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RESEARCHOpenAccessIxodesricinusticksremovedfromhumansinNorthernEurope:seasonalpatternofinfestation,attachmentsitesanddurationoffeedingPeterWilhelmsson1*,PontusLindblom1,LindaFryland2,DagNyman3,ThomasGTJaenson4,PiaForsberg2,5andPer-EricLindgren1,6AbstractBackground:ThecommontickIxodesricinusisthemainvectorinEuropeofthetick-borneencephalitisvirusandofseveralspeciesoftheBorreliaburgdorferisensulatocomplex,whicharetheetiologicalagentsofLymeborreliosis.
TherisktocontractbitesofI.
ricinusisdependentonmanyfactorsincludingthebehaviourofbothticksandpeople.
Thetick'ssiteofattachmentonthehumanbodyandthedurationoftickattachmentmaybeofclinicalimportance.
DataonI.
ricinusticks,whichwerefoundattachedtotheskinofpeople,wereanalysedregardingpotentiallystage-specificdifferencesinlocationofattachmentsites,durationoftickattachment(=feedingduration),seasonalandgeographicaldistributionoftickinfestationinrelationtoageandgenderofthetick-infestedhosts.
Methods:During2008–2009,1770tick-bittenpersonsfromSwedenandthelandIslandsremoved2110I.
ricinusticks.
Participantsprovidedinformationaboutthedateoftickdetectionandlocationontheirbodyofeachattachedtick.
Tickswereidentifiedtospeciesanddevelopmentalstage.
Thefeedingdurationofeachnymphandadultfemaletickwasmicroscopicallyestimatedbasedonthescutalandthecoxalindex.
Results:In2008,participantsweretick-bittenfrommid-Maytomid-Octoberandin2009fromearlyApriltoearlyNovember.
Theinfestationpatternofthenymphswasbimodalwhereasthatoftheadultfemaletickswasunimodalwithapeakinlatesummer.
Tickattachmentsiteonthehumanbodywasassociatedwithstageofthetickandgenderofthehumanhost.
Siteofattachmentseemedtoinfluencethedurationoftickfeeding.
Overall,63%ofnymphsandadultfemaleticksweredetectedandremovedmorethan24hoursafterattachment.
Olderpersons,comparedtoyoungerones,andmen,comparedtowomen,removed"their"ticksafteralongerperiodoftickattachment.
Conclusions:Theinfestationbehaviourofthedifferenttickstagesconcerningwhereonthehost'sbodytheticksgenerallywillattachandwhensuchticksgenerallywillbedetectedandremovedinrelationtohostageandgender,shouldbeofvalueforthedevelopmentofprophylacticmethodsagainsttickinfestationandtoproviderelevantadvicetopeopleonhowtoavoidorreducetheriskoftickinfestation.
Keywords:Ixodesricinus,Tickinfestation,Tickbite,Attachmentsite,Feedingbehaviour,Feedingduration,Host-seekingbehaviour,Seasonalactivity,Sweden,land*Correspondence:peter.
wilhelmsson@liu.
se1DivisionofMedicalMicrobiology,DepartmentofClinicalandExperimentalMedicine,LinkpingUniversity,Linkping,SwedenFulllistofauthorinformationisavailableattheendofthearticle2013Wilhelmssonetal.
;licenseeBioMedCentralLtd.
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Wilhelmssonetal.
Parasites&Vectors2013,6:362http://www.
parasitesandvectors.
com/content/6/1/362BackgroundTheEuropeantickIxodesricinusisavectorofseveralpathogensofhumansanddomesticatedanimals.
ThemedicallymostimportantofthesepathogensaretheTBEvirus(TBEV)causingtick-borneencephalitis(TBE)andtheBorreliaburgdorferisensulato(s.
l.
)spirochaetescausingLymedisease(=Lymeborreliosis,LB).
I.
ricinusisathree-hostticksotocompleteitslifecycle,whichconsistsofthreeactivestages(larva,nymphandadult),itmustingestbloodfromavertebratehostineachoneofthethreestages.
Humansare,therefore,potentialhostsforeachoneofthesestages.
ThenorthernlimitofI.
ricinusisdeterminedbyclimatefactorsandaccesstosuitablehosts[1,2].
InSweden,thegeographicaldistributionofI.
ricinuscoversthesouthernandcentralpartsofthecountryaswellasthecoastalareaofnorthernSweden.
Jaensonandco-workerssug-gestedthatawarmerclimatewithmilderwintersandaprolongedvegetationperiodhavepermittedimport-antI.
ricinusmaintenancehosts,particularlyroedeer(Capreoluscapreolus),tospreadtoandinhabitpreviouslyclimaticallysuboptimalareasinthenorthernpartsofSweden;thishasresultedinagradualspreadnorth-wardsofI.
ricinusinfestingdeer;inthismannertherangeandabundanceofI.
ricinusinnorthernSwedenincreasedconsiderablyduringthelast30years[2].
Fur-thermore,theannualincidenceofcasesofneuroinvasive,humanTBEincreasedsignificantlyinSwedenduringtheyears2000–2012.
Thiswaspresumablypartlyaconse-quenceofthehightickabundance[3].
Onmostislandsofthelandarchipelago(thelandIslands),locatedintheBalticSeabetweenSwedenandthemainlandofFinland,I.
ricinusisabundantandtickbitesarecommonlyre-portedbytheinhabitants[4].
Theseasonalhost-seekingactivitypatternofI.
ricinusisquitevariableandnotyetfullyunderstood.
How-ever,itisinfluencedbyseveralbioticandabioticfac-torsincludingvegetationtype,densityandvarietyofhosts,weatherandclimate,andthephotoperiod(whichisdependentonlatitude)[5,6].
Intwoinves-tigationsconductedinsouth-centralSweden,nymphsandlarvaeofI.
ricinususuallyexhibitedbimodalhost-seekingactivitypatternswiththehighestactivityinMay-JuneandAugust-September,andamidsummeractivitydepression[7,8].
Itisproposedthatthemid-summerdepressioninhost-seekingactivityofsubadultticksmaypartlybeduetotherelativelydryconditionsthatusuallyprevailatthistime[7].
Duringsuchare-ductioninhost-seekingactivity,onewouldexpectalowertickinfestationonanimalsandhumans.
Incon-trasttonymphs,adultticksexhibitedaunimodalhost-seekingpatternwithoutanymidsummerdepression[7].
Fromamedicalpointofviewitiscertainlyrelevanttostudythetickinfestationpatternonhumans.
Thisriskoftickinfestationisdependentonthebehaviourofbothticksandhumans,whichareinfluencedbyweatherconditions,climateandotherfactors.
Berglundandco-workerscarriedoutanextensiveepidemiologicalstudyofLBinsouthernmostSweden[9].
Theyrecordedasignificantlygreaterproportion(20%)ofneurologicalmanifestationsamongLBpatientswhohadbeentick-bittenontheheadorneck,thanamongLBpatientsbittenonotherpartsoftheirbody(7%).
There-fore,thetick's"preferred"siteforattachmentonthehu-manbodymaybeofclinicalimportance.
Attachmentsites"preferred"byI.
scapularisticks,themainvectorofLBspirochaetesinnorth-easternUnitedStates,are,toacertainextent,dependentonthedevelopmentalstageofthetick:adultfemalesofI.
scapularisattachmorefre-quentlytotheheadandneckarea,thantootherpartsofthehumanbody[10].
Suchabitingbehaviourhas,toourknowledge,notbeenreportedforI.
ricinusadultfemalesthatbitehumans.
However,"preferred"attachmentsitesonsheepareinfluencedbythedevelopmentalstageofI.
ricinus:larvaeattachmainlytothelowerpartsofthebodyandadultfemalesmainlytotheupperparts,whilenymphswillattachmainlytositesinbetweenthoseoflar-vaeandadults[11].
Berglundandco-workersalsorecordedtickbitesmoreoftenontheheadandneckareaofchildrenwithLBcomparedtothesamebodyregionofadultswithLB[9].
Thiscouldsuggestfundamentaldifferencesinhowticksrespondtohostsofdifferentsizesand/orages.
ItmayalsosuggestthattransmissionoftheBorreliaspirochaetesismoreefficientwhenthetickisattachedtoaparticularbodyregion.
Thedurationoftickfeeding,includingsalivationandbloodingestion,isimportantfortransmissionofpatho-gens.
ThevirionsofTBEV-infectedtickswillbegintobetransmittedwiththetick'ssalivatothevertebratehostalmostinstantaneouslyaftertickattachment(24hoursandhumanageclasses.
Participantswerecategorizedintooneoffollowingageclasses(years):(19–29),(30–39),(40–49),(50–59),(60–69),(70–79),and(>80).
StatisticalanalyseswereperformedandgraphsweredrawnusingGraphPadPrismversion5.
00forWindows(GraphPadSoftware,SanDiego,CA).
P-values≤0.
05wereconsideredstatisticallysignificant.
ResultsDescriptionofthetick-bittenparticipantsand"their"removedticksBetweenMay2008andNovember2009,1896participantsattendedtheirfirstvisittoaPHC.
Amongthem,6.
6%(n=126)wereexcludedfromfurtheranalysisduetoin-completeanswersinthequestionnaires.
Theremaining1770participants(648menand1122women)removedandhandedinatotalof2110attachedI.
ricinusticks(487adultfemaleticks[23%],15adultmaleticks[1%],1519nymphs[72%],and89larvae[4%]).
Noothertickspecieswerede-tected.
Theproportionofadultfemaletickswassignificantlygreateronmen(27%)thanonwomen(21%,P24hours(Table2).
Whenattachmenttimewasanalysedinrelationtoattachmentsite,asignificantlygreaterproportion(77%)ofadultfemaleticksattachedtotheskinofthegroin/genitalareawasre-moved>24hoursoffeeding,comparedtotheproportionofadultfemaleticksthatwasremoved>24hoursfromthearms(53%,P24hourswasanalysedinrelationtoparticipant'sgender,nosignificantdifferencewasdetected(P=0.
08)(Table3).
Amongnymphs(n=1300),37%hadbeenattached24hours(Table2).
Thehead/neckareaappearedtohavethegreatestproportionofnymphs(71%)attached>24hours,whilethearmsseemedtohavethelowestproportionofnymphs(54%)attached>24hours.
However,whennymphaldataonattachmenttimewasstatisticallyanalysedinrelationtositeofattachment,nosignificantdifferenceswererevealed.
Whenattachmenttime,i.
e.
nymphsremoved24hourswasanalysedinrelationtopartici-pant'sgender,womenremovedasignificantlygreaterproportionofnymphs(40%)within24hourscomparedtothatofmen(32%,P24hourswasanalysedinrelationtotheageclassesofparticipants,asignificant,positivecorrelationwasfound(r=0.
89,P24hoursincreasedwithincreasingageclassofparticipants;theageFigure3MonthlydistributionofdetectionofattachedI.
ricinuswithrespecttostageofdevelopment.
A.
during2008(n=959),andB.
during2009(n=1097).
Figure4Anatomicaldistributionof1881removedticks.
Percentagesrefertototalnumberofeachtickstage:adultfemales(n=459),adultmales(n=13)nymphs(n=1357),andlarvae(n=52).
Wilhelmssonetal.
Parasites&Vectors2013,6:362Page6of11http://www.
parasitesandvectors.
com/content/6/1/362class30–39yearsaccountedforthesmallestpercentage(46%)ofnymphsattached>24hours,whilepartici-pants>80yearsaccountedforthelargestpercentage(75%)ofticksattached>24hours(Table4).
Whentheproportionofadultfemaleticksattachedfor>24hourswasanalysedinrelationtotheageclassesofparticipants,nosignificantcorrelationwasfound(r=0.
36,P=0.
44).
Calculatedtimeoftickfeedinginrelationtotheparticipants'self-estimatedtimeoftickattachmentCalculatedtimesinhours(h)oftickfeeding,basedonthescutalandcoxalindices(intervals0–24h,24–48h,48–72h,and72–96h),togetherwiththeparticipants'self-estimatedtimesoftickattachmentwereavailablefor748ticks.
Forthecalculatedinterval0–24hoftickfeeding,participants'self-estimatedthetimeoftickattachmentto0–91h(median15h,interquartilerange[IQR]7–23h);forthecalculatedinterval24–48htheself-estimatedtimesrangedfrom0to92h(median20h,IQR9–33h);fortheinterval48–72hto0–91h(median20h,IQR10–40h);andforthecalculatedinterval72–96htheself-estimatedtimesoftickattachmentrangedfrom0to98h(median29h,IQR19–50h).
Figure5Anatomicaldistributionofticksreportedbytick-bittenparticipants.
Percentagesarebasedontotalnumberofticks(n=597)foundattachedtomen(leftside)andtotalnumberofticks(n=1051)foundattachedtowomen(rightside).
Table2AttachmentdurationsofI.
ricinusticksremovedfromdifferentanatomicalsitesofhumanhostsLocationoftickbitesonpeopleAdultfemaleticksNymphsNo.
%>24hNo.
%>24hLeg1515970164Torso/dorsum1406425465Arm515325154Groin/genital39776367Head/neck29693171Total41037130037Table3Attachmentduration(hours)ofI.
ricinusticksfeedingonmenorwomenIntervalsofattachmentinhoursNo.
(%)ofadultfemalesremovedfromNo.
(%)ofnymphsremovedfrommenwomenmenwomen0-2455(33)99(41)143(32)343(40)24-4868(40)99(41)204(46)366(43)48-7219(11)26(11)74(17)117(13)72-9618(11)10(4)20(4)29(3)>969(5)7(3)2(1)2(1)Total169(100)241(100)443(100)857(100)Table4Attachmentduration(>24hours)ofadultfemaleticksandnymphsofI.
ricinuswithrespecttoageclassesofthehumanhostsAgeclass(years)Totalno.
(%)ofparticipantsAdultfemalesaNymphsaNo.
%>24hNo.
%>24h19-2931(2)757205530-39100(6)1776764640-49192(11)56611275450-59398(22)93603035860-69621(35)135614696470-79351(20)775825372>8077(4)25845275Total1770(100)41062130063aRemovedadultfemaleticksandnymphswherecalculationoftickattachmenttimewaspossible.
Wilhelmssonetal.
Parasites&Vectors2013,6:362Page7of11http://www.
parasitesandvectors.
com/content/6/1/362DiscussionSeasonaldistributionofinfestingticksThegeneralseasonalinfestationpatternofnymphsduringbothyearswasbimodal,withpeaksinJune-JulyandAugust.
Asimilarbimodalseasonalactivitypatternofhost-seekingnymphswasrecordedduringmoststudyyearsintwodifferentfieldstudiesinsouth-centralSweden[7,8],andinoneyearaunimodalactivitypatternwasrecorded[8].
Thedepressioninnymphalinfestationonhumans,observedinthepresentstudymay,therefore,beaconsequenceofthedepressioninnymphalhost-seekingactivity.
Thegeneralseasonalinfestationpatternofadultticksduringbothyearswasunimodalratherthanbimodal.
Thereasonforthisisunknownbuttheadultticks,com-paredtonymphs,mayhaveagreaterresistancetorelativehumidityandasaresult,theymaynotexhibitadepres-sioninhost-seekingactivitypatternduringthehottestanddriestpartofthesummer.
Thenumbersoflarvaeinthisstudyweretoolowtoenableustodrawanyconclu-sionsabouttheirseasonalinfestationpattern.
InSouthernmostSweden,SouthCentralSweden,andonthelandIslandsthetickinfestationonparticipantsbeganonemonthearlierandendedonemonthlaterin2009comparedto2008(fromearlyApriltoearlyNovemberandfrommid-Maytomid-October,respectively).
AccordingtotheSwedishMeteorologicalandHydrologicalInstitute(SMHI),inSouthernmostSwedenandinSouthCentralSwedenthemeantemperatureduringApril2009washigherthanthemeantemperatureduringApril2008[24,25].
Inaddition,themeantemperatureforNovemberintheseregionswashigherin2009thanin2008[26,27].
Thismay,atleastpartly,explainwhytheparticipantsfromSouthernmostSwedenandSouthCentralSwedencontractedtickbitesduringanextendedtimeperiodin2009comparedto2008.
ForbothstudyyearsinNorthernSweden,thetickinfestationonparticipantslastedforashortertimeperiod(fromearly-Junetomid-Augustin2008andfrommid-JunetoearlySeptemberin2009).
TheshortertickinfestationperiodinNorthernSwedenmayreflectthegenerallylowerabundanceofticks[2],whichisafunctionofarelativelylowtickdensityandrelativelylowdielandseasonaltickactivities,whichreflectthegenerallylowerenvironmentaltemperatureandtheshortergrowingseasoninnorthernSwedencomparedtothesouthernregions.
However,toelucidatehowtemperaturesandotherclimateandweatherparame-tersmayinfluencetheseasonaltickinfestationpatterninacertainregion,frequentsamplingandanalysisofticksthathaveinfestedhumansovermultipleseasonsareneeded.
Moreover,theseasonaltickinfestationpat-ternsfoundinthisstudyareinfluencednotonlybythetick'sparticularseasonalactivitypattern,whichmaydifferamongdifferentregions,butalsobythevary-ingactivitiesofpeople,e.
g.
whenpeopletendtovisittick-infestedareasforberry-ormushroompickingorotherpurposes.
Ixodesricinusstagesand"predilectionsites"onhumansInthepresentstudy,onlyfewlarvaltickswereremoved.
However,thistickstageisconsideredtobeamuchlessimportantvectorofB.
burgdorferis.
l.
andTBEVinfectionstohumans;theunfedlarvaisalmostneverinfectedwithLB-causingbacteria[28],norwithTBEV[29].
Thema-jorityoftheI.
ricinusticksremovedwerenymphs.
ThisstageisconsideredtobethemostimportantstageinthetransmissionofborreliaeandTBEVtohumans.
Thisisbecauseinnaturenymphsaremuchmorenu-merousthanadultticks,andbecausenymphs,com-paredtoadultfemaleticks,aremoreeasilyoverlookedduetotheirsmallersizeandlessconspicuouscolour-ation.
Eveniffeweradulttickswereremovedfromtheparticipants,adultticks,comparedtoimmatureticks,are,ingeneral,moreofteninfectedwithBorreliabac-teria[30].
Menremovedagreaterproportionofadultfemaletickscomparedtowomen.
Incontrast,womenremovedagreaterproportionoflarvaltickscomparedtomen.
However,thisdoesnotnecessarilyimplythataparticulartickstageofI.
ricinushasapreferenceforacertaingenderofHomosapiens.
Rather,itmayreflectmorphological,behaviouralandphysiologicaldifferencesbetweenmenandwomen.
Suchdifferences,e.
g.
theusuallymorehairyskinofmenmayresultindifferencesbetweenmenandwomenintheircapacitytorapidlyde-tectatickontheskin.
Morphologicalandotherdiffer-encesbetweenmenandwomenandbetweenchildrenandolderpersonsmayresultinapparentlytick-stage-specific"predilectionsites"onthehumanbodywhichmaydependonhoweasyitisforacertaintickstagetofindasuitableattachmentsite.
Such"preferred"feedingsitesonthehost'sbodyare,mostlikely,muchdependentalsoonthehost'sgroomingbehaviour.
Themajorityofthetickshadattachedtothelegsoftheparticipants.
Thissiteofinfestationcorrespondstotheanatomicallocationwheretheclassicalsignoferythemamigrans(EM)thehallmarkrashofearlyLB,usuallyappears.
Bennetandco-workersrecordedthatthemostcommonanatomicallocalisationsofEMamongLBpatients(n=118)werethelegs(63.
6%)followedbytorso/dorsum(24.
6%),arms(10.
2%)andgenitalia(1.
7%)[31].
Theseproportionscorrespondarbitrarilytotheanatomicaldistributionoftickbitesrecordedbytheparticipantsinthepresentstudy.
Themostcommonlyinfestedanatomicallocation,i.
e.
legs,isapproximatelywithinthesameheightabovethegroundwherenymphsandadultsofI.
ricinusquestinthevegetation[32].
Thissuggeststhatmostticks,searchingforanoptimalattachmentsiteonarecentlyencounteredhumanhost,willwalkonlyashortverticaldistancebeforeWilhelmssonetal.
Parasites&Vectors2013,6:362Page8of11http://www.
parasitesandvectors.
com/content/6/1/362theywillattachandstarttofeed.
However,thesiteoftickattachmentwasrelatedtothestageofI.
ricinus.
Greaterproportionsofnymphs,comparedtoadultfemaleticks,wereremovedfromtheextremitiesoftheparticipants,i.
e.
fromlegsandarms,comparedtootherpartsofthebody.
Incontrast,greaterproportionsofadultfemaletickswereremovedfromtheskinofthetorso/dorsumarea,head/neckarea,andgroin/genitalareacomparedtootherpartsofthebody.
Falcoandco-workersrecordedasimilarbehaviour,i.
e.
anapparent"preference"forcertainbodypartsonhumanhostsbynymphsandadultfemalesofI.
scapularis[10].
Stage-relateddiffer-encesmay,atleastpartly,berelatedtothelevelatwhichaparticulartickstagequestsinthevegetation.
AdultsofI.
ricinususuallyquestatahigherlevelaboveground,comparedtonymphs[32].
Thus,thefirstcontactbyadultticks,comparedtonymphs,onhumanhostsshouldtakeplacefurtherupthehumanbody,closertothetorso/dorsumareaandhead/neckarea.
Tick-stagerelated"preferences"forsiteofattachmenthavebeenob-servedonothervertebratehosts.
Onthewhite-taileddeer,Odocoileusvirginianus,theadultI.
scapularisfeedmainlyontheanteriordorsalbodyregions:87%ofadultticksat-tachedtotheears,head,neckandbrisket[33].
Thesefeed-ingsitesbyadultsofI.
scapulariscorrespondarbitrarilytothefeedingsitesselectedbyadultfemalesofI.
ricinusonhumans(i.
e.
torso/dorsumandhead/neckareas),foundinthepresentstudy.
Onhorses,attachmentbyadultfe-maleI.
scapulariswaslargelyrestrictedtotheunder-bodyareas,whichwasconsideredtoreflectavoidanceofdirectsunlightbytheticks[33].
OntheEuropeanroedeer,larvae,nymphsandadultfemalesofI.
ricinusshowhighdegreesofintrastadialspatialaggregation[34]:larvaeaggregatemainlytotheforelegsandtotheheadofroedeer,nymphsaggregatemainlytothehead,andadultfemalesaggregatemainlytotheneckofroedeer.
Stage-specificdegreesoftoleranceofdesiccationmaybeoneamongfactors,whichexplainhowstage-specific"preferences"forattachmentsiteshaveevolved.
However,thehost'sgroomingbehaviourandcapacitytore-moveectoparasitesfromparticularpartsofthehost'sbodyshouldhaveagreateffectontheevolutionoffeedingsites"preferred"bytheectoparasites.
Thesiteoftickattachmentwasnotinfluencedbytheageofthebittenperson.
However,women,comparedtomen,removedagreaterproportionofticksfromtheheadandneckarea.
Berglundandco-workers[9]foundthatLBpatientsbittenontheheadorneckmoreoftenpresentedneurologicmanifestationscomparedtoLBpatientsbittenonotherpartsofthebody.
Thissuggeststhatthesiteoftickattachmentontheskinofthehumanhostmaybeofparticularclinicalsignificance.
Wealsofoundthatmen,comparedtowomen,removedagreaterproportionofticksfromthegroin/genitalarea.
SimilarresultswererecordedinthestudyofBerglundetal.
[9].
Conse-quently,thisbodyregionseemstobea"preferredsite"forblood-seekingticks,presumablysinceheretheyshouldberelativelywellprotectedfromsunlight,desiccationandhostgroomingactivity.
DurationoftickattachmentAmongadultfemaleticksandnymphs,63%wereremovedlaterthan24hoursofattachment.
Whenthecalculateddurationoftick-feeding(basedonscutalandcoxalindices)werecomparedtotheparticipants'self-estimateddurationsoftickattachment,wefoundthatthetick-bittenpersonsusuallyunderestimatedthedurationoftickattachment.
Apersonwhoremovesanattachedtickfromtheskinlaterthan24hoursoftickattachmentismorelikelytodeveloplocalisedandsystemicsymptomsoftick-bornediseasescomparedtoifthetickisremovedearlier[15].
Therefore,itisimportanttofindandremoveanytickfromtheskinasearlyaspossible.
Forinstance,evenifthevirionsofTBEVcanbetransmittedwithin1houraftertickattachment[12],itisadvisabletoremoveanytickasquicklyaspossiblesincetheamountofvirusparticlesintheticksalivaryglandseemstoincreasewithdurationoftickfeeding[35].
ItisreasonabletoassumethatthemoretimeaTBEV-infectedtickfeeds,thehigherwillbethevirusdosetransmittedtothehost.
Thelocationoftheattachmentsiteseemedtoinfluencehowsoonatickwasdetectedand,therefore,thedurationoftickattachment.
Ticksattachedtothegroin/genitalareaortothehead/neckareawereapparentlymoredifficulttodetectthanticksattachedtoothersites.
Wedidnotfindanysignificantdifferencesbetweenadultfemaleticksandnymphsregardingthetimefromattachmentuntilthetickwasdetectedandremoved.
Weexpectedthatthelar-ger,adultfemaletickswouldbedetectedsoonerthanthesmallerandmoreinconspicuousnymphs.
ThiswasreportedinastudyonI.
ricinusfeedingonhumansinSwitzerland[36].
Thediscrepancybetweenthatstudyandours,regardingthedurationoftickattachmentuntilthenymphsortheadultfemaletickswereremoved,couldbeduetodifferencesintheparticipants'awarenessaswellaspeople'sknowledgeabouttick-infestedhabitats.
Inthepresentstudy,themajorityoftickswerere-moved24–48hoursafterthebeginningofattachment.
Only5%oftheadultfemaleticksandnymphswerere-movedafter≥72hours.
Intransmissionexperimentsusingrodents,ahighlevelofBorreliatransmissionisreachedafter≥72hoursoftickattachment[13,14,37].
Severalstudieshaveshownthatfewpeople(6.
2-9.
0%)becomeinfectedwithBorreliawhentheyarebittenbyaBorrelia-infectedtick[18,38,39].
OneexplanationforthelowriskofcontractingaBorreliainfectioncouldbethatfewticks(only5%,inourstudy)arestillattachedtotheskinafter72hours.
Wilhelmssonetal.
Parasites&Vectors2013,6:362Page9of11http://www.
parasitesandvectors.
com/content/6/1/362Olderpeoplearelikelytohavepoorervisionandim-pairedphysicalsensitivitycomparedtoyoungerpeople.
Thismayexplainwhywefoundthatolderparticipants,comparedtoyoungerparticipants,detectedtheattachednymphsafteralongerattachmenttime.
Similarobserva-tionsonI.
scapularisnymphsremovedfromhumanswerereportedbyFalcoandco-workers[10].
Moreover,wefoundthatmen,comparedtowomen,usuallydetectedanyat-tachedtickafteralongerdurationoftickattachment;thissuggestsahigherriskofBorrelia-transmissiontomeniftheyarebittenbyBorrelia-infectedticks.
ConclusionsThisstudydescribesthetickinfestationpatternonhumansduringtwoconsecutiveyears.
Wefoundseasonalaswellasgeographicaldifferencesininfestation.
Wealsofoundthatthetickattachmentsiteofthehumanbodyinfluencedthetimeuntilthetickwasdiscoveredandremoved.
Thus,thesiteoftickattachmentmaybeofclinicalimportance.
Note-worthyisthatmostoftheticks(63%)wereremovedafter24hoursofattachment.
Olderpersonscomparedtoyoun-gerones,andmen,comparedtowomendetected"their"ticksslower,i.
e.
afteramoreextendedtickfeedingperiod,whichpotentiallywouldbemorepermissiveforpathogentransmission.
Informationabouttickinfestationpatternsprovidedbythisstudyshouldbevaluableforthedevelop-mentalofprophylacticmethodsagainsttickinfestationandforrelevantadvicetopeopleonhowtoavoidorreducetheriskoftickbites.
AdditionalfilesAdditionalfile1:Questionnaire1.
Additionalfile2:Questionnaire2.
CompetinginterestsTheauthorshavenocompetinginterests.
Authors'contributionsPEL,PF,andDNdesignedthestudy.
PWandPLperformedthelaboratoryanalyses.
DataanalysesandinterpretationofdatawereperformedbyPW,PL,TGTJ,LF,PF,DN,andPEL.
PWandTGTJwrotethemanuscripttowhichallauthorssubsequentlycontributed.
Thefinalversionofthemanuscriptwasapprovedbyallauthors.
AcknowledgementsWewouldliketothankallparticipantsofthestudyaswellasthestaffatthePHCsinvolvedintheTBDSTING-study.
WewouldalsoliketothanktheTBDSTING-studygroupmembersClasAhlm,JohanBerglund,Sten-AndersCarlsson,ChristinaEkerfelt,MatsHaglund,AnnaJHenningsson,PeterNolskog,MarikaNordberg,KatarinaOrnstein,andJohannaSjwallforallthevaluableworkandadviceonthestudydesign.
SpecialthankstoLiselottLindvall,SusanneOlausson,andMari-Annekesonfortheircontinuousworkwiththestudycollectionlogistics.
ThisstudywasfundedbyTheSwedishResearchCouncil(Medicine),TheMedicalResearchCouncilofSouth-EastSweden,ALFGrantsfromtheCountyCouncilofstergtland,andanEUInterregIVAsupportedprojectScandTick,andtheWilhelmandElseStockmannFoundationandtheFoundationforlandMedicalResearchofthelandCultureFoundation.
ResearchgrantstoThomasG.
T.
JaensonwerereceivedfromCarlTryggersStiftelse,LngmanskakulturfondenandMagnusBergvallsStiftelse.
Authordetails1DivisionofMedicalMicrobiology,DepartmentofClinicalandExperimentalMedicine,LinkpingUniversity,Linkping,Sweden.
2DivisionofInfectiousDiseases,DepartmentofClinicalandExperimentalMedicine,LinkpingUniversity,Linkping,Sweden.
3ThelandBorreliagroup,Mariehamn,Finland.
4MedicalEntomologyUnit,DepartmentofSystematicBiology,EvolutionaryBiologyCentre,UppsalaUniversity,Uppsala,Sweden.
5ClinicofInfectiousDiseases,UniversityHospital,Linkping,Sweden.
6DepartmentofMicrobiology,RyhovCountyHospital,Jnkping,Sweden.
Received:12September2013Accepted:16December2013Published:20December2013References1.
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doi:10.
1186/1756-3305-6-362Citethisarticleas:Wilhelmssonetal.
:IxodesricinusticksremovedfromhumansinNorthernEurope:seasonalpatternofinfestation,attachmentsitesanddurationoffeeding.
Parasites&Vectors20136:362.
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