Tropical Biomedicine 16: 25-29 (1999)

A comparative study on sampling techniques for Aedesaegypti(Diptera: Culicidae) surveillance in Madurai,South India (f)

N. Arunachalam, P Philip Samuel, J. Hiriyan and A. GajananaCentre for Research in MedicalEntomology,No.4, Sarojini Street, Chinna Chokkikulam,Madurai-625002,TamilNadu,India.

Abstract. Tin ovitrap fabricated from locally available materials was compared with Sheppard's singlelarva method, and resting and landing adult collections for surveillance of Aedes aegypti in Madurai,an urban area. Ovitrap index showed significant positive correlation with Breteau index as well asadult indices. During summer months, ovitrap could deteCt the presence of Ae. aegypti females whenno Ae. aegypti female adults could be collected.

INTRODUCTION

Dengue and its severe forms, denguehaemorrhagic fever (DHF) and dengueshock syndrome (DSS) are caused bymosquito transmitted dengue virusesbelonging to genus Flavivirus, familyFlaviviridae. Aedes aegypti (Linnaeus) isthe recognized major vector of dengue inubran localities and its invasion to ruralareas due to urbanization has beenrecorded. This species breeds in domesticand peridomestic water containers andadults rest inside the houses (Service,1992). Vector control is the only optioncurrently available to contain dengueoutbreak and therefore vector surveillance

forms an integral part of dengue controlprogramme for monitoring seasonalchanges in vector population. The recentdengue outbreaks in different parts ofIndia and elsewhere underline the need todevelop simple and reliable tools for Ae.aegypti surveillance.

World Health Organization hasrecommended adult and larval samplingmethods for surveillance of Ae. aegypti(Pant & Self, 1993). Disadvantages and

deficiencies of these methods have been

highlighted by investigators (PAHO, 1994;Focks & Chadee, 1997). In most studies onthe prevalence of Ae. aegypti, the methodof choice has been the single larva surveytechnique (Sheppard et al., 1969). But thismethod is inherently deficient in manyaspects, like omission of a single housewith high number of positive containerswill lower the larval index considerably(Tun-Lin et al., 1995). Larval survey provedto be less efficient system of evaluationwhen Ae. Aegypti adult population fall to

low levels (PARO, 1994). 1;he larval indexalso did not take into account the

containers that vary in the production ofadults (Focks &..Chadee, 1997).

Since adult mosquitoes are involved indisease transmission, it will be moreprudent to monitor adult population of Ae.aegypti over time. But the conventionaladult sampling methods (indoor restingand landing collections) are labourintensive and hence are not suitable for

routine use. On the otherhand, ovitraps aresimple devises to monitor adult populationeasily. The development of ovitrapprovided a potential new approach for

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Aedes surveillance (Fay & Perry, 1965).However, only a few studies have beencarried out to evaluate the ovitrap incomparison with larval and adultdensity measurement techniques (Kaul& Geevarghese, 1979). We designed anovitrap from easily available localmaterials and evaluated it in the field incomparison with larval and adult densitymeasurement mehtods.

MATERIALS AND METHODS

The study was carried out in tworesidential areas in Madurai town,Goripalayam and Mathichiyam, withdifferent socio-economic backgrounds.Goripalayam is mainly inhabited bypersons in the middle-income group withpopulation of about 5000 living in concretebuildings. Mathichiyam is a slum areaoccupied mainly by persons in low-incomegroup with a population of about 6000living in thatched huts. In both localitieswater is stored in cement cisterns and

plastic and iron drums placed outdoors.The mean monthly maximum

temperature was recorded in June (37.3°C)and December was the coolest month

(30.0°C). Rainfall was concentrated mainlybetween October and November (Fig. 1).

Between September 1997 and August1998, all the 164 houses in one block ofGoripalayam and 196 houses in a block inMathichiyam were surveyed fortnightlyfor Aedes breeding in water storagecontainers using single larva surveymehtod (Sheppard et al., 1969) and thelarval density was expressed as Breteauindex (No. of infested containers per100 houses).

Adult Ae. aegypti females restingindoors were collected in the morninghours (between 0900-1100 hrs) using flashlight and suction tube. Eight houses wereselected randomly in each study area andthe houses were covered by two insectcollectors in an hour. The resting densitywas expressed as number of Ae. aegypti

females collected per man-hour. Landingcollections were also conducted indoors inmorning hours. A collector collected themosquitoes landing on a human volunteer(one of the members of our team or amember of the house-hold in whichmosquitoes were collected) and a total ofeight randomly selected houses werecovered in two hours by two insectcollectors in each study area. The landingdensity is expressed as number of femalemosquitoes collected per man per hour.

The ovitrap consisted of a blackpainted tin container (500 ml capacity)into which a brown hard board strip (2" X6") was placed as oviposition paddle andfilled with 250 ml water. The paddle waskept in a slanting position with roughsurface facing up. After a few minutes, theupper half of the paddle became wet byabsorbing water through capillary action.Thirty traps were used for each study area.A total of 30 houses were selected atrandom and the traps were kept inshady and secluded locations like underthe roof, bathroom and backyard. Afterapproximately 48 hrs. the traps weretransferred to the laboratory andexamined for the presence of Aedes eggs.Eggs were counted in positive paddlesand the density was expressed as meannumb-er of eggs per trap. The paddlewith eggs were kept immersed in a traywith water to induce hatching and theimmatures were reared to adults forspecies identification.

Pearson's product ~oment correlationwas used to determine correlation

r ..'between ovitrap and' bther samplingmethods.

"\

........

---'-"

RESULTS AND DISCUSSION

Aedes aegypti was the only Aedes speciesencountered in all types of domestic andperidomestic water containers. Cementcisterns constituted 77.3% of totalcontainers in Goripalayam and 62.5% inMathichiyam. Discarded grinding stones

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Figure 1. Seasonal trend in Aedes aegypti population indices in Madurai.

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TempC I RH (%) Ra!nfall (mm)I 300100 I

:...-Tem. Max. ....R.H

801 I "'" l8Rainfall J I 1250200

I - - ----- -----. --- - I60

I - - .. I150

I . . . I40100

20 j I I I I Ilso0 ,- l1li.. - - - -'0

18 Breteau index16

nDGoripalayam

14 8Mathichiyam121086420

30 I_... ... ...... - 'OI/itrap Index'

25-Resting density

25 -i (liOnpalayam) .L/ \ ILanding density I- 20

201 //\ \ l-'-Ovitfap index j/ I- 15

15

': ] / \-\ A t:o0 I - ..-- ..- ..... .-r I 0

25 I I 35Adult.de nsity(9.9./M.H) ---Resting density OI/itrap Index(Mathichiyam)

-e-Landing density I- 3020-1

......Ovitrapindex J

r 2515 IN \\ 20

101 IV \\ \ f - r 15- 10

I .fI{ ,// "\\ \ II . I

55

0

"f, 1-0I I I I I . I I

&' ,.\ rl' cp « 1J ')<::- ') ,§0

o<:J ')tb-<::-

and mud pots with rain water collectionswere relatively more prevalent (21.14%)inMathichiyam than in Goripalayam (8.3%).Breteau index (Bl) ranged from 3.7(September 1998) to 15.9 (December1998) in Goripalayam and 1.5 (April 1998)to 7.1 (November 1997) in Mathichiyam.

Adult density increased during cool-wet months (October-January) followedby a sharp decline in hot-dry months(February-May) in both areas (Fig. 1).Indoor resting PMH density was highest(28.5 in Goripalayam and 20 inMathichiyam) in December and no adultscould be collected during March and Aprilin Goripalayam and March-May inMathichiyam. Landing PMH densitiesranged from 0 (March-April) to 25.3(November), in Goripalayan1 and 9 (March-May) to 17 (November) in Mathichiyam.

Aedes aegypti was the only Aedesspecies found in ovitraps. Ovitrap indexshowed the same seasonal trend as thatof adult indices (Fig. 1). A significantpositive correlation was observedbetween ovitrap index vs. Breteauindex. (Goripalayam, r=0.50, P<0.05,Mathichiyam, r=0.74, P<O.OI), ovitrapindex vs. adult resting density(Goripalayam, r=0.59, P<0.O25, Mathi-chiyam, r=0.88, P<0.005) and ovitrap indexvs. landing adult density (Goripalayam,r=0.63, P<O.025, Mathichiyam, r=0.88,P<0.O05).

This study demonstrated that ovitrapscould detect temporal changes in Ae.aegypti population. The ovitrap indexshowed significant positive correlationwith adult densities in both the studyareas. It was interesting to note that whenno Ae. aegypti adults were taken in bothresting and landing collections duringsummer months, ovitaps detected thepresence of adults during the sameseason. This was similar to a sutdy inPoona (Geevarghese et al., 1975) and Portof Spain, Trinidad (Rawlins et al., 1998).The ovitrap designed by us is made fromcheap materials like used tins and hardboard, whereas glass jar used by others is

expensive, liable to be stolen, easilybreakable and is difficult to transport inlarge numbers to the field.

Ovitrap surveillance offers economicalsampling method which could be executedwith limited resources and personnel. Acollector spent an hour for four houses foradult sampling whereas he covered thirtyhouses for placing or removing theovitraps within the same time. Because ofminimum involvement of personnel, theelement of human error is reduced inovitrap sampling. Community compliancefor use of ovitraps (which are placedoutdoors) is high unlike larval and adultsampling methods which often requires aperson to enter houses. In conclusion, thisstudy showed that the black tin ovitrap isa simple and effective contraption for Ae.aegypti surveillance.

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Acknowledgements. The authors arethankful to Shriyuts A Veerapathiran, VR.Alagan and R. Moorthy for the excellenttechnical assistance.

REFERENCES

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Geevarghese, G., Kaul, H.N. & Dhanda,V. (1975). Observations on thereestablishment of Aedes aegyptipopulation in Poona city and suburbs,Maharashtra state, India. IndianJournal of Medical Research 63: 1155.

Kaul, H.N. & Geevarghese, G. (1979). Acomparative study of ovitrap andsingle larva survey methods for thesurveillance methods surveillance of

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