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Background: Tularaemia is a geographically widespread disease affecting animals and humans. In Sweden, transmission patterns are complex, occurring mainly through mosquito vectors. We investigated human exposure and whether passive tularaemia surveillance (reported by the public) of deceased wild hares could be used to temporally and geographically predict outbreaks among humans. 

Methods: A survey was sent to the 830 cases of reported tularaemia in Norrbotten county, Sweden, between 2011-2021; and 313/415 (75.4%) respondents with laboratory-evidence of tularaemia were included. Geographic data from human infections in 2019 (n=54) and 2020 (n=77) was compared to data on deceased forest hares from the Swedish Veterinary Agency, matched by year and region.

Results: Respondents (n=313) rarely reported direct exposure to hares (8,6%) and/or other rodents (3.8%) during the 2-weeks prior to illness; while recreational activities (forest hiking 61.6%; mushroom/berry-picking 24.0%; fishing 11.5%; and hunting 3.8%) were more common. Peak incidence of reported deceased hares in 2019 and 2020 (n=84 and n=66; 11/15 [73.3%] and 19/21 [90.4%] PCR-positive for tularaemia, respectively) corresponded to peak incidence of symptom onset of human cases (median difference +6 days [2019] and -2 days [2020]; p=0.066 and p=0.695, respectively). Distribution of reported hares corresponded with municipalities with highest incidence of human tularaemia and location of self-reported suspected infection (Figure 1). Most reported their location of infection to be within their residential municipality (n=92/106, 86.8%).

Conclusion: Passive surveillance of tularaemia using deceased hares correlates with symptom onset in humans and could predict geographical outbreaks in the community. Surveillance of other affected/reservoir species should be considered.