An Emerging Role for Ticks as Vectors of Tularaemia in Sweden

The zoonotic bacterium Francisella tularensis, the causative agent of tularaemia, can be transmitted to humans via multiple routes, including through contact with infected animals, contaminated water or arthropod vectors. Ticks have not previously been described as transmitting the disease in Sweden. Recently, Ixodid tick species have expanded their latitudinal and altitudinal range in Sweden to areas where the disease is endemic. Tularaemia is a cause of growing concern, spreading to new areas in Sweden and infecting hares and humans.

To establish whether ticks could be a potential arthropod vector in the transmission of F. tularensis subsp. holarctica in Sweden.

Ticks were collected from northern Sweden and screened for F. tularensis. A follow-up study with ticks collected from F. tularensis-positive hares was performed. Ticks were analysed using real-time PCR and a pathological examination was performed on the hares.

F. tularensis subsp. holarctica was identified in ticks from one cat and three F. tularensis-infected hares. Two hares had skin lesions associated with tick bites with intralesional F. tularensis bacteria.

F. tularensis subsp. holarctica was isolated from ticks collected from the hares and cat, the first such reports in ticks in Sweden. Identification of the bacteria at the tick bite site and the more chronic character of the skin lesions compared to those of inner organs suggest that the ticks infected the hares. The cat showed no clinical signs of disease, suggesting that its tick was indeed the vector. These new findings suggest that ticks play a role in the transmission of F. tularensis to human and animal hosts in Sweden.