Francisella tularensis subspecies holarctica

Francisella tularensis, the causative agent of the zoonotic disease tularaemia, is highly contagious and potentially fatal for a wide range of wildlife species of the northern hemisphere, also humans. Although recognized as a pathogen for over 100 years, much still remains to be elucidated concerning the ecology and transmission of the bacterium, hence this literature overview aims at compiling data regarding the aquatic association and the role of mosquitoes in transmission of Francisella tularensis subspecies holartica, the sole subspecies in Sweden. While a linkage between the bacterium and natural waters stands beyond dispute, there is no consensus in the literature concerning its potential as a reservoir. However, two prevailing theories can be distinguished; one proposes the water association being mammaldependent and thus merely the result of contamination from semi-aquatic mammals living in close vicinity to the water source. The other, quite contrary, suggests mammal-independence and hence that water, possibly in association with protozoa, serves as an environmental reservoir for the bacterium. Regarding transmission of the infection in Sweden, it is assumed to be mediated by mosquitoes, yet their precise role and acquisition of bacteria is not well characterized. Again, two possibilities prevail. On the one hand, mosquitoes have been proposed to acquire the bacterium while feeding on infected animals and, on the other hand, already at their larval stages. Although no agreement is reached in literature, the involvement of larvae would inevitably bridge the gap between the aquatic and terrestrial cycle of the bacterium, especially when considering that mosquito invariably feed on protozoa as larvae in floodwater. Future determination of the natural reservoir of Francisella tularensis subspecies holarctica, as well as its relationship with mosquitoes, will shed new light upon its complex life cycle and thus significantly aid in prediction and prevention of new outbreaks.