The results of this study reinforce the notion that the spatial heterogeneity found in the occurrence of the disease is directly related to the living conditions of the population and environmental characteristics of household neighborhood . However, though these relations were possible to be characterized in this study, the validation sample results were not fully satisfactory, indicating that the prediction of areas at high risk for VL incidence is a more complex challenge than the simple identification of associations between environmental and socioeconomic factors and disease incidence, which has been already shown in Brazil and in the world [8–14].
A series of potential explanations for the deficient predictive ability of the model developed could be identified. First, the time interval between both periods analyzed implied in substantial modifications on the dimension and structure of the geographical area under study, including the incorporation on the validation sample of areas that were considered rural in the period of 1993–1996. Second, the data used to derive the predictive model refers to an epidemic period, while the validation sample entails endemic years [12, 15]. It is reasonable to suppose that in epidemic situations, due to the typical large magnitude of transmission during these periods, the disease spreads more largely in the geographic space, affecting population subsets that could have eventually been spared in endemic periods. However, having in mind that the VL was only introduced in Teresina in the beginning of the 1980s triggering two epidemics (1981–1985 and 1993–1996), there is still no set of complete data for a typical endemic period that not of 2001–2006. An alternative would be to restrict the analysis strictly to the years between 2001 and 2006, but that would bring difficulties related to the small number of annual cases observed in this period.
Third, the unsatisfactory performance of the model could be due to positive quantitative variations of the socioeconomic indicators, even if the distribution quartiles were used as cutoff points. Since the 1980s in Brazil there has been an improvement on the social and economic indicators. For example, a historical series analysis since the mid-1970s shows a substantial and unequivocal fall of inequality from 2001 to 2004, this last year having the smallest income inequality of the period analyzed . In Piauí the situation is not different, having occurred an increase of 12% of the average income (1985–2006), contributing to the increase in the human development index from 0.57 to 0.70 (1991–2005) and to the reduction of the illiteracy rate from 41.7% to 23.3% (1991–2007) [17, 18]. A possible alternative to minimize this problem would be employing a categorization of the indicators by strata of homogeneous areas according to living conditions [19, 20].
The VL transmission niches in urban environments not only present a heterogeneous distribution, but also constitute areas with varied landscape and epidemiologic characteristics, where distinct forms of occupation and soil coverage implicate in ecological and social processes which result in huge differences of magnitude on the incidence of the disease.
Therefore, it is possible to infer that the process of establishing and dissemination of VL in the urban environment of Teresina, having a markedly heterogeneous spatial distribution, results from the socio-territorial organization. Nevertheless, we believe that part of the predictive deficiencies of the model is owing to the lack of a better definition of the territory as a spatial-social-environmental unit favorable to VL occurrence.
The improvement on the predictive performance of area at high risk for VL could be attained with the use of more environmental indicators extracted from the satellite images of medium resolution [21, 22] or with the use of remote sensing images with high spatial resolution . The satellite images used in this study come from the Landsat Thematic Mapper 5 sensors, which have spatial resolution of 30 meters, in other words, it does not allow the discrimination of elements on the earth’s surface for areas smaller than 900 m2. However, VL studies that used medium spatial resolution were able to identify some elements related to vegetation coverage, soil use and patterns of urban occupation associated to the risk of the disease [12, 22, 24, 25], but the difficulties and limitations of these images to properly characterize the local features that determine the pattern of transmission of VL on the urban context are evident . In future studies, high spatial resolution images should be used to better define the local environmental features related to VL occurrence .
At last, considering the complexity of the factors involved in the VL dissemination in the urban environment, the results of this study demonstrate that the occurrence of VL on the outskirts of Teresina is intensely related to socioeconomic and environmental problems, arising from the urban expansion process and from the changes in the vector habitat, due to the environmental imbalance caused by deforestation and land occupation with lack of adequate urban infrastructure . In this perspective, focusing interventions in these considered high risk areas could be a useful strategy to improve the effectiveness of control measures while decreasing operational costs.