Factors Influencing Emergence of Tularemia, Hungary, 1984–2010

To the Editor: Francisella tularensis, the etiologic agent of tularemia, is a highly infectious zoonotic agent. F. tularensis subsp. holarctica (type B) is found throughout the Northern Hemisphere and is the only subspecies found in Europe (1). Lagomorphs and rodents probably serve as the primary mammalian reservoir hosts, and hematophagous arthropods, such as ticks, play a role as vectors and hosts (2,3). Although F. tularensis is a potential agent of biological warfare and several emergences and reemergences of tularemia have been reported around the world (1,4), the epizootiology of the disease is only partially understood. The aim of our study was to analyze factors that influence the emergence of tularemia in Hungary. 
 
The study area (15,475 km2) included 3 counties in eastern Hungary. The analyzed data represented a period of 25 years, March 1984–February 2010. Annual F. tularensis–specific seroprevalence data for the European brown hare (Lepus europaeus) population were obtained by slide agglutination testing during the winter (December and January) screening of 2,500–25,000 animals (Technical Appendix). Population density data (animals/km2) for hares were based on February line transect counts and were obtained from the Hungarian Game Management database (www.vvt.gau.hu/vadgazdalkodasi_statisztikak.htm). Common vole (Microtus arvalis) densities (calculated from the number of active burrows/hectare during November) for 1996–2010 were obtained from the Central Agriculture Office, Budapest, Hungary. Vole density was scaled from 0 (absent) to 10 (peak population). The annual number of tularemia cases in humans (based on clinical history and tube agglutination test results) was obtained from the National Center for Epidemiology, Budapest. 
 
The data were regrouped according to the yearly biologic cycle (March–February) for hares and voles (Figure), and relationships between these yearly data were quantified by using the Spearman rank correlation coefficient (5) at county and regional levels. A 2–3 year cycle was characteristic for the analyzed data. A significant positive correlation was found among the number of tularemia cases in humans and the seroprevalence of F. tularensis among European brown hares (Spearman ρ = 0.73; p<0.0001) and the population density of common voles (Spearman ρ = 0.77; p = 0.0081). A significant negative correlation was found between the population density of hares and the seroprevalence of F. tularensis in hares (Spearman ρ = −0.41; p = 0.0365). 
 
 
 
Figure 
 
Correlation between the seroprevalence of Francisella tularensis in the European brown hare (Lepus europaeus) population, the population density of European brown hares and common voles (Microtus arvalis), and the number of tularemia cases in humans eastern ... 
 
 
 
The comprehensive and long-term annual data used in this study provide clues regarding the factors shaping the intraannual epizootiology and emergence or reemergence of tularemia. The European brown hare is moderately sensitive to F. tularensis subsp. holarctica. The hares produce a heterogeneous response to infection, which means that some die of overwhelming bacteremia and others survive with a protracted course of infection, thereby contributing to the maintenance of tularemia over longer periods and serving as useful sentinels of disease activity. Other studies have concluded that hares, together with infected ticks, may serve as disease reservoirs between epizootics (2,3,6,7). 
 
However, we instead hypothesize that the 2–3 year cycling feature of the population dynamics for the common vole (2) determines the ecology of F. tularensis subsp. holarctica in eastern Hungary. The common vole is highly susceptible to F. tularensis subsp. holarctica (3,8). When population densities among voles are high, F. tularensis disease transmission and spillover to hares may be facilitated by stress-related aggression, cannibalism, and F. tularensis contamination of the environment by infectious body discharges (2). Enhanced transmission and spillover can expand local outbreaks to epizootic proportions, and infected hares may, in turn, further enhance the spread of disease through bacterial shedding in urine (6,7). 
 
The disease in hares often results in septicemia and death (7), thus decreasing the population density of these animals. Hares and especially voles are also hosts for different stages of several tick species (2,6), so it can be expected, that higher numbers of infected rodents and lagomorphs result in an increased proportion of infected ticks and, thus, increased transmission of F. tularensis subsp. holarctica. It can be concluded that a higher number of infection sources in the environment results in elevated numbers of cases in humans, mainly through the handling and skinning of hares, but also through tick bites and, potentially, the inhalation of infectious aerosols originating from, for example, hay or grain. 
 
Technical Appendix: 
Detailed data of the annual Francisella tularensis–specific seroprevalence in the European brown hare (Lepus europaeus) population, the annual population density of European brown hares and common voles (Microtus arvalis), and the annual number of tularemia cases in humans, Hungary, 1984–2010. Data are for 3 counties in eastern Hungary: Bekes, Csongrad, and Jasz-Nagykun-Szolnok)*. 
 
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To the Editor: Francisella tularensis, the etiologic agent of tularemia, is a highly infectious zoonotic agent. F. tularensis subsp. holarctica (type B) is found throughout the Northern Hemisphere and is the only subspecies found in Europe (1). Lagomorphs and rodents probably serve as the primary mammalian reservoir hosts, and hematophagous arthropods, such as ticks, play a role as vectors and hosts (2,3). Although F. tularensis is a potential agent of biological warfare and several emergences and reemergences of tularemia have been reported around the world (1,4), the epizootiology of the disease is only partially understood. The aim of our study was to analyze factors that infl uence the emergence of tularemia in Hungary.
The study area (15,475 km 2 ) included 3 counties in eastern Hungary. The analyzed data represented a period of 25 years, March 1984-February 2010. Annual F. tularensisspecifi c seroprevalence data for the European brown hare (Lepus europaeus) population were obtained by slide agglutination testing during the winter (December and January) screening of 2,500-25,000 animals (online Technical Appendix, wwwnc. cdc.gov/EID/article/18/8/11-1826-Techapp.xls). Population density data (animals/km 2 ) for hares were based on February line transect counts and were obtained from the Hungarian Game Management database (www.vvt. gau.hu/vadgazdalkodasi_statisztikak. htm). Common vole (Microtus arvalis) densities (calculated from the number of active burrows/hectare during November) for 1996-2010 were obtained from the Central Agriculture Offi ce, Budapest, Hungary. Vole All material published in Emerging Infectious Diseases is in the public domain and may be used and reprinted without special permission; proper citation, however, is required. density was scaled from 0 (absent) to 10 (peak population). The annual number of tularemia cases in humans (based on clinical history and tube agglutination test results) was obtained from the National Center for Epidemiology, Budapest.
The data were regrouped according to the yearly biologic cycle (March-February) for hares and voles ( Figure), and relationships between these yearly data were quantifi ed by using the Spearman rank correlation coeffi cient (5) at county and regional levels. A 2-3 year cycle was characteristic for the analyzed data. A signifi cant positive correlation was found among the number of tularemia cases in humans and the seroprevalence of F. tularensis among European brown hares (Spearman ρ = 0.73; p<0.0001) and the population density of common voles (Spearman ρ = 0.77; p = 0.0081). A signifi cant negative correlation was found between the population density of hares and the seroprevalence of F. tularensis in hares (Spearman ρ = −0.41; p = 0.0365).
The comprehensive and longterm annual data used in this study provide clues regarding the factors shaping the intraannual epizootiology and emergence or reemergence of tularemia. The European brown hare is moderately sensitive to F. tularensis subsp. holarctica. The hares produce a heterogeneous response to infection, which means that some die of overwhelming bacteremia and others survive with a protracted course of infection, thereby contributing to the maintenance of tularemia over longer periods and serving as useful sentinels of disease activity. Other studies have concluded that hares, together with infected ticks, may serve as disease reservoirs between epizootics (2,3,6,7).
However, we instead hypothesize that the 2-3 year cycling feature of the population dynamics for the common vole (2) determines the ecology of F. tularensis subsp. holarctica in eastern Hungary. The common vole is highly susceptible to F. tularensis subsp. holarctica (3,8). When population densities among voles are high, F. tularensis disease transmission and spillover to hares may be facilitated by stress-related aggression, cannibalism, and F. tularensis contamination of the environment by infectious body discharges (2). Enhanced transmission and spillover can expand local outbreaks to epizootic proportions, and infected hares may, in turn, further enhance the spread of disease through bacterial shedding in urine (6,7).
The disease in hares often results in septicemia and death (7), thus decreasing the population density of these animals. Hares and especially voles are also hosts for different stages of several tick species (2,6), so it can be expected, that higher numbers of infected rodents and lagomorphs result in an increased proportion of infected ticks and, thus, increased transmission of F. tularensis subsp. holarctica. It can be concluded that a higher number of infection sources in the environment results in elevated numbers of cases in humans, mainly through the handling and skinning of hares, but also through tick bites and, potentially, the inhalation of infectious aerosols originating from, for example, hay or grain.   (2) and subsequently in South Korea (3) and Taiwan (4). Public health agencies emphasize screening and strict contact precautions to control multidrug resistant Enterobacteriaceae (5). Routine testing for mechanisms of resistance facilitates detection of emerging carbapenem-resistant Enterobacteriaceae.
In Singapore's 1,000-bed National University Hospital during November 2010-January 2011, we identifi ed New Delhi metallo-β-lactamase 1-producing Enterobacteriaceae in 2 clinical specimens but none that produced KPC (I. Venkatachalam et al., unpub. data). We conducted a laboratory screening study to determine the prevalence and nature of carbapenem-resistant Enterobacteriaceae in April 2011. Ethics committee approval was waived for this study.
Testing of rectal swab samples is part of an established hospitalwide program for vancomycin-resistant enterococci screening. Using a scoring system to identify patients at high risk for vancomycin-resistant enterococci (6), we found that ≈2.5 specimens per 100 admissions were attained each month. During our study, we also tested these samples for carbapenemaseproducing Enterobacteriaceae.
Of the 201 nonduplicate samples processed, 79 microorganisms exhibited imipenem resistance and were isolated on CHROMagar KPC (Table). Among Enterobacteriaceae, carbapenem MIC >2 μg/mL was present in 1 E. aerogenes, 2 E. cloacae, and 4 K. pneumoniae isolates. One isolate (K. pneumoniae) had a positive combined disk test result with a pattern suggestive of serine carbapenemase production.