Environmental Monitoring and Surveillance of Rodents and Vectors for Francisella tularensis Following Outbreaks of Human Tularemia in Georgia

Tularemia is a re-emerging bacterial zoonosis, broadly distributed across the northern hemisphere. In Georgia, there is a history of human tularemia outbreaks dating back to the 1940s. In response to outbreaks, health officials initiated long-term field surveillance and environmental monitoring. The objective of our study was to obtain information from 57 years of field surveys to identify species that play a role in the occurrence Francisella tularensis subsp. holarctica in the environment in Georgia. We collected historical data on human outbreaks, field collections, population dynamics of the common vole ( Microtus arvalis ), and conducted surveys on small mammals and vectors from five regions in Georgia during 1956–2012. Bacterial isolation was conducted using standard culturing techniques, and isolation rates for species were obtained for a subset of years. We used a Spearman rank correlation to test for associations between the density of the common vole and isolation rates. From 1956 through 2012, there were four recorded outbreaks of human tularemia (362 cases). A total of 465 bacterial isolates of F. tularensis subsp. holarctica were obtained from 27 species and environmental samples. The number of isolations was highest in the common vole ( M. arvalis ; 149 isolates; 32%) and Dermacentor marginatus ticks (132 isolates; 28%); isolation rates ranged between 0–0.91% and 0–0.47%, respectively. Population dynamics of the common vole were not correlated with the isolation rate. Given the history of tularemia re-emergence in Georgia, continued field surveys and environmental monitoring may provide an early indication of outbreak risk in humans. In conclusion, our findings provide evidence of long-standing foci of F. tularensis subsp. holarctica that are likely maintained by the common vole–tick cycle.