Spatial distribution of the risk of dengue and the entomological indicators in Sumaré, state of São Paulo, Brazil

Abstract

Dengue fever is a major public health problem worldwide, caused by any of four virus (DENV-1, DENV-2, DENV-3 and DENV-4; Flaviviridae: Flavivirus), transmitted by Aedes aegypti mosquito. Reducing the levels of infestation by A. aegypti is one of the few current strategies to control dengue fever. Entomological indicators are used by dengue national control program to measure the infestation of A. aegypti, but little is known about predictive power of these indicators to measure dengue risk. In this spatial case-control study, we analyzed the spatial distribution of the risk of dengue and the influence of entomological indicators of A. aegypti in its egg, larva-pupa and adult stages occurring in a mid-size city in the state of São Paulo. The dengue cases were those confirmed by the city's epidemiological surveillance system and the controls were obtained through random selection of points within the perimeter of the inhabited area. The values of the entomological indicators were extrapolated for the entire study area through the geostatistical ordinary kriging technique. For each case and control, the respective indicator values were obtained, according with its geographical coordinates and analyzed by using a generalized additive model. Dengue incidence demonstrated a seasonal behavior, as well as the entomological indicators of all mosquito's evolutionary stages. The infestation did not present a significant variation in intensity and was not a limiting or determining factor of the occurrence of cases in the municipality. The risk maps of the disease from crude and adjusted generalized additive models did not present differences, suggesting that areas with the highest values of entomological indicators were not associated with the incidence of dengue. The inclusion of other variables in the generalized additive models may reveal the modulatory effect for the risk of the disease, which is not found in this study.

Figure 1. Municipality of Sumaré, São Paulo state, Brazil.

Legend of Figure 1: Municipality located in state of São Paulo, southeast region of Brazil. The area studied is shown highlighted.

Figure 2. Dengue cases and entomological indicators in stages: egg, larvae-pupae and adult forms.

Legend of Figure 2: The bar chart shows the number of dengue cases in the study area and the lines show the entomological indicators measured from February to December 2011. In order to compare the indicators seasonality in the same scale, the numbers of eggs were divided by 500 and the indicator of adult forms was divided by 5.

Figure 3. Ordinary Kriging maps of entomological indicators of Aedes aegypti and dengue cases.

Legend of Figure 3: The figure shows the estimated ordinary kriging of entomological indicators from February to July 2011 divided in four groups quarterly. The cases that occurred in the month corresponding to the middle of the period by each cluster are also plotted as black dots in the maps. The color gradient, corresponding to the variation range of the estimated entomological indicators, is shown for each map. For eggs indicator the values represent to the number of eggs. For indicators of larvae-pupae and adult mosquitoes, the values match to the percentage of positivity of blocks.

Figure 4. Spatial risk maps in crude and adjusted model (GAM).

Legend of Figure 4: The risk maps for the occurrence of dengue cases with the crude and adjusted models for the four quarterly groupings are shown with the odds ratio values defining the color gradient ranging of white (minor value) to red (highest values). The isolines show the values corresponding to the estimates generated by the GAM model. The dots and crosses plotted on maps are showing respectively controls and dengue cases for the period.