Are Rising Temperatures Responsible for the Rates of Tularemia in Sweden?

The number of reported human tularemia cases will steadily increase from 2010 to 2100 in the five high-endemic counties of Dalarna, Gävleborg, Norrbotten, Värmland, and Örebro.  The predicted increase in the incidences of human tularemia has been linked to the steadily warmer climate of Sweden enabling certain parasites to proliferate.  The parasites that spread the bacterium Francisella tularensis include ticks, mosquitoes, fleas, horse flies, and deerflies. Though the level of precipitation was predicted to remain stable and constant throughout the 2010–2100 periods, indicating that the increase in parasites is not due to increased rainfall, the rate of tularemia still increased as the climate became warmer (Rydén et al. 2009). The high localization of tularemia, the population patterns of its vectors and its incidences on the local population will need to be further researched in order to understand the reasons why increased tularemia epidemics correlate with increased temperature. Kevon White
Rydén, P., Sjöstedt, A., Johansson, A., 2009. Effects of climate change on tularaemia disease activity in Sweden. Citation: Global Health Action.  DOI: 10.3402/gha.v2i0.2063.

Patrick Rydén and colleagues from Umeå University analyzed data about human tularemia outbreaks in Sweden from 1997–2008. A 140-year model for simulation data was constructed and performed. Scenario data was taken using a climate model RCA3 and the IPCC Special Report on Emissions data. The data were arranged into 50 x 50 km square regions. Empirical data of disease onset from 379 individuals during 1981–2007 were collected from Dalarna County and used to determine the temperature and time parameters of human tularemia epidemics. The monthly average values were calculated for temperature and precipitation using the 50 x 50 km square regions.  The duration of a tularemia epidemic was determined using the mean temperatures from May to October and time plots from the first and last human tularemia case for each year, region, and local outbreak area.

Investigations on the five high tularemia-endemic areas showed that the incidences of human tularemia from 1997–2008 ranged from 40.1 to 81.1% of the total tularemia incidences in Sweden. All five counties contain 14.61% of the Swedish population at 1,352,558 human inhabitants. The number of incidences during the 1997–2008 range from 0 in the Värmland and Örebro counties in 1997 to 216 in Dalarna County in 2003. However, the distribution in the five counties was uneven, mainly due to the high localization of the disease. The tularemia prediction model indicated that the summer conditions will last longer due to an increase in temperatures by 2° C during 2010–2100. The precipitation was stable throughout the period and did not increase significantly. The duration of the temperate sensitive outbreaks will vary from 3.5 weeks in Norrbotten to 6.6 weeks in Värmland during 2010–2100. An exact relationship between the temperature and the rate of transmission has not been clearly documented; however, because precipitation is stable throughout the 2010–2100 periods then the amount of rainfall is insignificant to the rate of transmission. More analyses will have to be taken in order to determine the exact causes of the disease’s restrictive locality, the vector populations that house the bacterium Francisella tularensis, and the responses of the flora, fauna, and transmission rates to increased 

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