20 Distribution along the urban-rural gradient Generally, Ae. aegypti is highly adapted to the domestic environment and therefore the abundance is positively correlated with increasing urbanization. On the other hand, the distribution of Ae. albopictus is associated with vegeta- tion throughout rural and urban areas and the abundance is adversely affected by urbanization [3, 11, 33–39]. This difference in distribution along the urban-rural gradient is associated with the behavior related to blood feeding, resting, host preference and preference for vegetation. It was reported that habitat segregation in an environment where Ae. aegypti and Ae. albopictus have different niches plays an important role in the co-existence of dengue vectors even if the larvae have similar habitat requirements [3, 11, 39]. These studies suggest that the former species may be abundant in urban areas because the urban environment favors it, while the latter species may be abundant in rural areas because this environments usually has more vegeta- tion than urban areas and therefore favors the latter species. Since the distribution of Ae. aegypti and Ae. albopictus is highly influenced by climate, the response of the two species to environmental heterogeneity may be modified according to geographically different areas. Infestation of the two species along the urban-rural gradient may differ among areas. Therefore, it is hypothesized that, in Ae. albopictus (or Ae. aegypti) dominant regions, one of the species may be able to occupy a niche if it is unoccupied, even in urban (or rural) areas, because the population of Ae. aegypti (or Ae. albopictus) would be small. In regions where the two species co-exist, habitat segregation in which Ae. aegypti is abundant in urban areas and Ae. albopictus in rural areas would be conductive to co-existence. The infestation of Ae. aegypti and Ae. albopictus along the urban-rural gradient in northern, central and southern in Vietnam was examined [43]. In the north, Ae. aegypti was mainly collected from transition areas, while Ae. albopictus was collected throughout urban and rural areas. In the south, Ae. aegypti was dominant throughout urban and rural areas. In the central part of the country, Ae. aegypti was dominant throughout urban and rural areas while Ae. albopictus was observed in urban areas. These findings indicated that the infestation of Ae. aegypti and Ae. albopictus along the urban-rural gradient differed among geographic areas. In the central part of the country, however, the typical habitat segregation of the two species between urban and rural areas as hypothesized was not observed. A limitation of the study was that larvae were collected from used tires only [43]. The results of other studies revealed a variety of larval habitats throughout urban and rural and north to south gradients in Vietnam [12, 48–50]. The way in which People stored water affected mosquito larval occurrence. Moreover, Tropical Medicine and Health Vol.39 No.4 Supplement, 2011 Ae. aegypti and Ae. albopictus exhibited a different prefer- ence for containers [2, 3]. In Indonesia, differing coverage of piped water in urban and rural areas affected the infesta- tion of Ae. aegypti and Ae. albopictus [51]. The distribution in urban and rural areas differed among countries in which dengue control targeting a certain species affected the infestation [36]. These studies suggest that environmental factors created by humans as well as climatic factors greatly influence local distribution of dengue vectors. The distribu- tion along the urban-rural gradient can be regarded as a consequence of those factors. Since Ae. aegypti with a higher dissemination rate of dengue viruses than Ae. albopictus is abundant in urban areas, DF/DHF is referred to as an urban disease. The differ- ence in the infestation of Ae. aegypti and Ae. albopictus along the urban-rural gradient has been extensively studied in view of the ecological and epidemiological importance [11, 33, 34, 36, 37, 39, 43, 52, 53]. Thus, it is worthwhile to study the relationship between the distribution of Ae. aegypti and Ae. albopictus and the environment related to urbanization in order to understand population trends in changing environments and the ecological basis of the spatial distribution as well as to develop effective mosquito control measures [3, 33, 34, 36–38, 43, 54]. It will also be helpful to assess high-risk areas with high vector densities. It is difficult to describe the environment quantita- tively. Categories such as urban, suburban and rural are subjective, but the recent development of remote sensing technology with satellite imagery and the geographical information system (GIS) in which an environment is visible and can be estimated by indices such as the normalized difference vegetation index (NDVI) has opened new windows for analyses related to mosquito control strategy [34, 55– 58]. The quantitative estimation of an environment by freely accessible satellite imagery and meteorological data is applicable to various countries and regions. Although it is still difficult to estimate the degree of urbanization/domesti- cation quantitatively, Tun-Lin et al. (1995) proposed “the premise condition index” reflecting the house condition, yard condition and degree of shade, and they successfully related this to the presence of Ae. aegypti [59]. Studies such as this help to streamline dengue vector surveillance. Quan- titative estimation is considered significant especially for dengue epidemic countries experiencing uncontrolled urbani- zation. Recently, some studies have reported that Ae. albopictus is increasing in the indoors and urban cities [43, 60, 61]. Although the reasons remain unclear, it is likely that, when a society develops to some extent, parks with vegetation are constructed and people cultivate gardens at home. Since Ae. albopictus can breed from a wide range of habitats and feed