Study area
The study was conducted in the district of Três Fronteiras (DTF), ciy of Colniza, northwestern region of Mato Grosso, located on the border with the state of Rondônia and Amazônia, 1,530 km distant from the capital city of Cuiabá (Figure 1). The health service in the FTD is scarce, having only a health clinic that performs exams of malaria and basic nursing care. The community of the FTD has no basic sanitation, electricity, access to mobile phones or landlines. In the rainy period, there is limited access to the DTF, due to the precarious state of the roads between the district and neighboring cities. Its main economic activities are cassiterite mining, gold prospecting and the timber exploitation (COLNIZA, 2014).
The estimated population of DTF is 2000 people. However, there is an intense migratory flow between people of the three bordering states, due to their economic activities. During certain period of the year, many workers live in the DTF, usually during the non-rainy months. During the rainy period, they move to one of the larger cities in the region (Machadinho-RO).
For the analysis of the geographical distribution of malaria cases, the area of the DTF was divided into three strata: Mining Company (Cassiterite/Gold mining), Logging/Sawmills and Village. The Mining Company stratum contain the mining locations São Francisco, Fazenda Comil and Igarapé Grande Artisanal Mining. The Logging/Sawmills concentrates loggers, sawmills and rolling mills. Village comprises the Village of Três Fronteiras, in addition to the households located on Rodovia do Estanho, from BR 174 through Lagoa das Conchas and Vila T (Figure 1).
Type of study:
Descriptive study of the ecological type, comprising information on the prevalence of infection by plasmodium and the use and occupation of the land by the population from DTF in July 2011. The prevalence of infection was obtained from a hemoscopic investigation of all (100%) inhabitants of DTF, which provided records of sociodemographic, clinical and exposure characteristics.
Data collection and treatment
Epidemiological data
After consent of voluntary participation, the notification form of the official system of epidemiological surveillance of malaria was fulfilled, and an interview was performed to obtain demographic, socioeconomic and exposure data for each participant. Furthermore, the infection by Plasmodium was researched in each individual residing in the household by blood collection from the digital pulp for manufacture of thick blood smear for examination by optical microscopy. The thick drop microscopic slides were stained and examined by Microscopists from DTF and forwarded for review at the Central Laboratory of the Health State Department of Mato Grosso. Individuals with a positive result in the first microscopic examination received treatment as recommended by the Ministry of Health, within a maximum of 24 hours after blood collection.
The sum of the individuals participating in the hemoscopic investigation was used for the calculation of the prevalence of infection by plasmodium in each locality of the DTF. The Epidata software 3.1 was used for the construction of the data bank and exploratory data analysis.
Information on spatial distribution of malária cases
For collection of the geographic coordinates of the households where each individual was residing at the time of the study, a geographic positioning equipment Garmim etrex 10 - Garmim was used. The geographic coordinates obtained were later associated with the questionnaires of epidemiological data and recorded into the database. For the construction of a georeferenced database, the ArcGIS 10.1 software was used (ESRI, California, USA). The points of each household were superimposed on a map of use and occupation of land, elaborated based on visual classification (DE OLIVEIRA et al 2013) of a TM-Landsat 5 multispectral image in 2011, obtained in the catalog of images of the Instituto Nacional de Pesquisas Espaciais (National Institute of Space Researches).
Information on the use and occupation of the soil obtained according to De Oliveira et al., 2011 and in accordance with multispectral image displayed: dense arboreal vegetation, predominance of forest formations without indication of anthropic interference (deforestation); deforested area, in case of predominance of open areas for villages, pastureland, mining and/or prospecting; undergrowth vegetation and sparse trees, predominance of natural vegetation in regeneration phase and presence of isolated trees. This stratification did not allow determining the extensions for each type of use and occupation in the area of the DTF. Subsequently, a map of the distribution of malaria cases by each locality was built, incorporating information on the use and occupation of the soil.
Using the TerraView freeware, version 3.14, available on the website www.dpi.inpe.br/terraview, the kernel map was built, to check the density of cases and their location. It required using the spatial interpolation method, which allows estimating the density of the event under study, in this case, the prevalent cases of infection by plasmodium. For the identification of areas with greater intensity of prevalence, the maps were constructed with a grid of 385 columns on the events and quartic function algorithm with adaptive radius (CÂMARA et al, 2004).
The degree of smoothing was controlled with the adoption of the bandwidth of 30 meters. This width intended to detect a specific pattern of distribution of cases, once the radius of influence (bandwidth), which defines the neighborhood of interpolated point, if too small, can generate a very discontinuous surface and, if tool arge, the surface can be very smooth, preventing an adequate result analysis. Six individuals had to be excluded from the study, whose geographical coordinate of their homes was not collected.
Ethical considerations
The Research Ethics Committee of the University Hospital Júlio Muller - Federal University of Mato Grosso, approved this study under protocol no. 158.109.