12 comparison of biological attributes of culex quinquefasciatus (diptera_ culicidae) populations...

4/27/2015 ComparisonofBiologicalAttributesofCulexquinquefasciatus(Diptera:Culicidae)PopulationsfromIndia

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ResearchArticle

ComparisonofBiologicalAttributesofCulexquinquefasciatus(Diptera:Culicidae)PopulationsfromIndiaMangeshD.Gokhale,MandarS.Paingankar,andSachinD.Dhaigude

DepartmentofMedicalEntomologyandZoology,NationalInstituteofVirology,MicrobialContainmentComplex,130/1SusRoad,Pashan,Pune411021,India

Received27April2013Accepted2June2013

AcademicEditors:A.I.KehrandM.V.Micieli

Copyright2013MangeshD.Gokhaleetal.This isanopenaccessarticledistributedundertheCreative CommonsAttribution License, which permits unrestricted use, distribution, andreproductioninanymedium,providedtheoriginalworkisproperlycited.

Abstract

Understandingthepopulationdynamicsofmosquitopopulationsthroughlifetableanalysisandinsecticidesusceptibilityisimportanttoassessthelikelyimpactofvectorcontrolstrategiesaswellastoaidthedesignofnovelinterventions.Variationinthelifetablesandotherbiologicaldata was compared for two populations of Culex quinquefasciatus Say 1823 fromgeographicallyisolatedregions,GorakhpurandPunefromIndia.Underastandardizedrearingregimeandconstantlaboratoryconditions,mosquitoeswererearedandbiologicalattributesofthese populations were compared. Development and survival of immature and adult stagesofCulexquinquefasciatuswerefoundsignificantlydifferentinGorakhpurandPunepopulations.Principal component analysis of morphological data revealed that the two populations formsignificantly different clusters which can be differentiated from each other based on siphon,saddle, anal gills, and pecten teeth related variables. Insecticide susceptibility results suggestthatthelarvaefrombothareasweremoresusceptibletodeltamethrinascomparedtoDDTandmalathion. The current study provides baseline information on survivorship, morphologicalvariationand insecticidesusceptibilityofCulexquinquefasciatus.The resultsobtained in thisstudy suggest that different geographical areas with contrasting habitats have significantinfluenceonsurvivalandreproductivestrategiesofCulexquinquefasciatus.

1.Introduction

Feasible mosquito intervention strategies require indepth knowledge of the populationdynamicsandinsecticidesusceptibilitystatus.Tobetterunderstandthepopulationdynamicsofthevectorspecies,understandingtheparameterssuchasdevelopmenttime,rateofsurvival,andfecundity of particular importance. A life table describes the development, survival, andfecundityofacohortandprovidesbasicdataonpopulationgrowthparameters.Lifetablestudyalsoprovide,conciseinformationoninherentdifferencesin thesurvivorshipandreproductivestrategiesofpopulationsunderdifferentecologicalregimes[1]andcanhelpinexplainingwhycertainspeciesproliferateinparticularenvironments.

CulexquinquefasciatusSay isacosmopolitanmosquitospecies found in tropical, subtropical,and warm temperate regions. Cx. quinquefasciatusacts as a vector of filarial worms [2, 3],protozoan parasites [4], and various arboviruses [59]. In India, Cx. quinquefasciatus is the



principalvectorofbancroftial filariasis [10].Rapidurbanizationand industrializationwithoutproper drainage facilities are responsible for the establishment of theCx.quinquefasciatus invariouscitiesofIndia[11].Oftheestimatedglobal128millionlymphaticfilarialcases,91percentarecausedbyWuchereriabancroftiCobbold[12].Indiaalonecontributes40percenttotheworlds lymphatic filariasis disease burden [13]. The estimates of disease burden due tofilariasissuggestthat2.06milliondisabilityadjustedlifeyears(DALYs)arelostinIndia[14].

Numerous studies have been carried out on Cx. quinquefasciatus biology and insecticidesusceptibility [1528]. Recently, Suman et al. [25, 28] reported significant variations inmorphology among eggs and life table attributes from the four strains of Cx.quinquefasciatus. Comparison of biological attributes such as insecticide susceptibility, lifetableanalysis,andmorphologicalvariationofCx.quinquefasciatusofendemicandnonendemicareasoflymphaticfilariasisisnotwelldocumented.

Mosquitoes inhabiting in distinct location normally adapt to local conditions.Gorakhpur andPune are geographically well separated (~1500km) (Figure 1) and having different climaticconditions.UttarPradesh (Gorakhpur) isoneof the lymphatic filariasis endemic statesof theIndiawhereasMaharashtra(Pune)isnonendemictolymphaticfilariasis.Theenvironmentandclimaticfeaturesofthesetwoecotypesarealsodifferent.Itseemspossiblethatenvironmentalstressofthesetwoecoclimaticregionsinfluencetheirlifehistorytraits.Wehypothesizedthatthesemosquitostrainswilldifferintheirlifetableandbiologicalattributesbecausetheyappeartobeadaptedtotheirrespectivelocalenvironments.Totestthishypothesis,weinvestigatedthelife table analysis, morphological variation, and insecticide susceptibility between these twopopulationsofCx.quinquefasciatusfromgeographicallywellseparatedareasinIndia,namely,GorakhpurandPune.

Figure 1: The sampling map indicates thesamplinglocations.

2.MaterialsandMethods

2.1.EthicsStatement

All animals were handled in strict accordance with good animal practice as defined byInstitutional Animal Ethics Committee (IAEC) affiliated with National Institute of Virology(NIV), Pune, India. All animal work was approved by the IAEC. Animal experiments werecarriedoutinstrictcompliancewithCommitteeforthePurposeofControlandSupervisionofExperimentonAnimals(CPCSEA)guidelines,India.

2.2.MosquitoCollectionandRearing

Adultand larvaeofCx.quinquefasciatuswere sampled from twopopulations to examine thepossibility of population differences in the endemic and nonendemic areas of lymphaticfilariasis infection. Uttar Pradesh is an endemic area of JE virus and lymphatic filariasisinfection and Maharashtra is nonendemic to JE virus and lymphatic filariasis infections.Gorakhpur (2645N, 8324E) is situated in the Tarai belt near Himalaya ranges in UttarPradeshstateborderingNepalwhereasPune(1831N,7355E)issituatednearWesternGhats



of India, inMaharashtra state. Gorakhpur is situated in the IndoGangetic Plain region thatreceives12001300mmofrainfallperyear.Annualtemperaturerangesbetween8Cinwinterand42C in summer.Thevegetationof this region is tropical,moist, anddeciduous.Pune issituatedinasemiaridregionthatreceives700800mmofrainfallperyear.Annualtemperatureranges between 12C in winter and 38C in winter. The vegetation of this region is drydeciduous.Larvaewerecollectedfromtemporaryandsemipermanentgroundwaterpoolsandfields.Morphometricanalysiswascarriedoutonthesefieldcollectedmosquitoes.Inthepresentstudy, to better understand baseline parameters of the life cycle of Culex quinquefasciatus,colonies of this species from two different ecological areasweremonitored under laboratoryconditions for life table analysis and insecticide susceptibility. Individuals of Cx.quinquefasciatuswereidentifiedusingkeysofBarraud[29].VoucherspecimensweredepositedinNationalInstituteofVirologymuseum,Pune,India.

2.3.LifeTableAnalysis

Rearingofmosquito in laboratoryenvironment imposesacertaindegreeofselectionpressureon mosquito biology. Mosquito colonies were maintained in standardized environmentalconditions which were favorable to mosquito survival. Hence, we assume that lifetableparameters derived from data collected from the colonized wild strains represent maximumestimates of their lifetable parameters and are likely to reflect true differences betweengeographicallyisolatedstrains.SimilarassumptionsweremadeearlierbyReisenetal.[30]andSuman et al. [28] to compare the lifetable attributes of geographically distinct strainsofCulexmosquitoes.

ColoniesofeachpopulationweremaintainedatNationalInstituteofVirologyPuneinstandardlaboratoryconditionsat281C,705%relativehumidity(RH)andlight:dark(LD)12:12h.Thesecoloniesweremaintainedforuptosevengenerations(approximately4months).Theadultswereoffered10%glucosesolutionsoakedincottonpadsdaily,asasourceofnutrition.Afowlwasprovided as a sourceof blood twice aweek.Fowlsweremaintained in accordancewith approved guidelines of the Committee for the Purpose of Control and Supervision ofExperiments on Animals (CPCSEA) India. Egg rafts were collected from the individualmosquitoesinplasticcups(60ml)containingwaterandlinedwithfilterpapers,inthecage.Thesingle egg raft was transferred from the filter papers with the aid of mounting needles intoplastic bowls (10cm height and 25cm diameter) where they were held for 24 to 72h forhatching. Twentyfour hours after hatching, the larvae were maintained at a density of 100larvaeper tray (453010cm)containing2Lofdechlorinatedwater.The larvaewere fedwithmixtureofyeastextractanddogbiscuitpowder,sprinkledonthewatersurface.Oneveryalternateday,thewaterfromtheculturetraywaschangedcarefullyuntilpupation.Thepupaewere separated from the larvae daily and placed in plastic bowls (10cm height and 30cmdiameter)halffilledwithwater.Theseplasticbowlswithpupaewereplaced inadultholdingcages for emergence. Adults after emergence were offered 10% glucose solution soaked incotton pads daily. Fecundity was estimated based on the average numbers of eggs laid perfemale.Egghatchingratewasestimatedfromthenumberofeggshatchedperfemale.Durationofthepreadultdevelopmentperiodswasdeterminedbyobservingeachpandaily,andalllarvalskins were removed, scored to instar, and counted. There were two replicates for eachexperiment described above and the whole study was repeated three times, resulting in themonitoringofabout600larvaefromfirstinstarstagetoadulthood.

2.4.SurvivalAnalysis

Onehundrednewlyemergedlarvaewereplacedinbasins.Allbasinswerescreenedwithwhite,



insectproof,nylonnettingtopreventcolonizationbyothermosquitoesandpredators.Eachdaythenumberof larvae alive and their stageofdevelopmentwere counted and recorded.Adultmosquitoesthatemergedwerecounteddaily,andthesexeswererecorded.Standardlife tableanalysiswasperformedonthedata(Table1).Computationofstagespecificsurvivorshipwasaccordingtotheformula:

where isthenumberofindividualsenteringstage and isthenumberofindividualsthatenteredtheprecedingstage.

Table 1:Mean duration and survival rates of lifestagesofCx.quinquefasciatus.

2.5.MorphologicalAnalysis

Fieldcollectedlarvaewereusedformorphometricanalysis.Eightmorphologicalcharactersandthree ratioswere scored formorphological analysis of fourth instar larvae (Table3). Siphonindex, analgill index, and siphonsaddle ratioareused in traditional taxonomy todistinguishamongspeciesandsubspecies.However,FakoorzibaandVijayan[31]andKanojiaetal.[32]have used these ratios even to distinguish between different populations. The characters offourthinstarlarvaeweremeasuredusingmicrometricocularswiththeleastcount0.01mm.

2.6.AdultMosquitoBioassays

Insecticide susceptibility testswere carried out using the standardWHOprotocol, insecticidesusceptibility test kits, and impregnated papers. Two to threedayold nonbloodfed adultfemaleCx.quinquefasciatusweretested.Threebatchesof20mosquitoeseachwereexposedtotestpapersimpregnatedwithDDT(4%),Deltamethrin(0.05%),andMalathion(5%).Controlsincludedbatchesofmosquitoesfromeachsiteexposedtountreatedpapers.

2.7.LarvalBioassays

Batches of fourth instars were exposed to insecticide solution (DDT, Deltamethrin andMalathion)diluted indistilledwater.Twenty larvaeperconcentrationandfiveconcentrationspertest,providingmortalitiesrangingfrom0to100%,wererun.Temperaturewasmaintainedat 28C throughout the test, and larval mortality was recorded after 1h exposure and thencorrected for controlmortality [33].Three replicateswith insect larvae fromdifferent rearingbatchesweremadeatdifferent times,and theresultswerepooled.Thedataweresubject toalogprobitanalysis[34]todeterminetheLC andLC valuesaswellastheir95%confidenceintervals(CI).

2.8.AnalysisofData

Survivalwasevaluatedbyscoringthenumberofdeadmosquitoes/larvaeeveryday,toestimatemosquitostagespecificlifespan.Larvaethosewhoareenteredinpupalstagewereconsideredascensoreddata.Thesurvivalrateforeachmosquitopopulationwasdescribedusinglifetableanalysis.Thesesurvivalcurveswerethencomparedusingthelogranktest.Pairwisecomparison

50 95



of each character was carried out using test, with the Bonferroni adjustment to theprobabilities(as11comparisonsweremadethereforeweused asourcutoffvalue).Datawereanalyzedusingmultivariate statistics.Multivariateanalysisoffers theadvantageoftaking into account all the variables in a single analysis, thus making it possible to assessvariation in the morphological characters of larvae from different collection sites. Principalcomponent analysis (PCA) was done to create uncorrelated principal components from theoriginal variables. Factor scores estimate the actual values of individual observations for thefactors,whereascorrelationbetweenvariablesandfactorsiscalledasfactorloading.Principalcomponents were extracted from the covariance matrix. Mean values (centroids) and 95%asymptotic confidence limits of the scores of individual larvae on the first two principalcomponents were computed for each sample. First two principal factors, which explainedmaximumvariationinthedata,wereanalyzedtounderstandthevariationinthemorphologyofdifferentindividuals.

2.9.AnalysisofBioassayData

Bioassay datawere pooled and analyzed (LC and 95%LC values) by using SPSS ver. 16program(SPSSInc.,Chicago,IL).

2.9.1.AbbottsFormula

Bioassaydatawerenotconsideredifthecontrolmortalitieswere>20%.ActualmortalitieswerecalculatedagainstcontrolmortalitiesbyusingAbbottsformula[33].Correctedmortality=(%survival in control experiment % survival in treated experiment)/(% survival in controlexperiment)100.

2.9.2.LogProbitRegressionLines

Logprobit regression lines were obtained by plotting percentage mortalities ( axisprobability) against concentration ( axislog). LC (dosage which kills 50% of thepopulation)andLC (dosagewhichkills95%ofthepopulation)valueswereobtained.Fitnessoftheregressionlineswasstatisticallytestedusing values[35].

3.Results

3.1.PreadultDevelopmentandSurvival

The mean duration and survival rates of different life stages of Cx. quinquefasciatus ofGorakhpur and Pune are depicted in Table1. The percentage of egg hatching was lower inGorakhpur(86.57.3)ascomparedtoPune(91.53.8).LarvaltoadultsurvivalrateofCx.quinquefasciatusinPune(58.063.61)wassignificantlyhigherthanGorakhpur(28%0.76)( test ).PunepopulationofCx.quinquefasciatushadslightlylongerlarvaldevelopmenttime (14.57 days) when compared with that of Gorakhpur Cx. quinquefasciatuspopulation(13.33 days). Larval survival rate of Pune population (68%) was higher than Gorakhpurpopulation (42%) of Cx. quinquefasciatus ( test ). Pupal survival rates of bothpopulations were not significantly different ( test ). No significant difference wasobserved in male female emergence ratio ( test ). Survival rate of Cx.quinquefasciatusadultsofPunewashigherthanthoseofGorakhpur(logranktest on1degreesoffreedom, )(Figure2).

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Figure 2: Survivorship curves of Cx.quinquefasciatusofGorakhpurandPune.(a)Adultsurvivorship curve of Gorakhpur population. (b)Adult survivorship curve of Pune population. Thesurvival rate for each mosquito population wasdescribed using life table analysis and wascomparedusingthelogranktest.

3.2.Fecundity

Comparison of egg production of Gorakhpur population and Pune population of Cx.quinquefasciatusisreportedinTable2.Gorakhpurpopulationproducedbiggereggraft (151186eggs)ascomparedtoPunepopulation(121160eggs).ThenumberofeggsperraftwerehigherinGorakhpurpopulation(16914.5)ascomparedtoPunepopulation(139.7515.96)(Table2).Thenumberofeggs/femalelifespanwashigherinGorakhpurpopulation(67632)(Table2).Eggs/female/daywas ranged from32.19 (1.68) inGorakhpurpopulationand26.59(2.45)inPunepopulation(Table2).

Table2:ComparisonofbiologicalattributesinCx.quinquefasciatus.

Table 3: Comparison of morphological charactersoftwopopulationsofCulexquinquefasciatus.

3.3.PhenotypicVariationamongthePopulations

The pairwise comparison of morphological characters using unpaired test revealed thatcharacterslikelengthofanalgills,lengthofsiphon,denticlesontheapicalpectenteeth,siphonsaddle ratio, and anal gill index were significantly different in both populations studied(Table3).PCAofthemorphologicalanalysisextractedfourfactorswitheigenvaluesmorethanone.Cumulatively,thesefactorsexplained79.61%ofthetotalvariabilityinthedata.Thefirstfactor (F1) explained 31.97% of the total variability while the second factor (F2) explained19.73% of total variability and together the first two factors explained 51.70% of the totalvariability.CharacterswhichshowedhighfactorloadingonF1includedlengthofsiphon,widthofanalgill,siphonindex,andsiphon/saddleratio,whilecharacterswhichshowedhighfactorloading on F2 included length of anal gill, length of saddle, and anal gill index. PCA coulddistinguish two significant clusters forCx.quinquefasciatusofPune,Cx.quinquefasciatus ofGorakhpur (Figure 3). Our null hypothesis that there is no significant difference inmorphometryofCx.quinquefasciatuspopulationswasrejected.



Figure 3: Principal component analysis (PCA) ofmorphological data. (a) Clusters of differentpopulations. (b) Variables which discriminatebetweentheclusters.

3.4.InsecticideSusceptibility

BasedonWHOcriteria[36],Cx.quinquefasciatusfromGorakhpurandPuneshowedcompletesusceptibility to deltamethrin (100% mortality), potential resistance to malathion (~80%mortality),andcompleteresistancetoDDT(Lessthan80%mortality).Nosignificantdifferencewas noted in the toxicity of diagnostic concentrations of DDT, deltamethrin, and malathionbetween the Gorakhpur and Pune populations of Cx. quinquefasciatus ( test ). Thelarvicidal potency of DDT, deltramethrin, and malathion against Cx. quinquefasciatus isdepictedinTable4.GorakhpurpopulationshowedhigherLC valueforDDTandmalathionas compared to Pune population.No significant differencewas observed for LC values ofdeltamethrinforboththepopulations.

Table 4: Comparative toxicity of DDT,deltamethrin, and malathion against populationsofCx. quinquefasciatus tested at the larval stage(larvalbioassays).

4.Discussion

Theeffectsofgeographicalandenvironmentaleventsonlifetableattributesandmorphologicalstructure of local populations of Cx. quinquefasciatus were analyzed in this study. Our nullhypothesisthatthereisnosignificantdifferencebetweenthelifecycleparameters,morphology,andprofilesofGorakhpurandPunepopulationswasrejected.GorakhpurandPunepopulationsof Cx. quinquefasciatus are significantly different with respect to life table attributes andmorphologicalcharacters.

Gorakhpur andPune are~1500km apart, Gorakhpur area having the higher range of annualtemperatures, lower relative humidity for most of the year, and higher annual rain fall ascomparedtoPune.Accordingly,wemightexpectthatenvironmentalconditionsinPunewouldfavoursurvivalandreproductivepotentialofmosquitoes.ThepopulationfromGorakhpurareaexposedtoharshconditionsforsurvivalinnaturethanPunestrainhadashorterlifeexpectancy(2023 days in Gorakhpur vs 2427 days in Pune), laid eggs onmost days (1820 days inGorakhpur vs 2022days inPune), and constructed larger egg rafts (169 14.5 eggs/raft inGorakhpurvs139.7515.96eggs/raft inPune), indicatingthatGorakhpurstraininvestedthemostenergyinrapidreproduction.ThehighersurvivorshipofadultCx.quinquefasciatusinthePune compared to the Gorakhpur is likely caused because of favorable habitat andenvironmentalconditionsinPune.

The percentage of eggs hatching was not significantly different in Gorakhpur (86.5 7.3)populationascomparedtoPune(91.53.8)( test ).Thepercentageofeggshatchinghas been reported to be 82.1% in Culex pipiens fatigans (synonymous with Cx.

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quinquefasciatusGomezetal.[37]),79.6%inCx.quinquefasciatus(SulemanandReisen[38]),and 80.5%95.6% in Cx. quinquefasciatus (Suman et al. [28] five populations from India)whichareclose to thevalues reported in thepresent study (Table1).Themeandevelopmenttimefromeggtoadultwasfoundtovarylittle,withshowingarangeof1day(15.4daysinGorakhpur and 16.5 days in Pune). These valueswere higher than the developmental periodreported for Cx. quinquefasciatus collected from Bikaner, Jamnagar, and Bhatinda areas ofIndia[28]. It hasbeendocumented thatvariations in immaturemosquitodevelopmental ratesdependonhabitatconditions [28,3942].The resultsobtained in this studyalso suggest thathabitatandenvironmentalconditionsplayimportantroleinbiologicalattributesofmosquitoes.

Life expectancy is an important attribute of vector capacity as it plays important role intransmissionpathogens.Studiesonlymphaticfilariasistransmissionindicatethatmicrofilariaerequire1617daysinthemosquitovectortoreachtheinfectivestage[43].Thisperiodshortenswithhigher temperatureandhumidity [44]but increasesup to42days at lower temperatures[45]. As life table analysis suggests that Gorakhpur population invest more energy inreproduction it is likely to take bloodmealmore frequently. The repeated bitingmay causespread of diseases rapidly. Higher ambient temperatures and frequent biting nature of Cx.quinquefasciatusofGorakhpurmightberesponsibleforfrequentcasesoflymphaticfilariasisinthisarea.

Multivariate analysis of morphological characters revealed that two populations of Cx.quinquefasciatus differ significantly in morphological characters (Table 2Figure3(a)).Principalcomponentanalysisofthedatasuggestedthatsiphon,saddle,analgillsand pecten teeth related variables were the most important distinguishing characters(Figure3(b)).Anal gills are longer inGorakhpur population as compared to Pune population(Table3).Assaltconcentrationofwaterincreasesthelengthofanalgillsstartsreducing[46].ThesaltconcentrationofPunewatermightbeaprobablereasonbehind thereducedanalgilllength.Thecausesofmorphologicaldifferencesbetweenpopulationsareoftenquitedifficulttoexplain.Phenotypeisunderthedoublecontrolofenvironmentalconditionsandgenotype,butmorphological changes canbe rapidwhendifferent environmental conditionsoccur [47].WehypothesizethattransitionofsaddleandanalgilllengthinCx.quinquefasciatuspopulationsisaconsequenceofenvironmentalconditionsselection.

The history of insecticide resistance inCulex species suggests thatmembers ofCulex genushave a notorious reputation for developing resistance to insecticides, includingorganophosphates,carbamates,andpyrethroids[17,21,22,48,49]. In India, first evidenceofdevelopmentofDDTresistance inCx.quinquefasciatuswasobserved in1952 fromavillagenear Delhi [50]. Later, several areas like Nagpur, Pune, Patna, and Rajahmundry reportedresistance in Culex mosquitoes to various insecticides such as BHC, fenitrothion, DDT,malathion,and temephos[15,16,18,20,23,51]. InvestigationbyMukhopadhyayet al. [23]showed nomortality inCx.quinquefasciatus adults against the 4%DDT and 5%malathion.About 8%and14%mortalitywere found inCx.quinquefasciatus larvae in 0.125 and 3.125mg/l concentrationofDDTandmalathion, respectively.Thepresent studyconcludes that thelarvaeofboth thepopulationsofCx.quinquefasciatus arehighly susceptible todeltamethrin.However, Cx. quinquefasciatus populations in these areas showed resistance to DDT andmalathion. Therefore, as a baseline study it may be concluded that in future prudent use ofvector control strategies should be applied to reduce the Cx. quinquefasciatus nuisance inGorakhpurandPune.

Insummary,thisstudyprovidesbaselineinformationonsurvivorship,morphologicalvariation,



andinsecticidesusceptibilityofCulexquinquefasciatuspopulations.TheclimateconditionsofPuneseemsmorepermissive to thedevelopmentandsurvivalofCulexquinquefasciatus.Theinsecticide susceptibility results revealed that in the future different controlmeasureswill berequired to reduce the Cx. quinquefasciatus populations. This knowledge, coupled withenhanced understanding of the ecology ofCx. quinquefasciatus, will enable locally specific,efficientdeploymentofpublichealthresourcesinreducingtheburdenofCx.quinquefasciatus.

ConflictofInterests

Theauthorshavedeclaredthatnocompetinginterestsexist.

Funding

ThestudywassupportedbytheIndianCouncilofMedicalResearch,GovernmentofIndia.Thefundingagencyhadnoroleinstudydesign,datacollectionandanalysis,decisiontopublish,orpreparationofthepaper.

Acknowledgments

The authors thank Dr. A. C.Mishra, Director, National Institute of Virology, Pune, for thefacilitiesandencouragement.TheyappreciatethetechnicalassistanceprovidedbythefieldstaffofNIV.TheyarethankfultoDr.NeeleshDahanukar,IISER,Pune,forthevaluablesuggestionsandhelpinstatisticalanalysis.

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