The Spatiotemporal Distribution of Historical Malaria Cases in Sweden: A Climatic Perspective

Abstract

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 southern-most Sweden.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 meteorological series. Our results indicate that unusually warm and/or dry summers may have contributed to malaria epidemics, but with non-linear characteristics, highlighting the difficulties in modeling 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.

Temperate climate malaria in nineteenth century Denmark

“…atroparvus vector [ 44 , 52 , 53 ]. The geographical pattern of high malaria incidence in coastal wetlands has also been observed in south-east England and other parts of northern Europe [ 2 , 9 , 15 , 17 , 53 ].…”

“…In northern Europe, the size of malaria epidemics depended on the previous year’s summer temperatures. In Denmark, springtime malaria incidence was positively correlated with higher temperatures during the previous July–August (Table 1 ), while in Finland and Sweden, malaria peak incidence was more closely correlated with June-July temperatures [ 2 , 16 ]. Denmark’s lower latitude and warmer climate may explain the difference.…”

“…This could have expanded anopheline populations in the summer of 1873 and might explain the following epidemic in the spring of 1874. The parts of Sweden that experienced flooding during the storm surge [ 50 , 56 ] also experienced malaria epidemics in 1873 and 1874 [ 2 , 50 ]. Therefore, while it is clear that the 1873 and 1874 malaria epidemics in Denmark and Sweden were connected to the storm surge, the exact mechanisms that started the 1873 epidemic remains unclear.…”

“…The relatively low lethality of malaria in Denmark suggests that a large mortality spike that occurred in Denmark in 1831 was not caused by malaria [ 13 , 18 , 21 , 22 ]. Interestingly, malaria was a more lethal disease in Sweden and Finland in the 18th and early nineteenth centuries with a CFR of 0.85–3% in Finland but became less deadly during the nineteenth century [ 2 , 17 ].…”

“…In recent times, it has however been the domain of historians and demographers. Whereas the historical epidemiology of malaria is well-described in England [ 14 , 15 ], the Netherlands [ 9 , 10 ], Finland [ 16 , 17 ] and Sweden [ 2 , 18 ], the evidence from Denmark is more anecdotal. The medical doctor Carl Adam Hansen described Danish malaria in 1886 [ 13 ], and in 1921, the entomologist Carl Wesenberg-Lund described the Danish malaria vectors [ 19 ].…”