Background: Understanding of the impacts of climatic variability and change on human health, and the spread of diseases, remains poor despite an increasing burden of vector-borne diseases under global warming. Many confounding social 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-term data sets. Previous studies of malaria in historical times have revealed an association with climate 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 using several data sources, and to characterise the spatio-temporal variations at parish level during severe malaria years in Sweden between 1749 and 1859.
Methods: Pearson (rp) and Spearman’s rank (rs) correlation analyses were conducted to evaluate inter-annual relationship between malaria deaths, temperature and precipitation. 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.
Results: The number of malaria deaths showed the most significant positive relationship with warm-season temperature of the preceding year, but less significant against precipitation. 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. Most malaria hot-spots, during severe malaria years, concentrated in areas around big inland lakes and
Conclusions: Increases in malaria transmission, and hence malaria deaths, in Sweden was linked to high summer temperature during the preceding year. This relationship can be established with statistical confidence from parish records in tandem with the long series of meteorological series. Our results indicate that unusually warm and/or dry summers may have contributed to malaria epidemics, but with non-linear characteristics, making it difficult to model 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.