Figure 1.

The role that arthropods play in the transmission of F. tularensis type A and B. Arthropods maintain F. tularensis infections in nature by transmitting F. tularensis between small mammals, such as rabbits and beavers in order to maintain the reservoir. Type A strain of F. tularensis is commonly transmitted by ticks and by tabanid flies, whereas Type B strain is commonly transmitted by ticks, blood-feeding mosquitoes, and by tabanid flies. In USA, bites by ticks are the predominant mode of tularemia transmission. Transmission by biting flies is observed in western regions of the USA and in Russia. Transmission by mosquitoes is observed in the northern countries of Sweden, Finland, and Russia. Classification of hosts of F. tularensis based on the subspecies of F. tularensis associated with them indicates that there are two cycles of F. tularensis: terrestrial and aquatic. Type A has a terrestrial cycle with the main reservoirs being cottontail rabbits and ticks. Type B has mainly a water-borne cycle with semi-aquatic rodents as reservoirs of infection, such as muskrats and beaver in North America, and ground voles in the former Soviet Union. Type B infections have also been associated with rivers, streams, and temporarily flooded landscapes. Asterisk (*) indicates reservoirs of F. tularensis infections.