Climate Variability Reveals Complex Events for Tularemia Dynamics in Man and Mammals

Tularemia is caused by the bacterium Francisella tularensis, but the natural reservoir is unknown and environmental conditions for outbreaks in mammals and man are poorly understood. The present study analyzed the synchrony between the North Atlantic Oscillation (NAO) index, the number of human cases of tularemia reported in Sweden, and the density of hares. Climate variation at a lag of 2 yr explained as a single factor ~ 27% of the variation in the number of tularemia cases over time. A low NAO index, indicating cold winters, and low water flow in rivers during the coming summer were associated with high numbers of human cases of tularemia 2 yr later. The number of mountain hares was not related to NAO or to the number of cases of tularemia. The change in mountain hare numbers was negatively associated with the number of human cases, showing the sensitivity of this species to the disease. Low turnover in water environments may at some point in time trigger a chain of events leading to increased replication of F. tularensis via unknown reservoirs and/or vectors that affect humans and mammals. A possible increase in the NAO index with a future warmer climate would not be expected to facilitate a higher frequency of tularemia outbreaks in Sweden.