Epidemiology and Ecology of Tularemia in Sweden

Introduction: Sweden has one of the highest incidences of tularemia in the world. Our objective was to describe the epidemiology of tularemia from 1984 through 2012, to explore trends in the geographical distribution, and relationships with ecoregions (areas defined by the distribution of flora, fauna, geomorphology, climate, and soils). 

Methods: Data on age, sex, place, and time of infection of 3,524 eligible cases were analyzed. All cases were given geographical coordinates and the spatial distribution of cases was correlated to ecoregion and presence of inland water. Categorical variables were analyzed using X², proportion test, Wilcoxon rank sum test, or Spearman rank correlation test, as appropriate.

Results:The incidence of tularemia increased tenfold (0.26-2.47/100,000 population) between the first and the second half of the study period, a significant long-term change (p<0.05). The annual and local variations in incidence were great. The incidence by age group showed a bimodal distribution with a decrease in the young twenties. 57.7% of the study cases were men and the global relative risk of contracting tularemia was 1.39 times higher for men than women (p<0.05). Tularemia in Sweden is highly seasonal. There were more cases than expected in the northern half of Sweden, but a 7.8 times higher rate of increase between the early and late time period in the southern part (p<0.05). Based on population at risk, the observed/expected number of cases ratio was higher than expected in 4/6 and lower than expected in 2/6 ecoregions (p<0.05). At the municipality scale, there was a positive correlation between the incidence of tularemia and the relative area of inland water (rho=0.28, p<0.05).

Conclusions: This study shows an over-representation of tularemia cases in the Swedish north, a changing epidemiology with an increase of cases in the south, and positive correlation with the presence of inland water and specific ecoregions.