Very Low Serological Evidence of Exposure to Francisella spp. in Wild Boar and Red Deer from Central Portugal

Tularemia is a zoonotic disease caused by the highly contagious bacterium Francisella tularensis, with rabbits, hares, and rodents considered as primary reservoirs. Clinical manifestations in humans can range from mild to life-threatening, depending on the mode of exposure (ingestion, inhalation, skin contact, injection, or tick bite), with high fever being a common feature. The primary aim of the present research was to evaluate the circulation of F. tularensis and the potential infection among wild boar (Sus scrofa) and red deer (Cervus elaphus) from the central region of Portugal. A total of 368 samples (including serum and organ samples) were collected from 184 wild boar and 184 red deer. For serological analysis, two commercially available enzyme-linked immunosorbent assay (ELISA) kits were used to detect immunoglobulin M (IgM) and G (IgG) antibodies to F. tularensis. Antibodies to F. tularensis (IgG and IgM) were detected in one adult male wild boar, corresponding to a seroprevalence of 0.54% (1/184, 95% confidence interval [95% CI]: 0.01–3.0%). No antibodies to F. tularensis were detected in red deer. Molecular detection by PCR was negative in the seropositive animal, in which submandibular lymph node, liver, and spleen samples were analysed targeting the 16S rRNA gene. These findings indicate exposure of wild boar to F. tularensis in central Portugal, suggesting a sporadic presence of the pathogen. Although no evidence of active infection was detected in the analysed tissues, the presence of seropositive individuals highlights the need for further investigation. Despite the very low seroprevalence observed, the zoonotic potential of F. tularensis supports the importance of continued surveillance within a One Health perspective.