Epidemiology and Ecology of Tularemia in Sweden, 1984–2012 - Volume 21, Number 1—January 2015 - Emerging Infectious Disease journal - CDC

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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

Research

Epidemiology and Ecology of Tularemia in Sweden, 1984–2012

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• Methods
• Results
• Discussion
• Suggested Citation

Figures

• Figure 1
• Figure 2
• Figure 3
• Figure 4
• Figure 5
• Figure 6
• Figure 7

Tables

• Table

Technical Appendicies

• Technical Appendix

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Amélie Desvars, Maria Furberg, Marika Hjertqvist, Linda Vidman, Anders Sjöstedt, Patrik Rydén, and Anders Johansson 

Author affiliations: Umeå University, Umeå, Sweden (A. Desvars, M. Furberg, L. Vidman, A. Sjöstedt, P. Rydén, A. Johansson); The Public Health Agency of Sweden, Solna, Sweden (M. Hjertqvist)

Suggested citation for this article

Abstract

The zoonotic disease tularemia is endemic in large areas of the Northern Hemisphere, but research is lacking on patterns of spatial distribution and connections with ecologic factors. To describe the spatial epidemiology of and identify ecologic risk factors for tularemia incidence in Sweden, we analyzed surveillance data collected over 29 years (1984–2012). A total of 4,830 cases were notified, of which 3,524 met all study inclusion criteria. From the first to the second half of the study period, mean incidence increased 10-fold, from 0.26/100,000 persons during 1984–1998 to 2.47/100,000 persons during 1999–2012 (p<0.001). The incidence of tularemia was higher than expected in the boreal and alpine ecologic regions (p<0.001), and incidence was positively correlated with the presence of lakes and rivers (p<0.001). These results provide a comprehensive epidemiologic description of tularemia in Sweden and illustrate that incidence is higher in locations near lakes and rivers.

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.

Acknowledgments

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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Figures

• Figure 1. Mean incidence of tularemia (per 100,000 persons) and number of local outbreaks, Sweden, 1984−2012.
• Figure 2. Annual mean incidence of tularemia by age group and sex, Sweden, 1984−2012. Asterisks (*) indicate significant differences by sex.
• Figure 3. Cumulative number of tularemia cases, by week of onset, Sweden, 1984−2012.
• Figure 4. Distribution of 3,524 tularemia cases, Sweden, 1984−2012. Red dots indicate locations of reported cases; blue line indicates border between northern and southern Sweden, as defined by the southern border...
• Figure 5. Mean incidence (per 100,000 persons) of tularemia in 189 municipalities by 5-year* intervals, Sweden, 1984−2012. *Most recent interval, 2009–2012, was 4 years.
• Figure 6. Distribution of tularemia cases by ecologic region, Sweden, 1984−2012. Black dots indicate locations of reported cases. Region designations adopted from (24).
• Figure 7. Distribution of tularemia cases by altitude, Sweden, 1984−2012. Black dots indicate locations of reported cases.

Table

• Table. Human tularemia notifications, by study period and geographic location, Sweden, 1984–2012

Technical Appendix

• Technical Appendix.  46 KB

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

DOI: 10.3201/eid2101.140916