Dengue and its vectors in Thailand: introduction to the study and seasonal distribution of Aedes larvae.

Daniel Strickman Armed Forces Research Institute of Medical Sciences, Walter Reed Army Institute of Research, Bangkok, Thailand. daniel.strickman@na.amedd.army.mil

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Pattamaporn Kittayapong Armed Forces Research Institute of Medical Sciences, Walter Reed Army Institute of Research, Bangkok, Thailand. daniel.strickman@na.amedd.army.mil

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A study was conducted from 1989 to 1994 that included surveillance for dengue and its mosquito vectors in 3 villages and 2 schools in Chachoengsao Province, 100 km east of Bangkok, Thailand. The study is introduced, and results of larval surveys for Aedes mosquitoes (predominantly Aedes aegypti (L.)), with 94, 86, and 90% of positive containers infested by this species in the hot, wet, and cool seasons, respectively), are described. These surveys were conducted in 1990-1991 during each of the 3 principal seasons: hot (February-April), rainy (May-October), and cool (November-January). Indoor maximum and minimum temperatures were consistently greater than outdoor temperatures. The differences between maximum and minimum temperatures both indoors and outdoors varied seasonally, with the greatest differences in the cool season and the least differences in the rainy season. The most rain fell in September and October and the least rain in December, January, and February. The number of Aedes larvae in each container was categorized (no larvae, 1-9 larvae, 10-50 larvae, > 50 larvae) by dipping with a fishnet or by visual examination (for drinking water). A larval index was calculated for each house and school by summing the estimations from each container. These indexes showed that one village had more larvae in every season (mean larval index per house = 117) than the other 2 villages (larval indexes of 86 and 70). The larval index of each house was mapped for each season, and the distribution was spatially analyzed by producing kriged estimates of interpolated data. These analyses showed that larvae were significantly concentrated in particular areas of the villages, especially during the wet season. Even when larvae were least abundant, every part of each village had at least some larvae. The results of the study imply that dengue vector control programs in Southeast Asian villages could increase their efficiency by applying their most energetic efforts on schools and areas with the greatest abundance of larvae, as measured by calculation of larval indexes.

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