Epidemiology and Ecology of Tularemia in Sweden, 1984–2012 - Volume 21, Number 1—January 2015 - Emerging Infectious Disease journal - CDC
Volume 21, Number 1—January 2015
Epidemiology and Ecology of Tularemia in Sweden, 1984–2012
Tularemia is a zoonotic disease that causes geographically confined and seasonal outbreaks in many locations in the Northern Hemisphere (1–3). The highly infectious causative bacterial agent, Francisella tularensis, comprises 4 subspecies, but nearly all cases of tularemia are caused by subspecies tularensis (type A), the most virulent type, which is found in North America, or subspecies holarctica (type B), which is the most widespread species in Europe (4). F. tularensis can infect humans through bites of arthropods (e.g., mosquitoes, ticks, tabanid flies); inhalation of infectious aerosols; handling of infected animals; or ingestion of contaminated water (2,3).
Sweden, Finland, and Turkey have reported the highest incidences of tularemia worldwide (5). In Sweden and Finland, the most common form of the disease is ulceroglandular tularemia, which is characterized by a skin ulcer at the site of infection and adjacent swollen regional lymph nodes (6–8). A marked seasonality of tularemia has been reported in Sweden; most cases occur during late summer and early autumn (8–12). An exception was an outbreak affecting >2,700 persons during late fall and winter in 1966−1967 (13). In 2000, large numbers of cases were recorded outside the historically tularemia-endemic northern regions of Sweden, which could indicate a changing geographic pattern of disease (6).
F. tularensis subspecies holarctica naturally infects several mammalian wildlife species, in particular, mice, rabbits, hares, beavers, voles, lemmings, and muskrats (14). In Europe, the ticks Dermacentor reticularis andIxodes ricinus are vectors for the bacterium (15–17), although previous research has suggested that mosquito bites are the most frequent route of transmission to humans in Sweden (6,9,18). Furthermore, a relationship between exposure to F. tularensis and the presence of lakes and rivers has long been suspected and is repeatedly described in the literature on tularemia (4,19–23). However, the ecologic cycles and environmental reservoirs of tularemia remain largely unknown. Since the 1950s, observed disease patterns have suggested that tularemia foci in nature coincide with a suitable ecosystem at a particular place (21,24,25). According to this theory, disease vectors, hosts, and the pathogen are tied to a particular landscape—that is, an ecologic region—as the environmental determinant that controls disease distribution.
To determine the ecologic factors that contribute to the transmission of F. tularensis and the spread of tularemia in Sweden, we examined trends in the epidemiology of tularemia among humans during 1984–2012. We analyzed descriptive epidemiologic data, including the geographic distribution of cases during the study period, and investigated if changes in distribution occurred and if disease was associated with particular ecologic regions and inland water.
Dr. Desvars is a veterinarian and a postdoctoral scientist at Umeå University, Umeå, Sweden. Her research interests include epidemiology, animal health, and the ecology of zoonotic pathogens.
We thank all physicians in Sweden and the epidemiologists at the Public Health Agency of Sweden who provided and curated the epidemiological information in the national communicable disease surveillance system and the personnel at Statistics Sweden and at the Swedish Meteorological and Hydrological Institute for providing open data for research. We also thank Frauke Ecke for providing the map on ecological regions.
This work was supported by grants from the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (Formas no. 2012-1070) and Västerbotten County Council (Dnr. VLL-378261).
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Suggested citation for this article: Desvars A, Furberg M, Hjertqvist M, Vidman L, Sjöstedt A, Rydén P, et al. Epidemiology and ecology of tularemia in Sweden, 1984–2012. Emerg Infect Dis [Internet]. 2015 Jan [date cited]. http://dx.doi.org/10.3201/eid2101.140916