When I think of Finland, malaria just doesn’t normally come to mind. Although northern climes often have swarms of mosquitoes, its hard to imagine mosquito-borne infections gaining much traction in the short summer season. Yet defying imagination, malaria has thrived in northern Finland, Sweden and Russia near the arctic circle in the past. In the late 19th and early 20th century, Plasmodium falciparum and Plasmodium vivax caused outbreaks in northern Europe. Despite the outbreak of P. falciparium at Archangelsk in the 1930s, P. vivax is believed to be the primary malarial species in northern Europe.
Finnish researchers Lena Huldén, Larry Huldén, and Kari Heliövaara focused on the 1800-1870 period in southern Finland as having the ideal demographic, medical and temperature records before the advent of quinine to study malaria transmission in cold climates.
Medical records are available for Finland from annual reports of ‘district physicians’ and local ministers for most of the 19th century. Doctors were stretched thin across Finland but in the fifty years between 1826 to 1870 there were 542 reports of malaria. Ministers were required to record the cause of death of their parishioners from 1749. Digitization of parish records by the Finnish Genealogical Society has made this data available online for 1800-1850. Terms used for malaria were specific enough that general fever terms in the records did not correlate with malaria outbreaks or temperatures.
Malaria isn’t recorded in Finland until the 17th century, probably brought by migrant workers and gained traction among people gathered for summer infrastructure projects in southern Finland in the 18th century. Death records and physician reports indicate that during mid-19th century epidemics the mortality rate reached as high as 3% of the population with 7-20% infected. The worst epidemic occurred in 1862.
There are three Anopheles mosquito species found in Finland. All are believed to have been present in Finland since prehistory. It had been thought summer temperatures of 16 C (60.8 F) were required to maintain endemic malaria, but malaria has been recorded areas in of northern Sweden and Finland that don’t reach 16C in the summer. Males die shortly after mating and female Anopheles must hibernate from late summer until well into spring. Therefore, the female spends most of its life indoors hibernating with humans and sheltered domestic livestock. The female will take sporadic nocturnal blood meals over the winter but won’t lay her eggs until spring.
Huldén, Huldén, and Heliövaara correlated malarial deaths with annual, seasonal and monthly temperatures. The only significant correlation occurred between summer temperatures of the previous year, but not at all with annual or seasonal temperatures of the same year. Malarial deaths peaked in the spring rather than the usual late summer or autumn. So how does this work with a temperature correlation to the previous summer? Winter infections. The previous summer temperatures effect how many mosquitoes will be hibernating over the winter in homes. Sporadic blood meals over the winter in the confined space of the home spreads the infection to most of the humans and other hibernating mosquitoes causing infections that peak in the spring. Humans are the primary reservoir of infection in cold climates. It doesn’t matter that the malarial sporozoites won’t mature outdoors during the cool summer because they will mature in the cozy confines of winter homes. Fatal spring infections in 40-50 infants born in the winters from 1750 to 1850 supports the theory that the female mosquitoes were capable of transmitting malaria for the entire winter.
Age distribution among the malarial deaths was very similar to the total population indicating that all ages groups were equally vulnerable to infection. Huldén, Huldén, and Heliövaara interpret this as indicating infection occurred at a time when the entire family would be together in heated buildings, in the winter rather than in summer when occupations cause families to live apart by age and gender often in unheated buildings.
Epidemiological data can usually be explained but not necessarily predicted. They have provided another example of why epidemiology can’t always be definitive in ruling in or ruling out the diagnosis of a historic epidemic. Based on outdoor temperatures malaria should have never been endemic in Finland at all. This study highlights the importance of the indoor environment for malaria (and other zoonoses).
Huldén L, Huldén L, & Heliövaara K (2005). Endemic malaria: an ‘indoor’ disease in northern Europe. Historical data analysed. Malaria journal, 4 (1) PMID: 15847704