Dengue fever is a serious acute illness caused by infection with one of four viral strains. It is spread through the bites of infected mosquitoes, the primary vector in Venezuela being Aedes aegypti. The objective of this study was to determine whether temperature and other environmental factors limit the distribution of Aedes aegypti in high altitude areas, while controlling for socioeconomic determinants. During the summer of 2009, 24 randomly selected sites distributed across three cities (El Vigia, Ejido and Merida) ranging in altitude from 70 to 1950m were surveyed for mosquito breeding habitats in and around the homes. Occupant interviews were conducted to assess socioeconomic status and access to relevant infrastructure, such as trash collection and water services. For the highest and largest city (Merida), environmental variables were derived from an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite image. These included land surface temperature, elevation, the mean and standard deviations for each ASTER spectral band, the Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Built Index (NDBI). In order to confirm that Merida was located at the limit of Ae. aegypti distribution, we also surveyed two cemeteries in towns situated at altitudes higher than our survey sites (cemeteries are prime habitats for Ae. aegypti): Mucuchies (3000m) and Mucurubá (2400 m).
Results/Conclusions
El Vigia, the town at the lowest elevation, had a higher proportion of positive containers (17.7%) than Merida and Ejido (10.5% and 10.7%, respectively). Water holding containers were more common in El Vigia, while flower pots and vases – linked to better socioeconomic conditions, were more frequent in the other two towns. In Merida, the proportion of positive containers was higher in warmer and less vegetated areas (lower NDVI). Mean house size and number of inhabitants were also positive predictors, while access to water and trash services was not found to be predictive, in contrast with previous studies in coastal Venezuela. We did not find any Ae. aegypti larvae in the high altitude towns. The fact that climate, rather than socioeconomic factors, may limit the distribution of Ae. aegypti – and thus of dengue, at the altitudinal limit of its distribution suggests the potential for future expansion with climate change.
