Tularemia, caused by the bacterium Francisella tularensis, where F. tularensis subspecies holarctica has long been the cause of endemic disease in parts of northern Sweden. Despite this, our understanding of the natural life-cycle of the organism is still limited. During three years, we collected surface water samples ( ) and sediment samples ( ) in two areas in Sweden with endemic tularemia. Real-time PCR screening demonstrated the presence of F. tularenis lpnA sequences in 108 (32%) and 48 (20%) of the samples, respectively. The 16S rRNA sequences from those samples all grouped to the species F. tularensis. Analysis of the FtM19InDel region of lpnA-positive samples from selected sampling points confirmed the presence of F. tularensis subspecies holarctica-specific sequences. These sequences were detected in water sampled during both outbreak and nonoutbreak years. Our results indicate that diverse F. tularensis-like organisms, including F. tularensis subsp. holarctica, persist in natural waters and sediments in the investigated areas with endemic tularemia. 1. Introduction Tularemia is a zoonotic disease caused by the bacterium Francisella tularensis. At present, four subspecies of F. tularensis are suggested [1, 2], two of which are of clinical importance (subsp. tularensis and holarctica, [2]). Francisella tularensis subsp. tularensis strains only occur in North America [3, 4] whereas F. tularensis subsp. holarctica strains are found throughout the Northern Hemisphere [5]. Francisella tularensis is categorized as a category A potential bioterrorism agent. Recently, it was established that diverse Francisella-like bacteria exist in the environment (in soil, seawater, and fish) [6–10]. These Francisella-like organisms cluster in various genetic clades together with tick endosymbionts, fish pathogens, and bacteria detected in soil and sediment [5]. The epizootiology of F. tularensis is complex, involving numerous wildlife species and several potential vectors for its transmission as a disease-causing agent. Indeed, tularemia has been detected in approximately 250 wildlife species, giving F. tularensis a broader host range than any other known zoonotic disease-causing organism [11]. Various bloodsucking arthropods have been found naturally infected with the bacterium, like ticks, tabanid flies, midges, mites, fleas, lice, and mosquitoes [12]. Nevertheless, local tularemia outbreaks are often patchy, occurring around natural foci in geographically restricted areas, typically in association with just one or a few key mammalian and arthropod species.
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