A natural focus of Francisella tularensis holarctica

Francisella tularensis, the causative agent of Tularemia, is a Gram-negative bacterium that can be transmitted by aerosol, contact with mucous membranes or by the alimentary route. Most common in Germany and Europe is the vector-borne infection. While in Scandinavia mosquitoes are the dominating vector, in Germany most infections originate from tick-borne transmission. As for other vector borne diseases, i.e. Tick-Borne Encephalitis Virus, the occurence of natural foci is known, where a single strain of the pathogen circulates over time, indicated by the high genetic stability of isolates over time.

When a case of tularemia associated with a tick bite in Munich occured, we started to analyse ticks from this area for Francisella tularensis. Isolation of the bacterium was conducted with all samples positive in Francisella tularensis holarctica specific PCR. For this a selective medium comprising high cystein and iron content combined with antibiotics to eliminate companion bacteria was used. All isolates were tested for antibiotic susceptibility and showed no resistance against classes of antibiotics with therapeutical relevance for Tularemia. Moreover, NGS was performed and whole genomes compared to data from Francisella tularensis holarctica from Germany, Austria and Switzerland. All strains belong to Clade 33 of Francisella tularensis holarctica. Compared to other strains of Francisella tularensis holarctica the isolates only differ in very few single nucleotide polymorphisms. This low diversity was seen not only in isolates from the same year but for all isolates from 2022 to 2023. The sampling of the respective area will be further continued to monitor the behaviour of Francisella tularensis holarctica in a natural focus.