockholm. Pearson (rp) and Spearman’s rank (rs) correlation analyses were conducted to evaluate inter-annual relationship between malaria data and long meteorological series. The climate response to larger malaria events was further explored by Superposed Epoch Analysis, and through Geographic Information Systems analysis to map spatial variations of malaria deaths.ResultsThe number of malaria deaths showed the most significant positive relationship with warm-season temperature of the preceding year. The strongest correlation was found between malaria deaths and the mean temperature of the preceding June–August (rs = 0.57, p < 0.01) during the 1756–1820 period. Only non-linear patterns can be found in response to precipitation variations. Most malaria hot-spots, during severe malaria years, concentrated in areas around big inland lakes and southern-most Sweden.ConclusionsUnusually warm and/or dry summers appear to have contributed to malaria epidemics due to both indoor winter transmission and the evidenced long incubation and relapse time of P. vivax, but the results also highlight the difficulties in modelling climate–malaria associations. The inter-annual spatial variation of malaria hot-spots further shows that malaria outbreaks were more pronounced in the southern-most region of Sweden in the first half of the nineteenth century compared to the second half of the eighteenth century.
Highlights
Changes in climate, as well as in the frequency and duration of extreme weather events directly affect human health, e.g. by increased mortality during heatwaves [1].Chen et al Malar J (2021) 20:212Even comparably small changes in temperature and precipitation can result in measurable impacts on diseases [2]
Effect of anomalously warm temperatures The occurrence of severe malaria events in response to climate anomalies over the entire 1749–1859 period was further tested by Superposed Epoch Analysis (SEA) assessing the years with most recorded malaria-attributed deaths
The results indicate that the peak years of malaria-attributed deaths were significantly associated with positive anomalies in temperature of May–July and June–August of the preceding year (Fig. 3)
Summary
As well as in the frequency and duration of extreme weather events directly affect human health, e.g. by increased mortality during heatwaves [1].Chen et al Malar J (2021) 20:212Even comparably small changes in temperature and precipitation can result in measurable impacts on diseases [2]. Studies of human health vulnerability in relation to changing climate or extreme weather events need to include both direct and indirect impacts of climate change [7]. Evidences of outbreaks driven by temperature anomalies in Europe [10], and projected spread of vectors with the presence of warming [11], indicated that the expected increase in global mean temperature, and associated changes in regional climate patterns, will very likely have effects on human health and on the prevalence and distribution of various vector-borne diseases [12]. Understanding of the impacts of climatic variability on human health remains poor despite a possibly increasing burden of vector-borne diseases under global warming. This study seeks to examine the relationship between malaria and climate fluctuations, and to characterise the spatio-temporal variations at parish level during severe malaria years in Sweden 1749–1859
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