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RESEARCHOpenAccessImportedsubmicroscopicmalariainMadridGermánRamírez-Olivencia1*,JoséMiguelRubio2,PabloRivas1,MercedesSubirats3,MaríaDoloresHerrero1,MarLago1andSabinoPuente1AbstractBackground:Submicroscopicmalaria(SMM)canbedefinedaslow-densityinfectionsofPlasmodiumthatareunlikelytobedetectedbyconventionalmicroscopy.
Suchsubmicroscopicinfectionsonlyoccasionallycauseacutedisease,buttheyarecapableofinfectingmosquitoesandcontributingtotransmission.
Thisentityisfrequentinendemiccountries;however,littleisknownaboutimportedSMM.
Thegoalsofthisstudyweretwo-fold:a)toknowthefrequencyofimportedSMM,andb)todescribeepidemiological,laboratorialandclinicalfeaturesofimportedSMM.
Methods:Aretrospectivestudybasedonreviewofmedicalrecordswasperformed.
Thestudypopulationconsistedofpatientsolderthan15yearsattendedattheTropicalMedicineUnitofHospitalCarlosIII,betweenJanuary1,2002andDecember31,2007.
RoutinelydetectiontechniquesforPlasmodiumincludedFieldstainingandmicroscopicexaminationthroughthickandthinbloodsmear.
Asemi-nestedmultiplexmalariaPCRwasusedtodiagnoseortoconfirmcaseswithlowparasitaemia.
Results:SMMwasdiagnosedin104cases,representing35.
5%ofallmalariacases.
Meanage(IC95%)was40.
38years(37.
41-43.
34),andsexdistributionwassimilar.
Mostcaseswereinimmigrants,butsomecaseswerefoundintravellers.
EquatorialGuineawasthemaincountrywhereinfectionwasacquired(81.
7%).
Symptomswerepresentonlyin28.
8%ofallSMMcases,mainlyasthenia(73.
3%ofsymptomaticpatients),fever(60%)andarthromialgias(53.
3%).
Theassociatedlaboratoryabnormalitieswereanaemia(27.
9%),leukopaenia(15.
4%)andthrombopaenia(15.
4%).
Co-morbiditywasdescribedin75cases(72.
1%).
Conclusions:ResultsfromthisstudysuggestthatimportedSMMshouldbeconsideredinsomepatientsattendedatTropicalMedicineUnits.
Althoughitisusuallyasymptomatic,itmayberesponsibleoffever,orlaboratoryabnormalitiesinpatientscomingfromendemicareas.
ThepossibilityoftransmissioninSMMhasbeenpreviouslydescribedinendemiczones,andpresenceofvectorinEuropehasalsobeenreported.
Implementationofmoleculartestsinallasymptomaticindividualscomingfromendemicareaisnoteconomicallyfeasible.
Sore-emergenceofmalaria(Plasmodiumvivax)inEuropemaybespeculated.
Keywords:Submicroscopicmalaria,Paludism,Asymptomaticmalaria,Traveller,Immigrants,PCRBackgroundMalariaisusuallydiagnosedusingmicroscopicmethods(thickorthinbloodsmears)orimmunochromatography.
Theseproceduresareroutinelyusedinclinicalpractice,buthavelimitedsensitivity.
Submicroscopicmalaria(SMM)isdefinedaslow-densityinfectionsofPlasmodiumthatareunlikelydetectedbyconventionalmicroscopy.
Severalquestionsarisefromthisdefinition:IsSMMarealentityor"only"alaboratoryfindingIsSMMarareconditionorafrequentlyneglectedproblemAndfi-nally,canSMMplayaroleinmalariatransmissionorisitaproblemonlyforindividualpatientsSincethede-velopmentofpolymerasechainreaction(PCR)techni-ques,attemptshavebeenmadetoanswerthesequestions.
SMMhasbeenassociatedtoanaemiaandlowweightatbirthinseveralstudieswithpregnantwomen[1-3],al-thoughtheseresultsarestillcontroversial[4,5].
EventhoughSMMhasbeenassociatedtocerebralmalaria[6],SMMisonlyoccasionallyassociatedwithanyclinicalmanifestation.
*Correspondence:germanro.
76@gmail.
com1UnitofTropicalMedicine,InfectiousDiseaseDepartment,HospitalCarlosIII,1028029,Madrid,SpainFulllistofauthorinformationisavailableattheendofthearticle2012Ramírez-Olivenciaetal.
;licenseeBioMedCentralLtd.
ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttributionLicense(http://creativecommons.
org/licenses/by/2.
0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
Ramírez-Olivenciaetal.
MalariaJournal2012,11:324http://www.
malariajournal.
com/content/11/1/324SMMiscommoninendemiccountriesanditspreva-lenceiscomprisedbetween13%and33%insomeareas[7-10].
Thehigheristheprevalenceofmacroscopicmal-aria,thehigheristhedetectedprevalenceofSMM[11].
Inaddition,SMMcanbetransmittedbyinfectiousmos-quitobites,asithasbeendemonstratedinendemiccountries[12].
However,littleisknownaboutimportedSMM,namelyitsrelevanceintermsoffrequency,clinicalmanifesta-tions,orlaboratoryabnormalities.
IncountriesfreeofmalarialikeSpainorGreece,recentautochthonouscases[13-15],suggestthepossibilityofamalariare-emergencefromundetectedimportedcases.
Thisstudyhastwoobjectives:a)assessingthefre-quencyofimportedSMM,andb)describingtheepi-demiological,laboratoryandclinicalfeaturesofimportedSMM.
MethodsStudyareaanddesignInSpain,malariaisareportabledisease.
Lastautoch-thonouscasewasnotifiedin1961[16]andsincethen,allreportedcasesareimportedfromendemiccountries.
HospitalCarlosIIIisareferralunitfortropicaldiseasesatMadrid,Spain.
MostpatientscomebythemselvestotheemergencyunitorarereferredfromprimarycareorgeneralhospitalsinMadrid.
Averysmallproportioncomesfromotherregions.
Aretrospectivestudybasedonareviewofmedicalrecordswasperformed.
Thestudyincludedthosepa-tientsolderthan15yearsanddiagnosedwithmalariainHospitalCarlosIIIbetweenJanuary1st,2002andDe-cember31st,2007.
Thestudywasapprovedbythecor-respondingEthicsCommittee.
Exclusioncriteriawere:a)Unspecifieddiagnosismeth-ods;b)Medicalrecordswithlackofdata(>25%items):epidemiologicaldata(>5items),clinicaldata(>5items),oranalyticaldata(>7items).
Malaria,submicroscopicmalaria(SMM),andotherdefinitionsApatientwasdiagnosedwithmalariawhenPlasmodiumspp.
infectioncouldbedetectedbyconventionalmicros-copyand/orusingPCR,regardlessofthepresenceofsymptoms.
ApatientwasdefinedassufferingfromSMMifhe/sheproducedapositivePCRtesteveniftheexaminationofthickandthinbloodsmearsbylightmi-croscopywasnegative.
Casesdetectedbyconventionalmicroscopywereconsideredasmicroscopicmalaria.
WHOcriteriawerefollowedtoidentifytheseveremal-ariacases[17].
Casesofmalariawereallocatedintofourgroupsfol-lowingtheclassificationcriteriapublishedbyourstudygroup[18,19]:natives,native-travellers,residents,andtravellers.
Nativesarepersonsbornandlivinginzonesendemicformalaria,thatcometoanon-endemiczone.
Native-travellersareborninendemiczonesformalaria,liveinnonendemiczones(formorethan2years)andhavetravelledtoendemiczones(countryofbirthoran-other).
Residentsinanendemiczonearepeopleborninnon-endemiczones,whichhavebeenlivinginzonesen-demicformalariaforatleasttwoyears.
Thesethreegroupsareconsideredassemi-immunes.
Travellersaredefinedaspeoplebornandlivinginnon-endemiczonesthathavetravelledtozonesendemicformalaria(nolongerthantwoyears).
Thislastgroupisconsideredasnon-immune.
Thisclassificationismoreaccurateforthepurposesofourstudythantheclassicalsortingintonatives,immigrants,expatriates,andvisitingfriendsandrelatives.
DatacollectionandlaboratoryexaminationEverypatientunderwentacompleteclinicalhistoryandanexhaustivephysicalexamination.
Theywereroutinelyscreenedforhumanimmunodeficiencyvirus(HIV),hepatitis(A,B,C),syphilis,intestinalhelminthiasis,orprotozoaninfestation.
Thescreeningforfilarariasis(symptomaticorsymptom-free)wasperformedforna-tives,native-travellers,orresidentsinendemiczoneshavingtravelledtozonesendemicforfilariae.
Foreverypatientcomingfromzonesendemicformal-aria,routinedetectiontechniquesforPlasmodiumin-cludedField'sstainingandmicroscopicexaminationthroughthickandthinbloodsmears.
Forallpatientswealwayscollectedonebloodsample,independentlyofthepresenceoffever.
Ifthesamplewasnegative,asecondbloodsamplewascollectedonlyinfebrilepatients.
Asemi-nestedmultiplexmalariaPCR[20]servedtodiag-noseortoconfirmcaseswithlowparasitaemia.
DNAwasextractedfollowingtheChelexmethodwithminormodifications.
DetectionandidentificationofmalarialspeciesweresimultaneouslyperformedwithasequenceoftwoSnM-PCRs.
Thefirstreactionwasexpectedtoyieldtwoproducts:abandof231basepairs(bp)fromUNR-HUFproducedbytheamplificationofthesmallsubunitofthehumanribosomalgene(positivecontrol),whereasthesecondreactionyieldedabandof783to821bpfromUNR-PLFthatshoulddetectthepresenceofanymalariaspeciesofPlasmodiumspp.
Inthissec-ondreaction,infectionswithdifferenthumanPlasmo-diumspeciesyieldedproductsofdifferentsizes.
Abandof269bpindicatesaninfectionbyPlasmodiummalar-iae;abandof395bpevidencesaP.
falciparuminfec-tion;abandof436bpsuggestsaPlasmodiumovaleinfection;andabandof499bpindicatesaPlasmodiumvivaxinfection.
Themixedinfectionswouldshowthecorrespondingbands.
Ramírez-Olivenciaetal.
MalariaJournal2012,11:324Page2of8http://www.
malariajournal.
com/content/11/1/324Patientswithconfirmedmalaria(microscopicorsub-microscopic)weretreatedaccordingtotheWHOguidelinesatthetimeofthediagnosis.
Foreachcase,demographic,clinical,andlaboratorydataweredocu-mented(seeTable1).
Anaemiawasdefinedashemoglobinlevels1.
2mg/dl.
StatisticalanalysisDatawereanalysedusingSPSSpackageforWindows17.
0(SPSS,Chicago,IL).
Forunivariateanalysisofcat-egoricalvariables,Pearson'sChi-squaretestwasused(Fishertestwhenneeded).
Forcontinuousdata,t-Studenttestwaschosentocomparemeansbetweengroups,ex-ceptwhenthevariancesofthesampleswerenothomo-geneous.
Inthiscase(absenceofhomoscedasticity),thenon-parametricMann–Whitneytestwasused.
Ap-valuep25%information,itisexcluded.
Table2Characteristicsofcasesofmalaria(totalandbygroup)Characteristicsofcasesofmalaria(totalandbygroup)TotalTravelerNativeNative-travelerResidentinendemiczonen=293;(100%)n=67;(22.
86%)n=138;(47.
09%)n=14;(4.
77%)n=74;(25.
26%)Age.
Mean(SD)39.
66(14.
40)37.
55(10.
45)41.
38(16.
52)35.
39(9.
72)55.
29(16.
68)Malesn(%)155(52.
9)42(62.
7)66(47.
8)36(48.
6)11(78.
6)Mediantime(days)todiagnosis(range)11(0-1544)8(0-730)15(0-1544)12(0-317)8.
5(0-37)Antimalarialchemoprophylaxisn(%)36(12.
3)21(31.
3)n.
a15(20.
3)n.
aSymptomaticn(%)198(67.
6)62(92.
5)56(40.
6)11(78.
6)69(93.
2)AbnormallaboratorialtestsAnaemia96(32.
8)11(16.
4)53(38.
4)28(37.
8)4(28.
6)Leukocytopenia50(17.
1)12(17.
9)21(15.
2)12(18.
9)3(21.
4)Thrombocytopenia126(43)32(47.
8)35(25.
4)50(67.
6)9(64.
3)Renalfailure31(10.
6)12(17.
9)11(8)7(9.
5)1(7.
1)Jaundice9(3.
1)4(6)3(2.
2)2(2.
7)0(0)Comorbidityn(%)177(60.
4%)23(34.
3)109(79)36(48.
6)9(64.
3)SMMn(%)104(35.
5)13(19.
4)83(60.
1)6(8.
1)2(14.
3)PlasmodiumspeciesP.
falciparum255(87)52(77.
6)123(89.
1)70(94.
6)10(71.
4)P.
vivax13(4.
4)7(10.
4)3(2.
2)1(1.
4)2(14.
3)P.
ovale13(4.
4)4(6)7(5.
1)1(1.
4)1(7.
1)P.
malariae6(2)3(4.
5)2(1.
4)0(0)1(7.
1)Mixedinfections6(2)1(1.
5)3(2.
2)2(2.
8)0(0)Medianofparasitationindex-inmicroscopicmalaria-(range)0.
1(0.
1-50)0.
1(0.
1-50)0.
1(0.
1-15)0.
6(0.
1-30)0.
1(0.
1-2.
7)Abbreviations(inalphabeticalorder).
n.
aNotapplicable.
SD(standarddeviation).
SMMSubmicroscopicmalaria.
Ramírez-Olivenciaetal.
MalariaJournal2012,11:324Page3of8http://www.
malariajournal.
com/content/11/1/324cases)andasthenia(175cases).
Associatedco-morbidity(mainlyotherinfections)wasfoundin177cases(60.
41%).
AnalysisbygroupsTable3showstheepidemiologicalfeaturesofmicro-scopicmalariaandSMM.
Meanageandsexdistributionweresimilar.
Semi-immunegroup(natives,residentsinendemiczonesandnative-travellers)wasmorefrequentinSMMthaninmicroscopicmalaria(p<0.
001).
Anti-malarialchemoprophylaxishadbeentakenin15.
9%ofthemicroscopicmalariacases(30outof189),whereasonlyin5.
8%oftheSMMcases(6outof104)(p=0.
012).
Thesedatamaybesurprising,butfurtheranalysisrevealedthatonly10%ofthepatientssufferingfrommicroscopicmalariaachievedgoodadherencetochemoprophylaxis(3casesoutof30),comparedtothe66.
7%ofpatientsaffectedbySMM(4casesoutof6)(p=0.
008).
Otherparasiticinfectionsweremorefre-quentintheSMMgroup,butnodifferenceswherefoundconcerninginfectionsbyHIV,hepatitisBvirus(HBV)orhepatitisCvirus(HCV).
MalariawascausedbyP.
falciparuminfectionin171casesofmicroscopicmal-aria(90.
5%),andin90casesofSMM(86.
5%)(p=0.
301;Chi-squaretest).
InfectionwascausedbyotherPlasmo-diumspecies,suchasP.
ovale(8microscopicmalariacases;5SMMcases),P.
vivax(8microscopicmalariacases;5SMMcases),andP.
malariae(2microscopicmalariacases;4SMMcases).
Wefoundsomerelevantdifferencesinclinicalcharac-teristicsbetweenmicroscopicmalariaandSMMgroups(Table4).
Mostmicroscopicmalariacasesweresymp-tomatic,whileonlyoneoutofthreecasesreportedclinicalsignsintheSMMgroup.
Fever,astheniaandheadacheweremorecommoninpatientswithmicroscopicmalariathanintheSMMgroup.
Althoughtropicalsplenomegalywasrarelyfound,itwasmorefrequentlynoticedintheSMMgroup(13.
3%)thanintheMMgroup(1.
8%)(p<0.
011).
NoseveremalariawasfoundintheSMMgroup.
Table5showstheanalyticalvaluesofsomebloodtests.
ComparisonoflaboratoryabnormalitiescanbeseeninFigure1.
Morecasesofthrombocytopenia(p<0.
001)andrenalfailure(p=0.
047)weredetectedintheTable3EpidemiologicalfeaturesofimportedmicroscopicmalariaandSMMEpidemiologicalfeaturesbygroupMicroscopicmalariaSubmicroscopicmalariapvalueAge.
Mean(SD)39(13.
9)40(15.
2)0.
527Malesex.
N(%)104(55%)51(49%)0.
326Group.
N(%)Traveller(notimmune)54(28.
6%)13(12.
5%)<0.
001Semiimmune135(71.
4%)91(87.
5%)Native55(29.
1%)83(79.
8%)Residentinendemiczone12(6.
3%)2(1.
9%)Native-traveller68(36%)6(5.
8%)Prophylaxis.
N(%)30(15.
9%)6(5.
8%)0.
012DrugAtovaquone-proguanil3(1.
6%)4(3.
8%)Doxycycline3(1.
6%)0Mefloquine15(7.
9%)0Others9(4.
8%)2(2%)Comorbidity.
N(%)102(54%)75(72.
1%)0.
002Parasitologicalinfection25(24.
5%)55(73.
3%)<0.
001Filariae11(10.
8%)36(48%)<0.
001Intestinalhelmints11(10.
8%)26(34.
7%)<0.
001Others16(15.
2%)30(40%)<0.
001HIV16(15.
8%)11(14.
7%)0.
831HBV15(7.
9%)7(6.
7%)0.
708HCV14(7.
4%)9(8.
6%)0.
294Numberofdifferentcountriesofacquisition31(EquatorialGuinea69.
3%(114);Nigeria5.
3%(10);Cameroon3.
7%(7);Mali3.
2%(6))*15(EquatorialGuinea81.
7%(85);Cameroon2.
9%(3))*n.
a.
SD:Standarddeviation.
*Countriesrepresenting<2.
5%areomitted.
Ramírez-Olivenciaetal.
MalariaJournal2012,11:324Page4of8http://www.
malariajournal.
com/content/11/1/324patientssufferingfrommicroscopicmalariathaninSMMpatients.
Anaemia,leukocytopaenia,andjaundicecasesweresimilarinbothgroups.
Symptoms,laboratoryabnormalitiesandco-morbidityTable6showssymptomsandlaboratoryabnormalitiestoascertainwhethertheyarerelatedtomalariaortootherco-morbidities.
SMMintravellersAtotalof13SMMcasesweredescribedintravellers(nonimmune).
Atotalof4patientsweresymptom-freeanddidnotshowlaboratoryabnormalities.
Agoodadherencetoanti-malarialchemoprophylaxiswasde-scribedinonlyoneoftheseSMMcases.
IntheremainingninesymptomaticSMMpatients,onesufferedfroman-aemiaandonefromthrombocytopaenia.
Forthoseninepatients,goodadherencetoanti-malarialchemoprophy-laxiswasachievedinthreecases.
Whengoodadherencewasachievedthereportedsymptomswereasthenia(3cases),arthromyalgia(3),fever(2),headache(1),andcough(1).
Thesymptomsreportedwhennoadherencetoanti-malarialchemoprophylaxiswasachievedwereas-thenia(5cases),fever(4),headache(3),arthromyalgia(3),rash(1),ocularpain(1),diarrhoea(1)andliveren-largement(1).
Table5AnalyticalvaluesofimportedmicroscopicmalariaandSMMLaboratorialfeaturesbygroupsMicroscopicmalariaMean(SD)SubmicroscopicmalariaMean(SD)pvalueHaemoglobin(g/dl).
13.
1(1.
94)13.
2(2.
04)0.
85Whitebloodcell(cell/mm3).
5,625(2,008)6,091(2,218)0.
06Lymphocytes(cell/mm3)1,391(740)2,176(824)<0.
001Monocytes(cell/mm3)460(271)416(197)0.
131Platelets(cell/mm3)142,045(84,860)216,752(76,139)<0.
001Glycaemia(mg/dl)105(35)100(22)0.
222Urea(mg/dl)30(15)31(13)0.
497Creatinine(mg/dl)0.
97(0.
38)0.
94(0.
34)0.
539LDH(UI/ml)482(236)419(189)0.
024ALT(UI/ml)59(143)43(136)0.
392Cholesterol(mg/dl)132(39)164(44)<0.
001Abbreviations(inalphabeticalorder).
ALT:alanineaminotransferaseenzyme.
LDH:lacticdehydrogenaseenzyme.
SD:Standarddeviation.
Table4ClinicalfeaturesofimportedmicroscopicmalariaandSMMClinicalfeaturesbygroupMicroscopicmalariaSubmicroscopicmalariapvalueTimetodiagnosis(days)11(0-730)12(0-1544)0.
276(M)Symptomatic168(88.
9%)30(28.
8%)<0.
001Liverenlargement17(9%)11(10.
6%)0.
0659Spleenenlargement29(15.
3%)14(13.
5%)0.
663Asthenia153(91.
1%)22(73.
3%)0.
011(F)Headache119(70.
8%)12(40%)0.
001Ocularpain39(23.
2%)3(10%)0.
103Arthromyalgia116(69%)16(53.
3%)0.
093Vomiting36(21.
4%)2(6.
7%)0.
059Diarrhoea37(22%)2(6.
7%)0.
051Rash1(0.
6%)1(3.
3%)0.
281(F)Cough4(2.
4%)2(6.
7%)0.
226(F)Abdominalpain5(3%)2(6.
7%)0.
287Fever160(95.
2%)18(60%)<0.
001(F)Tropicalsplenomegaly3(1.
8%)4(13.
3%)0.
011(F)Severemalaria13(6.
9%)0(0%)0.
005(F)Abbreviations(inalphabeticalorder).
F:TestdeFisher.
M:TestdeMann–Whitney.
Ramírez-Olivenciaetal.
MalariaJournal2012,11:324Page5of8http://www.
malariajournal.
com/content/11/1/324DiscussionThisstudyaimedtodescribethefrequencyofimportedSMManditsepidemiological,clinical,andlaboratoryfeatures.
MostofthecasesofimportedmalariareportedinEuropeareofmicroscopicmalariaandafewreportedcasesareofSMM.
Twomainimportantreasonsexplainthisfact:a)Malariaissuspectedwhensymptomsarepresentandb)DiagnosisisusuallymadebymicroscopicexaminationbutPCRisnotroutinelyused.
ExaminationforSMMhasbeenprogressivelyimplementedinthedailycareschemesofourhospital,basedonseveralstudiesshowingthehighprevalenceofSMMinsomecountries[7,9-11]oronthefrequencyofsymptom-freecases[21,22].
ThisstudycouldnotestimatetheTable6LaboratoryabnormalitiesinSMMandcomorbidityAbnormalities(n)Co-morbidityCommentsAnaemia(29)24(82.
8%)Intestinalparasites(6cases),tuberculosis(1),thalassaemia-beta(1),chronickidneydisease(1)Leukocytopaenia(16)10(62.
5%)Tuberculosis(1case),HIVinfection(2),HBVinfection(1),Mansonellaperstans(6),andSalmonellaandSchistosomainfection(1)Thrombocytopaenia(16)15(93.
8%)HIVinfection(2cases),Salmonella(1)Renalfailure(6)6(100%)Notdemonstratedkidneydisease.
Jaundice(3)3(100%)HBVinfection(1case),tuberculosis(1),SalmonellaandSchistosomainfection(1)Liverenlargement(11)9(81.
8%)HCVinfection(2cases),HIVinfection(3),tuberculosis(1),alcohol-intake(1),drepanocyticanaemia(1),SalmonellaandSchistosomainfection(1)Spleenenlargement(14)14(100%)HCVinfection(2cases),alcohol-intake(1),tuberculosis(1),SalmonellaandSchistosomainfection(1),drepanocyticanaemia(1),Mansonellaperstans(5).
Notassociated(3)Asthenia(22)12(54.
5%)12differentinfectiousdiseasesHeadache(12)6(50%)Intestinalparasites(3cases)acuteBhepatitis(1),tuberculosis(1),SalmonellaandSchistosomainfection(1)Ocularpain(3)2(66.
7%)Intestinalparasites(2cases)Arthromyalgia(16)8(50%)AcuteBhepatitis(1case),tuberculosis(1),rickettsiainfection(1),pneumonianotfiliated(1),HCVinfection(1),Mansonellaperstans(2),intestinalparasites(1)Vomiting(2)0(0%)Diarrhoea(2)1(50%)MansonellaperstansRash(1)0(0%)Cough(2)1(50%)NotassociatedAbdominalpain(2)1(50%)NotassociatedFever(18)11(61.
1%)Infectiousdiseases(10),notassociated(1)Figure1Comparisonoflaboratoryabnormalities.
MM:Microscopicmalaria.
SMM:Submicroscopicmalaria.
Ramírez-Olivenciaetal.
MalariaJournal2012,11:324Page6of8http://www.
malariajournal.
com/content/11/1/324prevalenceofSMMinthepopulationcaredfor,becausePCRtechniqueswerenotsystematicallyperformedforallpatients(thisisnotaprospectivestudy).
However,theauthorsbelievethatSMMisafrequentdiseaseaccountingforuptothethirdofallcasesofimportedmalaria.
AlthoughSMMisaclinicalentityusuallysymptom-free,itcanbeassociatedtoasthenia,feverormusculo-skeletalpain.
Inaddition,anaemia,leukocytopaeniaorthrombocytopaeniawerereportedinmorethan10%ofthecases.
Symptomsandlaboratoryabnormalitiesmaybelinkedtootherdiseaseswhenco-morbidityispresent,butsometimesSMMistheonlycause.
SMMshouldbeenvisagedwhenconventionaltechniquesformalariade-tectiongavenegativeresultsbutsomeclinicaloranalyt-icalabnormalitiesremainunexplainedinpatientscomingfromzonesendemicformalaria.
NegativemicroscopictestscanproduceapositivePCRtestinthesecases:a)FalsepositiveofthePCRtech-nique,orb)Low-densityparasitaemias.
Inlow-densityparasitaemia,fivepossibilitieshavetobeconsidered:1)Infectioninsemi-immunepeople;2)Anti-malarialtreat-ment;3)Anti-malarialchemoprophylaxis;4)Firststagesofinfection;or5)Falsenegativemicroscopicmethod(microscopistswithalimitedexperienceinidentifyingmalaria,arealpossibilityinareaswithnotransmission).
Falsepositive(orPCRcontamination)isacommonproblem,withratesreportedbylaboratoriesbetween0.
7%and10%[11,23].
Infectionofsemi-immunepersonsiscommoninendemiccountries,aspreviouslymen-tioned,andhasalsobeendescribedinimmigrants[18,24].
Anotherexplanationcanbethedevelopmentofresistancetoanti-malarialtreatment,asaresultofanin-correctmanagementofthetreatmentorcausedbylowlevelsofmedication,aswellasthe"controlledinfection¨inpeopletakinganti-malarialchemoprophylaxis.
TheearlystagesofmalariainfectioncanbeidentifiedasSMM(eveninnon-immunepeople),andmayevolvetoa"microscopic"infectionifnotreatmentisgiven.
DatafromtheWHORegionalOfficeforEuropehaveshownthatthenumberofimportedmalariacaseshavenotchangedsignificantlyinthelasttenyears(exceptinFrance)[25].
AutochthonousmalariacaseshavebeenreportedinAzerbaijan,France,Georgia,Greece,Italy,RussianFederation,SpainandUkraineinthesameperiodoftime[13,15,25,26].
Thepossibilityoftransmis-sioninSMMhasbeendescribedinendemiczoneformalaria[8,12],andpresenceofasuitablevectorinEur-ope(Anophelesatroparvus,AnophelesclavigerorAnoph-elesmaculipennis)hasalsobeenreported[27,28].
Sincetheimplementationofmoleculartestsforallsymptom-freeindividualscomingfromendemicareaistooexpensive,thepossibilityofare-emergenceofmalaria(Plasmodiumvivax)inEuropecanonlybespeculated.
ConclusionsSMMisafrequentconditionthatshouldbeconsideredwhensomeclinicaloranalyticalabnormalitiesremainunexplainedinapatientcomingfromzonesendemicformalaria.
UndetectedanduntreatedSMMandthespread-ingofcompetentvectorsmightbethecausesofare-emergenceofmalariainEurope.
CompetinginterestsTheauthorsdeclarethattheyhavenocompetinginterests.
Authors'contributionsGRO,PR,MDH,MLandSPattendedthepatients,collectedthedataanddraftedthemanuscript.
MScarriedoutthemicroscopicexamination.
JMRcarriedoutthemoleculargeneticstudies.
Allauthorsreadandapprovedthefinalmanuscript.
AcknowledgementsThisstudywassupportedbyRICETRD06/0021/0003ISCIII-RETICS.
Authordetails1UnitofTropicalMedicine,InfectiousDiseaseDepartment,HospitalCarlosIII,1028029,Madrid,Spain.
2Malaria&EmergingParasiticDiseasesLaboratory,ParasitologyDepartment,NationalCentreofMicrobiology,InstitutodeSaludCarlosIII(ISCIII),28220,Madrid,Spain.
3MicrobiologyandParasitologyDepartment,HospitalCarlosIII,1028029,Madrid,Spain.
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