REVIEW ARTICLE The status of tularemia in Europe in a one … · Tularemia has a broad geographical...
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REVIEW ARTICLEThe status of tularemia in Europe in a one-health context:a review
G. HESTVIK1,2*, E. WARNS-PETIT3, L. A. SMITH4, N. J. FOX4, H. UHLHORN1,M. ARTOIS5, D. HANNANT6, M. R. HUTCHINGS4, R. MATTSSON1, L. YON6
AND D. GAVIER-WIDEN1,2
1Department of Pathology and Wildlife Diseases, National Veterinary Institute, Uppsala, Sweden2Department of Biomedical Sciences and Veterinary Public Health, Swedish University of Agricultural Sciences,Uppsala, Sweden3European Center for Disease Prevention and Control, Stockholm, Sweden4Disease Systems, SRUC, Edinburgh, UK5Université de Lyon, VetAgro Sup, Marcy l’Etoile, France6Faculty of Medicine and Health Sciences, School of Veterinary Medicine and Science, University ofNottingham, UK
Received 8 May 2014; Final revision 5 August 2014; Accepted 23 August 2014;first published online 30 September 2014
The bacterium Francisella tularensis causes the vector-borne zoonotic disease tularemia, andmay infect a wide range of hosts including invertebrates, mammals and birds. Transmission tohumans occurs through contact with infected animals or contaminated environments, or througharthropod vectors. Tularemia has a broad geographical distribution, and there is evidence whichsuggests local emergence or re-emergence of this disease in Europe. This review was developed toprovide an update on the geographical distribution of F. tularensis in humans, wildlife, domesticanimals and vector species, to identify potential public health hazards, and to characterize theepidemiology of tularemia in Europe. Information was collated on cases in humans, domesticanimals and wildlife, and on reports of detection of the bacterium in arthropod vectors, from 38European countries for the period 1992–2012. Multiple international databases on human andanimal health were consulted, as well as published reports in the literature. Tularemia is a diseaseof complex epidemiology that is challenging to understand and therefore to control. Manyaspects of this disease remain poorly understood. Better understanding is needed of theepidemiological role of animal hosts, potential vectors, mechanisms of maintenance in thedifferent ecosystems, and routes of transmission of the disease.
Key words: Animal pathogens, infectious disease, vectors, veterinary epidemiology and bacteriology,zoonoses.
Tularemia is a multi-host, contagious, vector-bornezoonosis caused by Francisella tularensis, a bacterium
with a broad host range, which includes invertebrates,mammals, and birds [1, 2]. There are four subspecies:F. tularensis subsp. tularensis (type A), F. tularensissubsp. holarctica (type B), F. tularensis subsp. media-siatica and F. tularensis subsp. novicida. Types Aand B are considered important for causing diseasein humans and animals. Type A is present almostexclusively in North America and type B is distributed
* Author for correspondence: Ms. G. Hestvik, Department ofPathology and Wildlife Diseases, National Veterinary Institute,SE-751 89, Uppsala, Sweden.(Email: [email protected])
Epidemiol. Infect. (2015), 143, 2137–2160. © Cambridge University Press 2014doi:10.1017/S0950268814002398
all over the Northern hemisphere, but predominatelyin Asia and Europe . Tularemia-like symptoms inhumans have been reported since the late nineteenthcentury in Europe, and it is believed that the so called‘lemming-fever’ was caused by F. tularensis [2, 4].However, in recent years, tularemia has been shownto have a much broader range of hosts and infectionroutes than originally recognized. It is a zoonosis ofcomplex epidemiology, and knowledge of reservoirhosts for this pathogen is still incomplete [3, 5].Tularemia can be transmitted to humans throughmultiple routes, including contact with animals orcontaminated environments, or through arthropodvectors; clinical presentation varies depending on theroute of infection. Various epidemiological cyclesappear to exist, which depend on the local topogra-phy, hydrography and presence of appropriate animaland vector species; each of these may present infectionroutes for humans [3, 4]. Tularemia is important inEurope for a number of reasons. In humans it causespotentially severe disease if left untreated. Tularemiaoften presents with non-specific symptoms, whichmay delay its diagnosis. Sporadic human cases areoften missed, particularly in areas which are assumedto have a low incidence of the disease ; this maylead to inefficient or delayed treatment, which mayresult in more severe manifestations of the disease.Tularemia has a broad geographical distribution,with sporadic cases and/or outbreaks occurring inmany European countries. In the last 20 years, its geo-graphical range has expanded to, or been increasinglyrecognized in, new areas, and the known host rangehas expanded to include species not previously linkedwith tularemia, such as the red fox (Vulpes vulpes), thewild boar (Sus scrofa) and the raccoon dog(Nyctereutes procyonoides) [7–10]. Tularemia hasalso re-emerged in a number of locations, notablycausing new human outbreaks in southern and centralEurope in recent decades [11–13]. Tularemia is, there-fore, considered to be a locally emerging and/orre-emerging infection in Europe. Studies of tularemiahave focused on different host species in differentparts of Europe. Studies of wildlife in centralEurope have involved targeted (or active) surveillanceconducted in hunted hares, foxes, wild boar andtrapped small rodents in endemic areas; these typesof studies are scarce in Northern Europe. In the restof Europe, studies are mainly based on passive surveil-lance of animals found dead or diseased, and these areprimarily hares. Arthropod vectors play a role in theepidemiology of tularemia. Much of the research on
the identification of possible arthropod vectors oftularemia has focused on ixodid ticks and on mos-quitoes, but few studies have investigated the preva-lence of F. tularensis subsp. holarctica throughoutnatural populations of different potential arthropodvectors in Europe. European studies have describedcertain aspects of tularemia or its presentations in lim-ited geographical regions, but to date there has beenno comprehensive overview of existing informationon this disease in Europe, covering all its knownhost species and vectors. The aim of this study is to re-view the published and reported information and toprovide an update on the geographical distributionof F. tularensis in humans, wildlife, domestic animalsand vector species, and to identify potential publichealth hazards and to describe the current state ofthe epidemiology of tularemia in Europe. However,the type of reporting differs, and the investigationsperformed are of variable design, therefore the datacompiled may not reflect the true situation of tulare-mia in Europe.
This review includes data from 38 countries: Memberstates (MS) of the European Union (EU), EU candi-date and potential candidate countries and the fourEuropean Free Trade Association (EFTA) countries,hereafter called ‘the countries’. The review includesreported data and publications from 1992 to 2012on human, wild and domestic animal cases of tula-remia, and pathogen presence detected in arthropodvectors.
In the EU, tularemia in humans is reportable based onthe Commission decision of 19 March 2002, which es-tablished case definitions for reporting communicablediseases to the Community Network under DecisionNo. 2119/98/EC of the European Parliament and ofthe Council, consolidated version 20120927 . Forthe current review, the database of the EuropeanCentre for Disease Prevention and Control (ECDC), created in 2005 was utilized. This database wasqueried for data on human cases of tularemia reportedby EU MS, Norway, Liechtenstein and Iceland be-tween 2006 and 2012. Complementary data werealso extracted from the World Health Organization
2138 G. Hestvik and others
Centralized Information System for InfectiousDiseases (WHO-CISID) database  for 1992–2012, which yielded yearly case numbers for 1992–2005 for the above-mentioned countries, and for thewhole study period for Albania, Bosnia &Herzegovina, Croatia, Montenegro, Serbia, the FYRof Macedonia, Switzerland and Turkey. Case databy county/region and by year were also extractedfrom several national Public Health Institute websites.Population data produced by Eurostat  were usedfor incidence calculations. A literature review wasconducted in order to find additional case data andimportant information on surveillance system, typeof clinical presentation, and source of infection andexposure, in order to assess the disease risk in humans.The literature from January 1992 to December 2012was searched using Pubmed, EMBASE, theCochrane Library, OpenGrey and the DARTEurope e-Theses Portal. The following search strategywas utilized (free text terms/medical subject headingsand title/abstract): tularemia incidence, prevalence oroutbreaks in humans, with the study setting ‘the coun-tries’. Information on type of study, event description,frequency of clinical forms, routes and sources ofinfection, exposures and/or identified risk factors wasextracted (Table 1). Thus, all countries reporting tothe ECDC database have a comprehensive surveil-lance system for humans, but while reporting hasbeen compulsory in most countries since 2003, it isstill voluntary in Belgium and the UK. InSwitzerland and Turkey, reporting of cases has beencompulsory at the national level since 2004 [18, 19].The disease is not notifiable in Denmark, TheNetherlands, Portugal and Liechtenstein. No infor-
mation was available for Albania, Bosnia & Her-zegovina, Croatia, Montenegro, Serbia and the FYRof Macedonia.
Wild and domestic animal data
In animals, the disease is not reportable at the EUlevel (Council Directive 82/894/EEC on the notifica-tion of animal diseases within the Communityamended and consolidated version 20130101 ),but is reportable at the international level (WorldOrganization for Animal Health, OIE) according tothe OIE listed diseases 2013 . For the current re-view, information about tularemia and F. tularensisinfection in wild and domestic animals was compiledfrom OIE databases and from published articles.Because the OIE databases only included data from1996 onwards, data for the period from 1992 to1995 was only available from published articles. TheOIE databases used for this review were HandistatusII (1996–2004) , and the World Animal HealthInformation Database (WAHID, 2006–2012) .However, it is important to note that notification(an official declaration to the OIE) is only mandatorywhen a new disease event occurs in a country; when adisease is endemic in a country, there is no longer anyobligation to declare it. Thus, the information in OIEdatabases on the incidence of tularemia may not be anaccurate reflection of the true extent of this disease.Further, this means that the information available dif-fers among countries in the level of detail of the geo-graphical location of outbreaks, and indeed, inwhether all of the cases or outbreaks which occurredhave even been reported. In this review the data are
Table 1. Clinical presentations, routes of infection, type of exposure and risk factors of human tularemia in Europe
Route ofinfection Exposure Risk factors
Countries with frequentoccurrence*
Oropharyngeal Ingestion Contaminated food orwater
Drinking untreated water,presence of rodent faeces infood storage
Norway (northern andcentral part)
Contact with infectedanimals (scratch, cut) andinsect bites)
Hunting and farmingManipulatingcontaminated game or fish,outdoor activities
Sweden (mostly emergentareas), Spain, France andFinland, Norway (south)
Pneumonic Inhalation Contaminated dust (hay)or aerosols (crop, carcassskinning, eviscerating)
Hunting and farming Occasional outbreaks inFrance and Finland,probably Slovakia,Sweden (endemic areas)
* In Germany and Czech Republic, all three types seem to have occurred in different years and regions.
Tularemia in Europe in a one-health context 2139
shown only as notification or no notification of tular-emia at the country level. The literature from January1992 to December 2012 was searched using the follow-ing databases: Web of Science, Cab Abstracts,Pubmed, Scopus and ProQuest. The search strategywas: tularemia cases, prevalence or outbreaks in wild-life or domestic animals, and the study setting ‘thecountries’. Information on type of investigations con-ducted (active and passive surveillance, outbreakdescriptions and reports of individual cases), diseasestatus in wild and domestic animal species, estimatedprevalence, source of infection, route of sheddingand diagnostic methods used was extracted.
For vectors there are no official databases or obligationsto report. Databases and information are solelyextracted from studies investigating different types ofarthropod species. For the purpose of this review, a vec-tor is any arthropod which can introduce F. tularensisinto a susceptible host. A literature search of the data-bases Web of Knowledge, Science Direct and GoogleScholar was conducted using the following search strat-egy: tularemia cases, incidence, prevalence or outbreaksin vectors or arthropods, and the study setting ‘thecountries’. Reference lists of all reviewed articles werealso assessed. Data extraction on prevalence rates ofF. tularensis in European vectors was restricted to pub-lications from January 1992 to December 2012. Dataextracted included arthropod species, country, numberof arthropods sampled, number of arthropods forwhich the test gave a positive response, prevalence ofF. tularensis in arthropods studied, and the diagnosticmethod used. All studies regarding arthropod vectorswere of the type active surveillance.
Mapping of data
Maps were compiled to compare the reported distri-bution of F. tularensis in Europe in humans, wildlife,domestic animals, and arthropod vectors, and the dif-ferent types of F. tularensis surveillance in humansacross Europe. Due to restrictions in the resolutionof the available data, all information is shown at thecountry level. Information on the reported presenceof F. tularensis in humans was obtained from theECDC and the WHO-CISID databases. Maps ofthe reported presence of F. tularensis recorded in wild-life and domestic animals were prepared based ondata obtained from OIE databases and from the
literature search. Maps of the recorded presence ofF. tularensis in arthropod vectors were based on theinformation obtained from the literature. Maps ofthe type of human surveillance which have been con-ducted at the national level were based on the infor-mation recorded by MS in the ECDC database andin the literature for Switzerland and Turkey. Theyconcern coverage of surveillance (comprehensive orsentinel), notification type (compulsory or voluntaryreporting), data sources (laboratory, hospital, generalpractitioner, health services, others) and case defini-tion reference (for details see online Supplementarymaterial). Country base layers were downloadedfrom Global Administrative Areas , and mapswere created in ArcGIS.
RESULTS AND DISCUSSION
Tularemia in humans
During the twentieth century, according to Tärnvik, there were large outbreaks of several hundredtularemia cases. These were associated with wartimeconditions in various European countries during andafter World War II (Austria, France, Hungary,Bulgaria, Germany), and with farming activities inFinland during the 1980s and 1990s. Four tularemiaepidemics were reported from three different regionsof Turkey between 1936 and 1953 . After a long in-terval, a new tularemia epidemic was reported fromthe area around Bursa in the northwestern part ofTurkey in 1988, followed by small epidemics in variousnorthwestern Turkish regions over a 10-year period. Inrecent years, European country reports of tularemiacases to international databases has largely improvedthe overall knowledge of the extent of the disease; in1992 only 12 countries reported tularemia to theWHO, while in 2010, 31 countries reported to theECDC/WHO. However, public awareness of the dis-ease, and the surveillance systems used to detect thedisease are not identical in the different Europeancountries. Furthermore, the regularity and level of de-tail of reports submitted to international databasesvaries among the countries. Therefore, the currentinformation on tularemia which is present in theinternational databases should be interpreted withcaution.
Symptoms of tularemia are principally related to thesite of entry of the bacteria, the virulence of the
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F. tularensis strain, and the immune status of the host.Tularemia has several clinical forms in humans,including ulceroglandular, glandular, pneumonic, oro-pharyngeal, oculoglandular and systemic (typhoidal,intestinal). In the ulceroglandular form, a local skinlesion is often considered the route of entrance (scratch,cut, insect bite) and the disease progresses to the swel-ling of the regional lymph node, which may ulcerateand suppurate. The glandular form of this disease issimilar to the ulcerglandular form, but no primaryskin lesion is detected in these cases. The pneumonicform results from inhalation of F. tularensis, and affectsone or both lungs. The oropharyngeal form of tular-emia is linked to ingestion of contaminated food orwater. Other clinical presentations of tularemia, suchas septic or typhoidal, are infrequent in Europe .
Case numbers, incidence and outbreaks
Overall, 18 343 human cases of tularemia werereported to the WHO-CISID or ECDC databases be-tween 1992 and 2012. Sweden reported 25% of allreported cases, Finland 22%, Turkey 13% and theCzech Republic and Hungary around 9% each.
Highest overall case numbers in Europe for thestudy period were observed in 2000, 2003 and 2010
with 1657, 1865 and 2458 cases respectively (Fig. 1).However, the number of reporting countries hasincreased over time, and the reported case numbersrepresented 19, 27 and 31 reporting countries, respect-ively. When analysed by country, high case numberswere reported by Finland and Sweden in 2000 (917and 464 cases, respectively) and 2003 (823 and 798cases, respectively), by Sweden in 2010 (484 cases),Norway in 2011 (178 cases), Spain in 1997 and 2007(585 and 493, cases respectively), and Turkey in2010 (1531 cases). The average incidence rate in thestudy area for 2006–2012, i.e. number of cases per100 000 inhabitants, was 0·04; however, notablyhigher average incidence rates were seen in some coun-tries. The highest average incidence rates wereobserved in Finland, followed by Sweden (4·84 and3·78, respectively); relatively high average rates werealso observed in Norway and Slovakia (1·16 and0·93, respectively). When the data was examined byyear, it was determined that Turkey experienced ahigh incidence rate in 2010 (2·11), Hungary experi-enced relatively high rates in 2006 (1·38) and 2010(1·26), and Spain also had a high incidence rate in2007 (1·11). Large variations of incidence rates oc-curred between years for each country, but also withineach country, and the latter can be visualized at higher
Year1992 1993 1994 1996 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
BulgariaBosnia Herzegovina CroatiaLithuania
Fig. 1. Number of cases of tularemia by country and by year reported to the European Centre for Disease Prevention andControl (ECDC) and WHO databases between 1992 and 2012. Each country is represented by a different colour. Data forthis figure come from Austria, Bosnia & Herzegovina, Bulgaria, Croatia, Czech Republic, Estonia, Finland, France,Germany, Hungary, Italy, Latvia, Lithuania, Norway, Poland, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden,Switzerland and Turkey, but not all countries reported for the entire period.
Tularemia in Europe in a one-health context 2141
geographical resolutions. For example, very high inci-dence rates (above 82/100 000 inhabitants) wereobserved in Norrbotten (northern Sweden) in 2012and in Finnmark (northern Norway) in 2011.
Additional epidemiological data compiled from theliterature indicated that between 1992 and 1998 inSweden, about 80% of the cases occurred in the north-ern part of the country, while in 1999 and particularlyin 2000, a large proportion of cases also occurred inthe middle and southern regions of the country .During the period from 1995 to 2001, cases of tular-emia in Finland were reported predominantly in thesouthern and western parts of the country (NationalPublic Health Institute of Finland), while the northernand western parts of Finland have reported outbreakssince 2000 . The whole country of France seems tobe affected, but some areas more heavily than others,a pattern which was similarly seen in the casesobserved in hares . Similarly, more cases occurredin the western and southern parts of Germany, withan apparent increase in the number of cases after2004 . In Norway, only the southern part of thecountry seems to have remained unaffected. Two war-related outbreaks occurred in Kosovo in 1999 and2001 , (WHO 2000). As indicated in Reintjeset al. , an outbreak of ulceroglandular tularemia,suspected to be associated with infected hares, wasalso reported in central and western Bosnia in 1995,in the aftermath of warfare. Serbia was also repeatedly
affected in the early 2000s, in the region borderingBulgaria. In Spain, in Castilla y Leon, two outbreakperiods occurred at a 10-year interval (1997 and2007); one isolated event happened in 1998 inCastilla la Mancha . In Turkey, outbreaksoccurred in the northern half of the country in 2005, but during subsequent outbreaks in 2010–2012,the affected zone expanded to the more central partsof the country .
Outbreaks in humans are often sporadic and likely tobe spatially and temporally variable, in addition tospecific sites with potential for clusters of infection.Based on the reporting of human cases the major hot-spots for tularemia at the European level are located inScandinavia and Central Europe. According to datafound in the literature, southeastern Europe (Slovakia,Serbia, Kosovo, Turkey) also show elevated incidences.
Data reporting has been inconsistent through theyears and therefore trend analysis cannot be per-formed on all of ‘the countries’ at the same time.No clear trend was apparent in the data from 12European countries which consistently reported casesbetween 1992 and 2012. The same was true for 23countries which consistently reported between 2003and 2010, and for EU MS, Norway and Iceland dur-ing the last 5 years (Fig. 2). However, trends were
Number of cases
12-month Moving Average
0Jan. 2008 July 2008 Jan. 2009 July 2009 Jan. 2010 July 2010 Jan. 2011 July 2011 Jan. 2012 July 2012
Fig. 2. Number of cases of tularemia by month recorded in the European Centre for Disease Prevention and Control(ECDC) database, 2008 to 2012 (n= 4388), with a 12-month moving average. Data for this figure come from Austria,Belgium, Cyprus, the Czech Republic, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,Latvia, Lithuania, Luxembourg, Malta, Norway, Poland, Romania, Slovakia, Slovenia, Spain, Sweden and the UK.
2142 G. Hestvik and others
apparent in individual countries: Sweden, Norway,France, Germany and possibly Turkey (no recentdata is available, but data from 2005 to 2010 wasexamined) showed a trend towards an increasing num-ber of cases, while a trend of decreasing case numberswas apparent in Finland, Hungary, Bulgaria andSlovakia, and no trends appeared in data fromAustria and the Czech Republic (Fig. 1, Table 2).An increase in case numbers was also reported forSwitzerland .
Tularemia in wild animals
Much of the basis of existing knowledge of tularemiain wild animals in Europe stems from research fromthe 1950s and 1960s, with the most extensive studiesperformed in the former Soviet Union. Olsufjev discussed tularemia in many species of wild animalsincluding lagomorphs, murines, cricetines, carnivores,mustelids, artiodactyls and aves, providing infor-mation on ecology, epidemiology, bacteriology, anddifferences in host susceptibility. During the 1970sand 1980s, the mountain hare (Lepus timidus) inSweden  and a variety of rodent species in severalEuropean countries were investigated. Tularemia inthe European brown hare (Lepus europaeus) was stud-ied in Hungary through serological surveys . Fromthe 1990s onwards, there was a broadening of therange of wildlife species examined for tularemia, andan increasing number of countries that conducted stud-ies (Table 3). During the period 1992–2012, F. tular-ensis infection was most commonly investigated in theEuropean brown hare, and in a variety of murine andcricetine species. Other species were also investigated,though less extensively; these included red fox, wildboar, mountain hare, lynx (Lynx lynx) and migratoryshore birds. The investigations were conducted ondead animals obtained from hunters (hare, red fox,wild boar, lynx), through ad hoc sampling (hare,lynx, small rodents) and by trapping (murine and cri-cetine species, migratory shore birds). The methodsused to detect either the bacterium F. tularensis itself,or antibodies to it, are shown in Table 3. The geo-graphical distribution of F. tularensis at the countrylevel is shown in Figure 3.
According to previous publications, when infectedwith F. tularensis, the European brown hare developschronic lesions , while the mountain hare typically
develops acute fatal septicaemia [33, 36–38].However, studies from the last two decades indicatethat the European brown hare can also developacute disease [35, 39]. In acute disease, necroses aredistributed in multiple organs [33, 39], while inchronic cases granulomatous inflammation can beseen in a more restricted number of organs, e.g. thekidney and lungs . For other wildlife species,descriptions of the pathology and clinical presenta-tions in naturally infected animals are still notablyabsent from the literature.
Serological investigations 1992–2012
Most of the serological investigations were conductedon samples from apparently healthy hunted ortrapped animals. Results are presented in Table 3.Among the studied species, antibodies have beendetected in European brown hare, red fox and wildboar. In Hungary, hares have been serologically sur-veyed yearly since 1984. For the period 1992–2010,the overall serological prevalence in 140 935 harestested was 6·6% . Studies finding serologicallypositive animals have also been published fromAustria, the Czech Republic and Germany. In thesestudies, the prevalence in European brown hare, redfox and wild boar varied between 1·4% and 10·8%(Table 3). Serological investigations conducted duringthis time period in German and Hungarian smallrodents, and in Swedish lynx did not detect any anti-bodies [40–42]. However, studies before 1992 diddetect antibodies in Norwegian lemmings  andSwedish beavers [44, 45].
Detection of the bacterium F. tularensis 1992–2012
Table 3 summarizes results from investigations on thepresence of the F. tularensis bacterium either in activesurveillance studies or for diagnostic purposes, in sin-gle animals or small groups of animals. Many of thestudies only diagnosed F. tularensis to species level,while others also determined the subspecies, whichwas holarctica in all reported studies. Apparentlyhealthy hunted European brown hares were tested inGermany, with a detection rate for the bacterium vary-ing between 0·3% and 2·9% [46, 47]. F. tularensis wasdetected in hares found dead in Germany ,France  and Switzerland . In Germany,Runge et al.  also reported that 2·4% of wildEuropean rabbits (Oryctolagus cuniculus) found deadtested positive for F. tularensis. Small rodents wereinvestigated, primarily by trapping, in Austria,
Tularemia in Europe in a one-health context 2143
Table 2. Summary of data* on tularemia in humans in Europe
Presence ofF. tularensisin vectors
Presence ofF. tularensisin wildlife
Presence ofF. tularensisin domesticanimals
Austria ECDC 0·05 4 ‡ Winter Yes Yes NoBosnia &Herzegovina
WHO 0·01 0 ‡ ‡ No No No
Bulgaria ECDC [98–101] 0·06 5 ‡ Spring-summer Oropharyngeal Food orwater
No Yes Yes
Croatia WHO 0·07 3 ‡ ‡ No No YesCzechRepublic
ECDC [102, 103] 0·63 65 −0·2 All year, more inautumn-winter
Hares Yes Yes Yes
Oropharyngeal WaterEstonia ECDC 0·05 1 ‡ ‡ No No NoFinland ECDC [27, 28,
104]4·84 257 −15·6 Summer-autumn/
summerPneumonia Hay, rabbits
and volesFarming No Yes Yes
France ECDC [29, 105–109]
0·08 50 3·7 All year, more inwinter/summer-autumn-winter
Yes Yes Yes
August Pneumonia Dust Countrysidestay
Germany ECDC [46, 110–112]
0·02 16 1·0 Winter Oropharyngeal Meat Yes Yes YesPneumonia Hare Hunting,
skinning gameWinter/summer Ulceroglandular/glandular
Hungary ECDC 0·54 54 −3·0 Summer Yes Yes NoItaly ECDC 0·01 7 ‡ Spring 2008 No Yes YesKosovo [13, 31,
Autumn to spring Oropharyngeal Food orwater, fieldmice
Farming No Yes No
Latvia ECDC 0·04 1 ‡ ‡ No No NoLithuania ECDC 0·04 1 ‡ ‡ No No NoNorway ECDC [87, 88,
1·16 56 3·9 Autumn-winter/winter
Oropharyngeal Water,lemmingsand deadhares
No Yes Yes
Poland ECDC 0·01 3 ‡ ‡ No Yes YesRomania ECDC 0·00 1 ‡ ‡ No No No
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Table 2 (cont.)
Presence ofF. tularensisin vectors
Presence ofF. tularensisin wildlife
Presence ofF. tularensisin domesticanimals
Serbia WHO  0·26 19 −5·3 Oropharyngeal Yes No NoSlovakia ECDC  0·93 20 −2·1 All year/summer-
autumnHare, hayand straw,vectors
Yes Yes Yes
Slovenia ECDC 0·04 1 ‡ ‡ No No NoSpain ECDC [11, 75,
0·18 81 ‡ Summer-autumn2007/autumn-winter
Yes Yes Yes
Summer Red crayfish,positivewater
Sweden ECDC [27, 77,127–130]
3·78 351 21·9 Summer-autumn/Summer-autumn
Yes Yes Yes
Switzerland WHO  0·13 10 2·3 Ulceroglandular/glandular
Ticks Yes Yes Yes
Turkey WHO [12, 18,25, 131–141]
0·7 509 Rising? Autumn to spring Oropharyngeal Water,rodents
No No Yes
* Data extracted from the European Centre for Disease Prevention and Control (ECDC) and WHO databases or the literature review (in italic) between 1992 and 2012, bycountry (only countries reporting cases in the last 6 years/with published articles during the study period are included).†Number of cases per 100 000 population per year.‡ Insufficient case numbers to calculate trends.
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Table 3. Wild and domestic animal species investigated for infection with F. tularensis in Europe 1992–2012
Animal species Country Method
Seroprevalence(s)*Type of study†(%/no. investigated)
Prevalence(s) of agent*Type of study†(%/no. investigated) References
LagomorphaEuropean brown hare(Lepus europaeus)
Austria Culture,FBFST, MAT
S (7·1/311) — I I [7, 143]
Bulgaria PCR, IFA — Ih Czech Rebublic MAT, SAT S (1·4–6·6/1124) — [142, 144]France Organ
— S (3·3/3236) — Ow 
Germany WB, PCR,culture
S (0/299) — — S (0·3–2·9/2121) — Ih — I 
Hungary SAT S (0·5–23·0/140 935) I [34, 35, 40]— I [35, 40]
Switzerland PCR, culture — S (1·2/167) Mountain hare (Lepustimidus)
Norway PCR, culture,IHC
— I [36–38]
European rabbit(Oryctolagus cuniculus)
Germany PCR — S (2·4/41) 
RodentiaYellow-necked mouse(Apodemus flavicollis)
Austria-Slovakia Culture‡ — S (0·7–2·2/579) [73, 74]Germany PCR — Sh (2·9/69) Hungary SAT, PCR S (0/110) S (0/110) 
Wood mouse (Apodemussylvaticus)
Austria-Slovakia Culture‡ — S (1·6–1·8/120) [73, 74]France PCR, culture — Sw (0/ (5) Kosovo ELISA — Sh (0/2) 
House mouse (Musmusculus)
Austria-Slovakia Culture‡ — S (0/53) Bulgaria PCR — S (20·8/24) Kosovo ELISA — Sh (0/23) 
Striped field mouse(Apodemus agrarius)
Bulgaria PCR — S (0/9) Kosovo c-ELISA — Sh (7·7/26) Hungary SAT, PCR S (0/13) S (0/13) 
Pygmy field mouse(Apodemus microps)
Austria-Slovakia Culture‡ — S (0/6) 
Bank vole (Clethronomysglareolous)
Austria-Slovakia Culture‡ — S (1·1–2·9/514) [73, 74]Germany PCR — Sh (4·5/178) 
Common vole (Microtusarvalis)
Austria-Slovakia Culture‡ — S (4·5–4·9/170) [73, 74]Germany PCR — Sh (8·0/25) Hungary SAT S (0/37) S (0/3) 
Water vole (Arvicolaterrestris)
Germany PCR — Sh (15·0/40) 
Field vole (Microtusagrestis)
Germany PCR — Sh (10·0/10) 
European pine vole(Pitymys subterraneus)
Austria-Slovakia Culture — S (0/1) 
Black rat (Rattus rattus) Bulgaria PCR — S (23·5/136) Kosovo c-ELISA — Sh (18·1/11) 
Common hamster(Cricetus cricetus)
Hungary SAT S (0/900) S (0/100) 
Lemming (Lemmini) Norway PCR, ELISA — Ih Unspecified small rodentspecies
Germany ELISA, WB Sh (0/186) — Sweden PCR, culture — S (2·1/97) 
2146 G. Hestvik and others
Bulgaria, Germany, Hungary, Kosovo, Slovakia andSweden, and the detection rates varied greatly, from0·7% up to 20·8%. Positive individuals were foundamong yellow-necked mice, wood mice, house mice,striped field mice, bank voles, common voles, water
voles, field voles and black rats [41, 50]. In one studyin Austria, F. tularensis was detected in 1·3% of thesubmandibular lymph nodes of apparently healthyhunted red foxes . In Portugal, 212 migratoryshore birds of varying species were examined for
Table 3 (cont.)
Animal species Country Method
Seroprevalence(s)*Type of study†(%/no. investigated)
Prevalence(s) of agent*Type of study†(%/no. investigated) References
InsectivoraCommon shrew (Sorexaraneus)
Austria-Slovakia Culture — S (0/2) Hungary PCR S (0/6) S (0/6) 
Bicoloured white-toothedshrew (Crociduraleucodon)
Austria-Slovakia Culture — S (0/1) 
CarnivoraRed fox (Vulpes vulpes) Austria PCR, culture,
MATS (7·5/385) — — S (1·3/1152) 
European lynx (Lynxlynx)
Sweden TAT S (0/106) — 
ArtiodactylaWild boar (Sus scrofa) Czech Republic MAT S (10·8/204) — 
Germany ELISA, WB S (3·1/763) — 
France PCR, culture — Sw (0/1) 
Migratory shore birds Portugal PCR — S (0·5/212) 
Domestic ruminantsCattle Bulgaria Unknown Ih — 
Hungary SAT, TAT S (0/50) — Turkey MAT Sh (70·4/27) — 
Buffalo Hungary SAT, TAT S (0/50) — Sheep Bulgaria Unknown Ih — 
Hungary SAT/TAT S (0/100) — Turkey ELISA, MAT,
TATS (0–7·8/1431) — 
Goat Italy TAT S (0·5/320) — 
Domestic carnivoresDog Bulgaria Unknown Ih — 
France Unknown Ih — [106, 150]Slovakia Unknown S (16·4/548) — Turkey MAT S (2·0–5·3/270) — 
Domestic lagomorphsRabbit Turkey MAT Sh (4·0/25) — 
FBFST, Fresh blood fast staining test; IFA, immunofluorescence test; IHC, immunohistochemistry; MAT, microagglutina-tion test; PCR, polymerase chain reaction; SAT, slide agglutination test; TAT, tube agglutination test; WB, Western blot.* In surveillance type investigations percentage, or range of percentage, are presented. For other study types occurrence isshown.†Type of study: S, active or passive surveillance; O, outbreak description; I, individual cases. If an outbreak in humans orwildlife was the reason for conducting the study, this is shown as a subscript to S, O or I (h, outbreak in humans; w, outbreakin wildlife).‡ Samples pooled from several individuals.Most of the studies only diagnose F. tularensis to species, some also to subspecies holarctica. This is not detailed in the table.
Tularemia in Europe in a one-health context 2147
bacteria in their blood, which resulted in the identi-fication of F. tularensis in one little stint (Calidrisminuta) .
Tularemia in domestic animals
Outbreaks of tularemia in domestic animals have beenreported to the OIE by 11 European countries (Fig. 3).Of the eight countries that have stated the affected spe-cies, seven reported tularemia in farmed hares/rabbitsand only one country (Bulgaria) has reported tularemiain other species, specifically in cattle, sheep and dogs.Only a few papers or case reports on tularemia in dom-estic animals were found in the literature, and theseonly presented information on seropositivity for tulare-mia in domestic animals in European countries(Table 3). These studies often resulted from investiga-tions of possible sources of infection causing outbreaksin humans, and presented no information on clinicaldisease in domestic animals.
Tularemia in vectors
Arthropods have long been associated with the trans-mission of Francisella tularensis, with many of theearly studies focusing on deer flies  as potential vec-tors. The range of arthropod vectors connected withtularemia has expanded to include ticks [19, 40, 53–60], mosquitoes [56, 61], horse flies , fleas  andgamasid mites . In Europe, much of the researchon the role of arthropod vectors in the transmission
of F. tularensis subsp. holarctica has focused on ixodidticks (hard ticks) in central Europe and on mosquitoesin northern Europe (e.g. Sweden and Finland)(Table 4). These geographical differences in researcheffort and the role of these two vector species in thetransmission of tularemia are largely based on the re-gional abundance of the two vector groups, with mos-quitoes being in far greater abundance in northernEurope, while ticks are relatively rare . However,there have been surprisingly few studies investigatingthe prevalence of F. tularensis subsp. holarctica in natu-ral populations of vectors in Europe, and they cover arelatively narrow range of countries (Table 4).
There is strong evidence that the tick is a biological vec-tor of F. tularensis. The bacteria has been found in themidgut and salivary glands  of ticks and has alsobeen shown to replicate within the tick . Tickshave been shown to be capable of transmitting the in-fection during a blood meal [63, 64] and mechanicallyvia interrupted feeding . Furthermore, transstadialtransmission of F. tularensis has been demonstratedexperimentally in ticks [64, 65] and field studies haveisolated F. tularensis from a number of adult maleDermacentor reticulatus ticks . Given that adultmale D. reticulates ticks do not take blood meals, thissuggests that transstadial transmission may have oc-curred. Transovarial transmission of F. tularensis hashowever not been shown to occur experimentally in
Presence in wildlifeand domestic animals Presence in vectors
Type of human reportingat national level
Presence in humans
Not reportedReported in wildlifeReported in dometic animals
Not reportedMandatory reportedVoluntary reportedUnknown reported type
Fig. 3. Reported presence of tularemia in different host types across Europe 1992–2012, (a) presence in animals (reportedin OIE database and literature), (b) presence in vectors (reported in literature), (c) presence in humans [reported inEuropean Centre for Disease Prevention and Control (ECDC) and WHO-CISID databases for all countries exceptSwitzerland and Turkey for which literature reports were used], and (d) type of human reporting at national level.
2148 G. Hestvik and others
ticks, although there have been reports of infectedunfed larvae in the field .
The prevalence rates detected in ticks have beenmostly between 0% and 3% (Table 4). However, itshould be noted that some studies estimated prevalencebased on the testing of individual ticks,while others esti-mated prevalence by testing a pooled set of ticks. Thelatter type of studymay not accurately reflect the preva-lence of the bacterium in the tick population. This ishighlighted in two studies which reported unusuallyhigh prevalence rates of 16%  and 8%  whichwere based on pools of 8–13 ticks, and therefore maynot be an accurate estimation of prevalence in the popu-lation of ticks tested. Furthermore, ticks have also beenfound to harbour Francisella-like endosymbionts(FLEs). FLEs are non-pathogenic bacteria which areclosely related to F. tularensis . These FLEs pose achallenge for accurate identification of F. tularensis by
PCR as the 16S rDNA gene sequence is highly con-served between these two species . Thus, studiesthat have not used tools capable of discriminatingbetween F. tularensis and FLEs may overestimate theprevalence of F. tularensis in ticks.
In contrast to ticks, there is less information on therole of mosquitoes as biological vectors of F.tularensis.
Lundström and colleagues isolated F. tularensissubsp. holarctica from adult mosquitoes that were col-lected from the field as larvae and reared in the labora-tory . This suggests that the larvae acquired F.tularensis in the field, and maintained the bacteriauntil they were tested as adults. Furthermore, mos-quito larvae predate on protozoa that have been
Table 4. Studies estimating prevalence of F. tularensis in European arthropods 1992–2012
Arthropod species Country MethodPrevalence*(%/no. investigated) References
TickDermacentor reticularis Austria Culture 2·3 /1098 Ixodes ricinus France PCR† 1·0/551 
Germany PCR 1·4/636 Germany PCR 0/1000 Hungary PCR† 0/1800 Luxembourg PCR 0/1394 Serbia PCR 3·8/287 Switzerland PCR† 1·2/6071 
Dermacentor reticularis, Ixodes ricinus, Haemaphysalisconcinna
Culture† 16·5/1987 
Slovakia IWM 0·8/4542 Ixodes ricinus, Haemaphysalis concinna Hungary PCR 0·1/2014 Dermacentor marginatus, Dermacentor reticulate; Ixodesricinus, Hyalammoa marginatu, Rhipicephephalus sanguineus
Portugal PCR 0·9/110 
Dermacentor marginatus, Ixodes ricinus, Hyalammoamarginatum, Rhipicephephalus sanguineus, Rhipicephephaluspusillus, Rhipicephephalus bursa, Hyalomma lusitanicum,Haemaphysalis hispanica
Spain PCR 0·5/1480 
MosquitoAedes spp. Czech
RepublicPCR† 0/8449 
Aedes communis, Aedes intrudens, Aedes punctor, Aedescinereus, Aedes sticticus, Aedes vexans, Culiseta alaskaenis,Culiseta annulata, Culex pipens, Culex torrentium
Sweden PCR† 8·3/334 
FleaCtenophtalmus assimilis, Nosopsyllus fasiatus Hungary PCR 0/25 
IWM, Inoculation on white mice; PCR, polymerase chain reaction.* All studies are of active surveillance type.† Samples pooled from several individuals.Most of the studies only diagnose F. tularensis to species, some also to subspecies holarctica. This is not detailed in the table.
Tularemia in Europe in a one-health context 2149
shown to harbour F. tularensis subsp. holarctica ,suggesting that it is possible that mosquitoes may ac-quire the infection as larvae. Transstadial trans-mission was also confirmed in experimental studiesby Thelaus et al. , where 25% of the adults ofAedes aegypti infected with F. tularensis subsp. holarc-tica as larvae maintained the bacteria. When unin-fected mosquitoes fed on infected mice they wereable to acquire the bacteria. In contrast, laboratoryexperiments by Triebenbach et al. , infecting larvaefrom A. aegypti and Anopheles gambiae with F. tular-ensis subsp. novicida concluded that there was no evi-dence of transstadial transmission, that mosquitoeswere unable to acquire the bacteria during a bloodmeal and that infected mosquitoes were not able totransmit the bacteria during a blood meal. Neitherthe study by Thelaus et al. or Triebenbach et al.could show transmission of the bacteria from infectedmosquitoes to uninfected mammal hosts. They alsoagree that there is no evidence for active replicationof the bacterium within the mosquitoes [68, 69].
There have been relatively few studies investigatingthe prevalence of F. tularensis in mosquitoes (Table 4).However, when prevalence of F. tularensis was esti-mated in mosquitoes, it was reported to be greaterthan that observed in ticks (a prevalence of 8–21%),although it should be noted that this was only basedon one study .
Linking humans, wildlife and arthropod vectors
Susceptibility to disease in different animal species
Sensitivity to infection, and thus the severity of theresulting disease, differs between animal species. Inthe WHO Guidelines on tularemia , a host’spotential sensitivity to F. tularensis spp. (includingsubspecies tularensis and holarctica) infection isclassified as ‘very sensitive’, ‘medium sensitive’ or‘resistant’, based on data from experimental infec-tions. According to the WHO Guidelines , severalsmall rodent species are considered very sensitive tothe pathogen, which can be exemplified by the exper-imental infection conducted on common hamster(Cricetus cricetus) leading to septicaemic disease anddeath . Experimentally infected lagomorphs,have been considered to be very sensitive and developacute disease . In Europe this has been shown inexperimentally and naturally infected hares withF. tularensis subsp. holarctica [33, 39, 71]. However,studies of naturally infected European brown hares(L. europaeus) have also described chronic disease
, which suggests that the disease in hares doesnot always have an acute and fatal presentation.Antibodies against tularemia have been detected inEurasian beavers (Castor fiber) , but unfortunatelythere are no studies published on the disease in nat-urally infected Eurasian beavers in Europe. Olsufjev(Olsufjev, 1970, cited in ) suggests that some resist-ant genera are Canis, Felis, Meles, Nyctereutes, Vulpesand Mustela. There are, however, occasional descrip-tions of naturally infected and diseased domesticcats and dogs . Birds have been experimentallyinfected, and most species seem to be able to becomeinfected, but are relatively resistant to developingclinical disease . Humans are considered to behighly susceptible to infection, resulting in a diseasethat ranges from mild to severe [2, 26]. It is importantto take into account the differences in sensitivitybetween species in order to understand the epidemio-logical role of the various possible host species, and todesign appropriate surveillance strategies. However,the information available on the susceptibility ofEuropean hosts to F. tularensis subsp. holarctica islimited and there are few descriptions of experimentalinfections and natural disease. A large part the infor-mation available (see above) is outdated and oftencontains no information on the subspecies of F. tular-ensis involved.
Geographical distribution of tularemia in Europe1992–2012
Tularemia is widely distributed in humans, wildlifeand arthropod vectors in Europe. Figure 3 maps thedistribution of F. tularensis subsp. holarctica acrossEurope; it clearly has a broad distribution, withareas of reported presence spanning from the north-ernmost European countries to the southernmostones, and from the east across to the west. It can beclearly seen from Figure 3 that tularemia is presentin both humans and wildlife in the majority ofEuropean countries. However, while human caseshave been reported from Estonia, Latvia, Lithuania,Slovenia, Croatia, Bosnia & Herzegovina, Serbia,Turkey and Romania, there have been no reportedcases in wildlife. In Belgium there have been reportsof tularemia in wildlife, but not in humans, either inthe databases or the literature. In Denmark, the onlywildlife case reported was in a European brownhare, and there was a single human case described inthe literature . Tularemia cases in domestic ani-mals have been reported from Bulgaria, the Czech
2150 G. Hestvik and others
Republic, Germany, Finland, France, Italy, Norway,Poland, Slovakia, Spain, Sweden and Switzerland.Many of these were in domestic lagomorphs. F. tular-ensis subsp. holarctica was only reported in arthropodvectors in some of the countries where outbreaks inother hosts were also observed. One possible expla-nation for these differences is that the only source ofinformation on F. tularensis in vectors is publishedarticles, while international databases have further in-formation on the pathogen in humans, wildlife anddomestic animals. It is difficult to compare the preva-lence of tularemia, both between species (humans,wildlife, domestic animals and vectors) and betweencountries. There have been differences in the levelsof surveillance between countries, and in the level ofreporting engaged in by each country, ranging fromno reporting to compulsory reporting, which affectsthe number of cases in the databases. Notification ofanimal cases is mandatory at the international level.The purpose of official notification of tularemiacases is to enable the veterinary authorities of atularemia-free country to prohibit importation or tran-sit of live hares through their territory from countriesconsidered infected with tularemia. As a result, somecountries are reluctant to report cases, as they mightsuffer economic disadvantages. A further explanationfor differences in levels of tularemia in the differentspecies may be attributable to the different methodsof surveillance utilized: Human cases are recordedthrough passive surveillance systems, while wildlifeand domestic animal reports result from both passiveand active, and vectors from active surveillance.
Seasonality of diseaseThe majority of human cases reported between 2007and 2012 in EU countries, Norway and Iceland oc-curred from July to November, with a peak typicallyoccurring in August or September. However, somecases were also reported in February and March.
Tularemia thus appears to have a clear seasonalpattern in humans in the EU, with most cases occur-ring in the summer and early autumn months(Fig. 4, see also trend graph in Fig. 2). However,when examined at the individual country level, thepattern of seasonality appeared highly variable. InItaly, cases were more often reported in spring;in Hungary, in summer; in Sweden, Finland andSpain, in summer/autumn; in Norway, in autumn/winter; in Germany, in winter; and in Slovakia,France and the Czech Republic, all year round witha slight peak in winter (see Table 2). Data was eithermissing or insufficient to assess seasonality in theother countries. According to the literature reviewed,outbreaks were reported: in spring/summer/autumnin Bulgaria, Finland, Slovakia and Sweden; inautumn/winter/spring in Kosovo and Turkey; and inall seasons in France, Germany, Norway and Spain(see Table 2). Thus, in some countries, outbreaks oc-curred in periods during which the Europe-wide aver-age reported incidence was low.
In wildlife, previous studies have reported seasonalpatterns of outbreaks in Northern Europe, whichresearchers have associated with the peak of the mos-quito season . From 1992 to 2012, a study wasconducted on Norwegian mountain hares, which
Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.
Fig. 4. Number of cases of tularemia by month recorded in the European Centre for Disease Prevention and Control(ECDC) database (n= 5715), 2007 to 2012. Data for this figure has been reported by Austria, Belgium, Cyprus, CzechRepublic, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg,Malta, Norway, Poland, Romania, Slovakia, Slovenia, Spain, Sweden and the UK.
Tularemia in Europe in a one-health context 2151
were found dead between May and November, andhad died of tularemia [36–38]. These hares had septi-caemic (acute) disease; as this form of tularemiaresults in death within a few days, it is likely theywere infected shortly before death. Since the mosquitoseason spans from about June to September inScandinavia, the timing for these infections wouldbe consistent with the hypothesis that the mosquitoesplayed an important role in spreading the infection.Information from studies on European brown haresin Hungary, by contrast, have not been as informativewith respect to the time of infection and identificationof possible vectors. In Hungary, hares have beenexamined primarily during hunting season; in thoseinfected with tularemia, the lesions were consistentwith subacute to chronic disease, making it difficultto establish at what time point the hares contractedthe infection . Efforts have been made to detectF. tularensis in small rodents in endemic areas inBulgaria, at the Austrian-Slovakian border, and inHungary and Sweden (Table 3). Only two studies,both on the Austrian–Slovakian border, presentedresults by season [73, 74]. The combined resultsshowed a bacterial detection rate of 0% in January–June, 1·2–2·6% in June/July–September and3·9–8·3% in October–December. It is not possible todetermine if these results reflect a true seasonality,since the studies were small, and limited in bothtime and geographical location.
Reports of tularemia in domestic animals have beenalmost exclusively about farmed hares/rabbits [22, 23]and give no information on seasonality.
Human disease and risk factors
The most frequent clinical presentations of tularemiain humans in Europe, routes of infection, type of ex-posure, and risk factors are all summarized inTable 1. Oropharyngeal outbreaks were more frequentin autumn/winter/spring in most of the countries,associated with exposure to contaminated food orwater, while ulceroglandular forms occurred mostlyin summer or autumn, associated with exposure toinfected vectors or hares (see also Table 2). Themost frequently reported information on wildlife,associated with all forms of human outbreaks, wasobservations of increased numbers of rodents ordead hares, or contact with these (Table 2). Infectioncan indeed occur through direct contact, for examplewhen hunters skin infected hares. Two outbreak inves-tigations in Spain in 1998–1999 and 2007 mentioned
handling of red crayfish as a risk factor for infection[75, 76]. From the literature reviewed in the currentstudy, the most frequently mentioned risks for humansin Europe to contract tularemia were from: consump-tion of contaminated water or food (mentioned 20times), direct (skin) contact with infected animals –frequently wild ones (mentioned 19 times), vectorbites from infected arthropods (mentioned 13 times),and exposure to infected environments via air (men-tioned 10 times). Males in general and rural popula-tions were found to be more often occupationallyexposed (mentioned 11 times), urban populations tobe affected through recreational outdoor activities,like hunting and fishing (mentioned 10 times).Contact with domestic animals has on occasion beenimplicated as a risk factor in connection with tular-emia outbreaks in humans [18, 76–79]. The few papersor case reports on seropositivity to F. tularensis indomestic animals in European countries (seeTable 3) often resulted from investigations of potentialrisk factors for human outbreaks. Apart from thosestudies, evidence of the link between tularemia indomestic animals and humans in Europe was oftencircumstantial, and traced to reports of humans inwhom infection was detected after they were scratchedor bitten by cats [80, 81] or had been fleecing sheep. The information regarding tularemia in zoo ani-mals is scarce and describes mostly outbreaks or singlecases in non-human primates. However, recentlyKuehn et al.  performed serological investigationsof 1122 German zoo animals representing 46 differentspecies, most of them ungulates. Three (0·3%) testedpositive for antibodies against tularemia. Contact be-tween wild animals, e.g. small rodents, and zoo ani-mals located in the same geographic area mightenable transmission of tularemia between these animalgroups. This study showed low seroprevalence andtherefore the risk for humans to contract infectionfrom zoo animals was likely low.
Sources of infection and routes of dissemination
Wild animals and arthropods appear to be importantsources of infection with F. tularensis for humans andother animals. In theory, species that are moderatelysusceptible to tularemia, and maintain the infectionfor a prolonged time, may serve as reservoirs. Inother hosts, bacterial amplification can occur, andthey may serve as a source of infection to others. It has been shown that sensitivity to F. tularensisinfection, and susceptibility to develop disease, varies
2152 G. Hestvik and others
between animal species. The sources of infection foranimals have not been well investigated, but it isreasonable to hypothesize that they could be infectedby routes similar to those for humans, i.e. through di-rect contact with secretions from infected animals,from ingestion of contaminated food or carcasses,via vectors, or from inhalation of the bacteria. Onlya few studies have investigated the distribution oflesions in wildlife to determine in which organs lesionswere present. Similarly, few studies have investigatedpossible routes of infection and shedding of F. tularen-sis in wildlife. In Hungary, hunted European brownhares with tularemia had subacute to chronic lesionin the lungs, pericardium, and kidneys . Becauselung involvement was a common finding, the authorspostulated a respiratory route of infection. In terms oftransmission of the disease, hares with chronic diseasein the kidney could, in theory, shed bacteria in urineand thereby serve as a source of infection. This hasnot been shown in hares, but in experimental infectionof meadow vole (Microtus pennsylvanicus) the animalsdeveloped bacteriuria . Olsufjev  reported thatthe organs and blood of small rodents contained nu-merous bacteria, which could pose a risk for trans-mission to other animals either through shedding bythe rodents during the acute phase of the disease, oras contaminated carcasses once the animals died.Most of the descriptions of transmission from smallrodents to humans resulted from contact with deadinfected animals, for example as reported throughskinning of hamsters . There is very little infor-mation about their importance as live shedders or asreservoir hosts. Naturally infected small rodentshave only been investigated for the presence of theagent, but not for lesions [31, 40, 41, 50, 73, 86, 87].The fact that these small rodents were alive while har-boring the bacterium might reflect that they acted ashealthy carriers, had chronic disease, or had been re-cently infected and had not yet developed disease.Small rodents have also been implicated in the spreadof tularemia through the contamination of food andwater. In Norway, outbreaks of tularemia in humanshave been attributed to drinking water from water-wells contaminated by dead lemmings or their excreta. Similarly, in Kosovo an outbreak of oropharyn-geal tularemia in humans was attributed to contami-nation of food and water with rodent faeces .From the studies conducted to date, it is not clear ifsmall rodent species act as amplifiers of tularemia in-fection or might be reservoirs, or if their role differsin different areas, given that multiple factors are
involved in the eco-epidemiology of tularemia. Redfox, wild boar and raccoon dog may contract infection(Table 3, ) when eating infected prey such as haresand small rodents and develop antibodies. Howeverthese species are not considered to be particularly sus-ceptible to developing disease. Their potential role asasymptomatic carriers, shedding and/or spreadingthe infection in nature, has not been investigated[1, 7, 9, 89, 90].
Eco-epidemiology of tularemia
The epidemiology of F. tularensis subsp. holarctica in-fection is complex and varies with ecosystem and geo-graphical region. There are considerable gaps in theknowledge of the role of arthropod vectors, wildlife,and the environment in maintenance of the infectionin natural habitats. Both a terrestrial and an aquaticecological cycle of tularemia have been described,each involving their respective populations of mammaland arthropod vector hosts and their environments. DNA from F. tularensis has also been found inwater and sediment, suggesting that the bacteria persistsilently in the environment . Highly endemic areasor natural foci of tularemia have been known for sev-eral decades [1, 73]. In these areas, cases often occurevery year, and in higher numbers, than in less heavilyaffected endemic areas . For example, areas of highendemicity have been identified in both Germany andSweden [47, 91]. In Sweden, the theory of nidality/focality has been applied to the outbreaks of tularemia,where molecular typing of F. tularensis subsp. holarc-tica in human cases revealed highly localized sub-populations of F. tularensis of identical genotypesdetected over multiple years . This suggests thatthe human cases of tularemia may have originatedfrom distinct local sources of infection, and that over-wintering of the disease may have occurred at thesesites. In laboratory experiments F. tularensis subsp.holarctica has been shown to survive in water for sev-eral months , and given the strong association ofF. tularensis subsp. holarctica with water, it is possiblethatmosquitoes play a role in transmission of tularemiafrom a water reservoir to humans. Although there is noexperimental data to support this assertion, the timingof human outbreaks tend to be localized to summer andautumn, when mosquito numbers are at their greatest,and this suggests theremay be a connection. Rydén andcolleagues  recently reported a significant corre-lation between mosquito abundance and human casesof tularemia. At present, transmission of F. tularensis
Tularemia in Europe in a one-health context 2153
from infected mosquitos has not been proven sinceit has not been shown experimentally [68, 69].However, Thelaus et al.  suggest that the trans-mission from infected mosquitoes occurs at a verylow frequency and therefore could not be detected intheir experiment.
It has often been reported that ticks play a signifi-cant role in maintaining F. tularensis in nature andmay be a reservoir species for tularemia [3, 65, 93].There are aspects of tick biology which provide sup-port for this possibility. Ticks are capable of transsta-dial transmission. Furthermore, the life-cycle of mostixodid ticks is completed over 2–3 years and they gothrough three life stages, each requiring a bloodmeal (larva, nymph, adult) . Studies on the long-term prevalence rates of F. tularensis in ticks providesfurther support for the theory that ticks may be areservoir species for tularemia. In the USA, therehas been evidence that F. tularensis subsp. tularensispersisted in tick populations at prevalence rates of2–4% for 4 years in Martha’s Vineyard .Similarly, in Europe, Gurycová et al.  found thatF. tularensis subsp. holarctica persisted at prevalencerates of 0·2–1% in ticks in western Slovakia for 6years in an area in which tularemia was endemic. Ithas been proposed that the long-term existence of F.tularensis in ticks is consistent with the theory of natu-ral nidality/focality where infections are maintained innatural nodi or foci, and spillover to other hosts/envir-onments occurs during periods of disruption, whichmay cause amplification [92, 94, 95]. However, thepoint at which these spillover events happen is un-known. There is conflicting evidence for the theorythat ticks maintain the tularemia infection in nature,as a number of studies have found that infectionwith F. tularensis negatively affects the survival ofticks [64, 94], which would impact the ticks’ abilityto maintain the infection in the population.
Surveillance of wildlife and humans
Surveillance data, based on an official declaration ofcases in animals or in humans, can be used to mapthe presence of tularemia over time. This informationcan then be used to estimate the risk of exposure forvarious populations in different locations. However,because the current level of awareness of tularemiais poor, it is quite possible that the risk of exposurefor humans or domestic animals has not beenadequately assessed. In recent years, tularemia hasbeen found in areas not previously known to be
affected; for example, in a hare in Thuringia inGermany 2006  and in humans in Kosovo during1999–2000 . Re-emergence has occurred inhumans in several regions of Turkey after decadesduring which there had been no outbreaks of the dis-ease [25, 96].There have been outbreaks of tularemiain humans in areas where the disease had previouslybeen rare, both in Sweden in 2000  and inGermany in 2004 . In some locations, there wasevidence that the peaks of infection coincided inhumans and in wild animals (e.g. hares) [4, 34]. Thispattern was not, however, consistently seen duringthe study period from 1992–2012. This may be be-cause this pattern of concurrent human and wildlifeoutbreaks only occurs in particular geographical loca-tions, or it could be because detection of such patternsrequires studies specifically aimed at simultaneouslyinvestigating tularemia in humans and wildlife. Thislatter type of study has not often been undertaken.Recent studies which used a more comprehensiveapproach to surveillance identified F. tularensis orantibodies to F. tularensis in species which had notpreviously been well-investigated, such as raccoondog, wild boar and red fox [7, 9, 10]. Thus, tularemiamay be more widespread in the environment than hasbeen generally recognized, which may be resulting inemerging and re-emerging disease in humans whoare exposed to contaminated areas of the environmentthrough high risk activities. Red foxes, wild boar andraccoon dogs are all wildlife species that can contracttularemia infection (usually by preying on otherinfected wildlife), but rarely develop disease. Thesespecies might therefore serve as useful indicators forthe presence of tularemia in areas where small mam-mals are infected. Furthermore, they can be used forserological surveillance of trends in tularemia andfor the detection of spread of the pathogen to newgeographical areas [7, 9, 10, 89, 90]. Serology inhares has been used for wildlife surveillance of tular-emia in Hungary , where results from regular test-ing of hunted hares for almost three decades hasidentified variation in prevalence of the pathogenover time in different parts of the country. Becausehares often succumb to acute disease, early detectionof outbreaks in this species can provide an early warn-ing for regional health authorities. These authoritiescould then help people reduce their occupational orrecreational exposure, by providing advice regardingprecautions which might be taken to avoid sourcesof infection such as avoiding contact with haresand bites by arthropod vectors. This strategy was
2154 G. Hestvik and others
employed during an outbreak of tularemia in hares inFrance in 2011; health officials enhanced local surveil-lance efforts, and warnings were issued to local peopleat risk (e.g. hunters and people walking in the forest)and to physicians to aid in prevention and early detec-tion of disease . In Lower Saxony in Germany, sys-tematic testing of hares which were hunted or founddead, revealed highly endemic areas of tularemiawhich were previously unknown . Small rodentspecies are also useful for active surveillance, as sev-eral studies [50, 73, 74, 86] have demonstrated thatthey are potential carriers of the bacterium and maybe reservoirs of infection .
The results of this review and conclusions drawn mustbe interpreted with caution. The results in humans andanimals are based on different types of reporting withvariable level of detail and differ between countries, insome reporting of tularemia is compulsory and inothers it is not. Moreover, the published articles areof different types, ranging from large surveillance stu-dies to descriptions of single cases. Therefore, this re-view study has limitations and may not necessarilyreflect the true situation of tularemia in Europe.
An increase in the breadth and depth of studies ontularemia, in combination with the development ofnew and better diagnostic methods, have increasedthe knowledge and detection of tularemia in humans,wildlife and arthropod vectors over the past 20 years,although many aspects of this disease still remainpoorly understood. There is a lack of knowledge andunderstanding of the epidemiological role of animalhosts, mechanisms of maintenance in the differentecosystems, and routes of transmission.
Reporting of tularemia has increased in manyEuropean countries over the past 20 years. This maybe attributable to the increase in the mandatoryreporting of this disease but also to better awarenessand diagnostic possibilities. Over the past 5 years,there has been no overall trend of infection in theEU, Norway and Iceland when taken as a whole,but there is considerable variation at the individualcountry level.
Tularemia is widely distributed throughout most ofEurope and has repeatedly shown signs of localemergence and re-emergence in humans and wildlife.The disease shows clear seasonality in humans butthis pattern has not been definitively demonstratedin wildlife.
Different geographical areas have different eco-systems, which influence the epidemiology andpresentation of the disease. Such factors include tem-perature and humidity, the presence of differentkinds of arthropod vectors, and the variety of smallrodent species and other wildlife species present.This makes it difficult to make direct comparisonsacross all of Europe.
Increased active surveillance of tularemia, simul-taneously investigating humans, wildlife, domestic ani-mals and vectors is recommended. This will contributeto the understanding of the role of the different animalspecies and vectors in the development, transmission,and maintenance of the disease. Development of sim-ple, field-based diagnostic methods, applicable to vari-ous animal species, would facilitate field investigations.
Better communication among veterinarians, re-searchers, physicians and public health authoritiesin a ‘one-health’ approach is crucial. Recognition ofoutbreaks in animals may provide an early warningto enable implementation of preventive measures toreduce human exposure, and for early recognitionand prompt treatment of outbreaks in humans.Conversely, humans may act as sentinels of tularemiain a given area.
For supplementary material accompanying this papervisit http://dx.doi.org/10.1017/S0950268814002398.
This work should be attributed to: the Department ofPathology and Wildlife Diseases, National VeterinaryInstitute, Uppsala, Sweden; Department of BiomedicalSciences and Veterinary Public Health, SwedishUniversity of Agricultural Sciences, Uppsala, Sweden.
The study was supported by theWildtech project (EU7thFrameworkProgramforResearchandTechnologicalDevelopment, grant agreement no. 222633).
DECLARATION OF INTEREST
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