A nationwide seroprevalence study of Francisella tularensis in Denmark

Francisella tularensis is the causative agent of tularaemia. The incidence of tularaemia has increased in Europe in recent years. Due to the non-specific symptoms and diverse clinical manifestations, potential underreporting may exist. Additionally, the recent expansion in tick-borne pathogens could have increased the risk of F. tularensis transmission. We therefore aimed to estimate the seroprevalence of F. tularensis in Danish blood donors and tick-exposed individuals. Plasma samples from healthy blood donors collected in 2022 (n=999) from all five administrative regions in Denmark, samples from blood donors from the Capital region collected in 2024 (n=200), and samples from tick-exposed individuals diagnosed with Lyme borreliosis (n=200), were screened for F. tularensis IgG using a commercial ELISA test. Positive samples for IgG were further tested using an in-house agglutination diagnostic assay measuring total F. tularensis antibodies and tested for Brucella antibodies to control for cross-reactivity. A total of 31/1399 samples were confirmed for F. tularensis antibodies resulting in an overall seroprevalence of 2.2% (CI [1.5;3.3]). All F. tularensis positive samples were negative for Brucella antibodies. The seroprevalence ranged from 1.0%-4.0% within the Danish regions with no significant regional differences. In the Capital region the seroprevalence was 1.0% in 2022 compared to 2.5% in 2024 (P-value 0.45). No significant difference in seroprevalence between blood donors and tick-exposed individuals was observed. The seroprevalence of F. tularensis in Denmark was modest and comparable to other European countries. The study indicates that tularaemia may be underdiagnosed in Denmark.