Determination of Breeding Habitats and Seasonal Prevalence of Larvae of Aedes aegypti (L) and other Possible Vector of Dengue in Kathmandu Valley

Abstract

Summary: DF/DHF occurs primarily in tropical and subtropical areas of the world, prevalent in over 100 countries and threatens the health of approximately 100 million cases of DF, 500,000 cases of DHF, and several thousand deaths occur annually worldwide. During the past decades, dengue virus has emerged in southern Asia; DF/DHF epidemics have occurred in Bhutan, India, Maldives, Bangladesh, and Pakistan. The incidence of dengue has been increasing in recent years in densely populated areas in Nepal. Dengue viruses are transmitted from viremic to susceptible human beings by various mosquitoes of the subgenus Stegomyia, notably day-biting mosquitoes Aedes aegypti and Aedes albopictus, vectors throughout tropical and sub-tropical areas. In this study, house-to-house surveys oflarval breeding places and larvalprevalence were conducted covering all three districts of Kathmandu valley during April,May, June, July, August, September and October in 2009. The sample sites includedKoteshwar, Manahara, Gongabu-Balaju, Tokha, Jorpati and Kalanki (Kathmandudistrict), Tikathali, Lokanthali, Kaushaltar, Thimi, Darbar square and Bode (Bhaktapur district). Satdobato, Thalchkhel, Mahalaxmisthan, Dhobighat, Sanepa, Bagalamukhi, Dholahiti, Chibahal, Thecho, Chapagaon, Badegaon, and Godawari (Lalitpur district) of Kathmandu valley. Collection sites were selected in both town and rural residential areas. Nine major Water-filled containers discarded tires, metal /plastic drums, plastic buckets, flower pots, mud pots, cement tank, metal containers, plastic pots and miscellaneous small, discarded items such as tin cans, jars and plastic food containers were searched for he presence or absence of Aedes larvae and/or pupae. Adult Aedes mosquitoes were examined with the help of aspirators and flash-lights. All live mosquito larvae and pupae collected were reared until adult emergence and identified. Main purpose of the selecting the areas for the study is to know if there are distribution, abundance of Aedes aegypti and other possible vector of dengue in urban areas of Kathmandu valley. No information is available regarding seasonal abundance of Dengue vector, Aedes aegypti in these districts. Primary end points of this study were the Breteau index and the house index; the secondary end point was the container index.Larval survey techniques were used to obtain the House Index, Container Index and Breteau Index. The container preferences of Aedes mosquitoes were assessed by calculation of breeding preference ratio (BPR). The highest HI, CI and BI recorded for Aedes aegypti were 4.42, 3.63 and 9.73 respectively in October, and 9.91, 12.45 and 31.40 respectively for Aedes albopictus in September, 2009 (Kathmandu district). Whereas, HI, CI and BI recorded for Aedes aegypti in Lalitpur district were 5.50, 4.06 and 12.80 respectively in October, 2009. For Aedes albopictus, the highest HI, CI and BI recorded for Aedes albopictus in Lalitpur district were 15.64 in August, 9.83 and 25.30 in September, 2009. The highest BPR recorded for Aedes aegypti andAedes albopictus in Kathmandu district were 1.89 and 1.89 in August, 2009. Similarly, the highest BPR recorded for Aedes aegypti and Aedes albopictus in Lalitpur district were 1.69. It was observed that discarded tires lying outdoors were the preferred breeding habitats. No breeding of Aedes aegypti was observed in other containers during this seven months survey. However, breeding of Aedes albopictuswas also observed in metal drum in Mangal bazar and in metal container in Gwarko of Lalitpur district . From the present entomological investigations, it can be concluded that Aedes aegypti andAe. albopictus are established within the urban agglomeration of Basundhara, Gongabu-Balaju area of Kathmadu district and Satdobato-Gwarko, Thalchikhel, Mahalaxmithannad Sanepa area in Lalitpur district showing larval indices. Overall, this study brings out an idea regarding the current distribution of Aedes aegypti and Aedes albopictus in Kathmandu valley for implementation of vector control operation. From a previous study on Kathmandu (Darsie and Pradhan, 1990,1994), Aedes aegyptihad not been previously recorded inthe Kathmandu valley. And, the breeding habitats occupied by immature mosquitoes had not been previously determined. In other words, information on seasonal distribution and breeding habitats of Aedes aegypti and Aedes albopictus in Kathmandu valley was lacking. Since Aedes aegypti has been newly emerged increasing its range throughout the country, an entomologicalinvestigation has been directed toward detecting its presenceor absence in Kathmandu district by the principal investigator. The Herewith, the principal investigator report the first (1st) collection and identification of Aedes aegypti in Kathmandu and Lalitpur district in 2009. Indeed, this finding constitutes the first record from the districts and the collection represents the Basundhara and Gongabu-Balaju and Gwarko–Satdobato area for this species within the Kathmandu valley. There has been a significant increase inthe human population, demographic and vehicular movement of the people and accommodation-based tourism facilities in Nepal. Continued and increased urbanization and vehicular movement and use in Kathmandu valley have been resulted in increased amount of non-biodegradable objects (tires). The accumulation of all sorts of discarded or unused tires in dwellings has resulted in occurrence of Aedes aegypti and Aedes albopictus. As a result, crowded urban human populations live in intimate contact with large mosquito populations, thus creating ideal conditions for increased mosquito-borne disease transmission. Finally, increased and more rapid travel possibly lead to increased movement of dengue viruses within the area. Alternative strategies are needed which can be implemented through participation continuous participation of people in the community and which compliment the efforts of the vector control teams. The Aedes aegypti control programme is needed to make people aware of the threat poses to their health, and to educate them on how they can reduce this threat by either eliminating the potential breeding habitats which harbour the mosquito larvae or by preventing mosquitoes from having access to water-holding containers for egg-laying that are use din or around houses.