A comparative study on sampling techniques for Aedes aegypti(diptera: Culicidae) surveillance in Madurai,

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Deborah Carpenter
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1 ~ Tropical Biomedicine 16: (1999) A comparative study on sampling techniques for Aedes aegypti(diptera: Culicidae) surveillance in Madurai, South India (f) N. Arunachalam, P Philip Samuel, J. Hiriyan and A. Gajanana Centre for Research in MedicalEntomology,No.4, Sarojini Street, Chinna Chokkikulam,Madurai-6252, TamilNadu,India. Abstract. Tin ovitrap fabricated from locally available materials was compared with Sheppard's single larva 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 as adult indices. During summer months, ovitrap could detect the presence of Ae. aegypti females when no Ae. aegypti female adults could be collected. INTRODUCTION Dengue and its severe forms, dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS) are caused by mosquito transmitted dengue viruses belonging to genus Flavivirus, family Flaviviridae. Aedes aegypti (Linnaeus) is the recognized major vector of dengue in ubran localities and its invasion to rural areas due to urbanization has been recorded. This species breeds in domestic and peridomestic water containers and adults rest inside the houses (Service, 1992). Vector control is the only option currently available to contain dengue outbreak and therefore vector surveillance forms an integral part of dengue control programme for monitoring seasonal changes in vector population. The recent dengue outbreaks in different parts of India and elsewhere underline the need to develop simple and reliable tools for Ae. aegypti surveillance. World Health Organization has recommended adult and larval sampling methods 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 on the prevalence of Ae. aegypti, the method of choice has been the single larva survey technique (Sheppard et al., 1969). But this method is inherently deficient in many aspects, like omission of a single house with high number of positive containers will lower the larval index considerably (Tun-Lin et al., 1995). Larval survey proved to be less efficient system of evaluation when Ae. Aegypti adult population fall to low levels (PARO, 1994). 1;he larval index also did not take into account the containers that vary in the production of adults (Focks &..Chadee, 1997). Since adult mosquitoes are involved in disease transmission, it will be more prudent to monitor adult population of Ae. aegypti over time. But the conventional adult sampling methods (indoor resting and landing collections) are labour intensive and hence are not suitable for routine use. On the otherhand, ovitraps are simple devises to monitor adult population easily. The development of ovitrap provided a potential new approach for 25

2 Aedes surveillance (Fay & Perry, 1965). However, only a few studies have been carried out to evaluate the ovitrap in comparison with larval and adult density measurement techniques (Kaul & Geevarghese, 1979). We designed an ovitrap from easily available local materials and evaluated it in the field in comparison with larval and adult density measurement mehtods. MATERIALS AND METHODS The study was carried out in two residential areas in Madurai town, Goripalayam and Mathichiyam, with different socio-economic backgrounds. Goripalayam is mainly inhabited by persons in the middle-income group with population of about 5 living in concrete buildings. Mathichiyam is a slum area occupied mainly by persons in low-income group with a population of about 6 living in thatched huts. In both localities water 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 (3. C). Rainfall was concentrated mainly between October and November (Fig. 1). Between September 1997 and August 1998, all the 164 houses in one block of Goripalayam and 196 houses in a block in Mathichiyam were surveyed fortnightly for Aedes breeding in water storage containers using single larva survey mehtod (Sheppard et al., 1969) and the larval density was expressed as Breteau index (No. of infested containers per 1 houses). Adult Ae. aegypti females resting indoors were collected in the morning hours (between 9-11 hrs) using flash light and suction tube. Eight houses were selected randomly in each study area and the houses were covered by two insect collectors in an hour. The resting density was expressed as number of Ae. aegypti females collected per man-hour. Landing collections were also conducted indoors in morning hours. A collector collected the mosquitoes landing on a human volunteer (one of the members of our team or a member of the house-hold in which mosquitoes were collected) and a total of eight randomly selected houses were covered in two hours by two insect collectors in each study area. The landing density is expressed as number of female mosquitoes collected per man per hour. The ovitrap consisted of a black painted tin container (5 ml capacity) into which a brown hard board strip (2" X 6") was placed as oviposition paddle and filled with 25 ml water. The paddle was kept in a slanting position with rough surface facing up. After a few minutes, the upper half of the paddle became wet by absorbing water through capillary action. Thirty traps were used for each study area. A total of 3 houses were selected at random and the traps were kept in shady and secluded locations like under the roof, bathroom and backyard. After approximately 48 hrs. the traps were transferred to the laboratory and examined for the presence of Aedes eggs. Eggs were counted in positive paddles and the density was expressed as mean numb-er of eggs per trap. The paddle with eggs were kept immersed in a tray with water to induce hatching and the immatures were reared to adults for species identification. Pearson's product ~oment correlation was used to determine correlation r..' between ovitrap and' bther sampling methods. RESULTS AND DISCUSSION Aedes aegypti was the only Aedes species encountered in all types of domestic and peridomestic water containers. Cement cisterns constituted 77.3% of total containers in Goripalayam and 62.5% in Mathichiyam. Discarded grinding stones " \ '-" 26

3 1 TempC I RH (%) I :...-Tem. Max....R.H 81 I 6 4 "'" l8rainfall J Ra!nfall (mm) I 3 I I I I I 15 I... I 2 j I I I I Ilso,- l1li ' 18 Breteau index 16 DGoripalayam 14 n 8Mathichiyam I_ ' 25 -Resting density OI/itrap Index' 25 -i (lionpalayam).l/ \ ILanding density I- 2 l-'-ovitfap index j 21 //\ \ / I ': ] / I - \-\ A 1 t:o r I 25 I I 35 Adult.de nsity(9.9./m.h) ---Resting density OI/itrap Index (Mathichiyam) -e-landing density I Ovitrapindex J r IN \\ 2 11 IV \\ \ f - r I.fI{,// "\\ \ II. I " I I I I I f,. I I 1- &' o,.\ rl' cp «1J <:J ')tb-<::- ')<::- '), 5 Figure 1. Seasonal trend in Aedes aegypti population indices in Madurai. 27

and mud pots with rain water collections were relatively more prevalent (21.14%)in Mathichiyam than in Goripalayam (8.3%). Breteau index (Bl) ranged from 3.7 (September 1998) to 15.

4 and mud pots with rain water collections were relatively more prevalent (21.14%)in Mathichiyam than in Goripalayam (8.3%). Breteau index (Bl) ranged from 3.7 (September 1998) to 15.9 (December 1998) in Goripalayam and 1.5 (April 1998) to 7.1 (November 1997) in Mathichiyam. Adult density increased during coolwet months (October-January) followed by 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 2 in Mathichiyam) in December and no adults could be collected during March and April in Goripalayam and March-May in Mathichiyam. Landing PMH densities ranged from (March-April) to 25.3 (November), in Goripalayan1 and 9 (March- May) to 17 (November) in Mathichiyam. Aedes aegypti was the only Aedes species found in ovitraps. Ovitrap index showed the same seasonal trend as that of adult indices (Fig. 1). A significant positive correlation was observed between ovitrap index vs. Breteau index. (Goripalayam, r=.5, P<.5, Mathichiyam, r=.74, P<O.OI), ovitrap index vs. adult resting density (Goripalayam, r=.59, P<.O25, Mathichiyam, r=.88, P<.5) and ovitrap index vs. landing adult density (Goripalayam, r=.63, P<O.25, Mathichiyam, r=.88, P<.O5). This study demonstrated that ovitraps could detect temporal changes in Ae. aegypti population. The ovitrap index showed significant positive correlation with adult densities in both the study areas. It was interesting to note that when no Ae. aegypti adults were taken in both resting and landing collections during summer months, ovitaps detected the presence of adults during the same season. This was similar to a sutdy in Poona (Geevarghese et al., 1975) and Port of Spain, Trinidad (Rawlins et al., 1998). The ovitrap designed by us is made from cheap materials like used tins and hard board, whereas glass jar used by others is expensive, liable to be stolen, easily breakable and is difficult to transport in large numbers to the field. Ovitrap surveillance offers economical sampling method which could be executed with limited resources and personnel. A collector spent an hour for four houses for adult sampling whereas he covered thirty houses for placing or removing the ovitraps within the same time. Because of minimum involvement of personnel, the element of human error is reduced in ovitrap sampling. Community compliance for use of ovitraps (which are placed outdoors) is high unlike larval and adult sampling methods which often requires a person to enter houses. In conclusion, this study showed that the black tin ovitrap is a simple and effective contraption for Ae. aegypti surveillance. Acknowledgements. The authors are thankful to Shriyuts A Veerapathiran, VR. Alagan and R. Moorthy for the excellent technical assistance. REFERENCES Fay, RW. & Perry, AS. (1965). Laboratory studies of ovipositional preferences of Aedes aegypti. Mosquito News. 25: Focks, D.A. & Chadee, D.D. (1997). Pupal survey: An epidemiological significant surveillance method for Aedes aegypti: An example using data from Trinidad. American Journal of Tropical Medicine and Hygiene 56: Geevarghese, G., Kaul, H.N. & Dhanda, V. (1975). Observations on the reestablishment of Aedes aegypti population in Poona city and suburbs, Maharashtra state, India. Indian Journal of Medical Research 63: Kaul, H.N. & Geevarghese, G. (1979). A comparative study of ovitrap and single larva survey methods for the surveillance methods surveillance of ~ "". 28

5 Aedes aegypti in Pune city. Indian Journal of Medical Research 69: Pan American Helath Organization. (1994). Dengue and dengue haemorrhagic fever in the Americas: Guidelines for prevention and control. Scientific Publication 548, Washington, DC. Pant, C.P. & Self, 1.S. (1993). Vector ecology and bionomics. In: Monograph on dengue and dengue haemorrhagic fever. World Health Organization Regional Publications SEARO No. 22, Rawlins, S.C., Martinez, R, Wiltshire, S. & Legan, G. (1998). A comparison of surveillance systems for the dengue vector Aedes aegypti in Port of Spain, Trinidad. Journal of American Mosquito Control Association 14: Service M.W. (1992). Importance of ecology in Aedes aegypti control. Southeast Asian Journal of Tropical Medicine and Public Health 23: Sheppard, P.M., Macdonald, M.W. & Tonn, RJ. (1969). A new method of measuring the relative prevalence of Aedes aegypti. Bulletin of World Health Organization 4: 467. Tlm-lin, W., Kay,B.H. & Barnes, A. (1995). Understanding productivity, a key to Aedes aegypti surveillance. American Journal of Tropical Medicine and Hygiene 53:

Aedes aegypti Larval Habitats and Dengue Vector Indices in a Village of Ubonratchathani Province in the North-East of Thailand

254 KKU Res. J. 2015; 20(2) KKU Res.j. 2015; 20(2) : 254-259 http://resjournal.kku.ac.th Aedes aegypti Larval Habitats and Dengue Vector Indices in a Village of Ubonratchathani Province in the North-East