Browsing by Author "Van Wijk, Charles M."

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    Malaria risk mapping and prediction in Cote d'Ivoire
    (Stellenbosch : Stellenbosch University, 1999-12) Van Wijk, Charles M.; Zietsman, H. L.; Teuscher, T.; Stellenbosch University. Faculty of Arts and Social Sciences. Dept. of Geography & Environmental Studies.
    ENGLISH ABSTRACT: Malaria, being the most important vector-borne disease in Africa, is still leaving an ever-increasing trail of health and economic impediments within the developing world. Since malaria's spatial distribution and intensity are influenced by numerous environmental variables, medical geography, the study of the spatial human-environmental interrelationship of disease, can make a significant contribution to modelling disease risk for different geographic locations, as based on environmental determinants. The investigation consequently included the development of malaria risk maps within northern Cote d'Ivoire, an analysis of its spatial variation through exploratory spatial data analysis and Geographical Information System (GIS) functionality, and the search for predictive models using environmental variables. Malaria transmission intensities of the Anopheles fonestus and Anopheles gambiae mosquito species have been studied as factors having a direct influence on the malaria transmission cycle, whilst indirect environmental influencing factors which were studied included: wetland rice production, water bodies and their flooding pattern, temperature and relative humidity ranges, amount of rainfall and insolation, distances between aquatic breeding grounds and villages, and vegetation cover. Data have been obtained from ongoing surveys carried out by Institut Pierre Richet/OCCGE and the WARDA Health Research Consortium within a random selection of 24 villages in northern Cote d 'Ivoire, district level meteorological observations, and GIS and satellite imagery computations. The data sets have been manipulated, standardised and reduced to three malaria risk indicators, and five uncorrelated environmental measures through principal component analysis. The application of exploratory spatial data analysis, in which the spatial capabilities of GIS played a significant role, involved: a) Mapping the disease's spatial distribution and extreme risk areas by means of proportional circle and probability maps. b) Exploring and modelling first and second order spatial disease variation through Triangulated Irregular Network (TIN) interpolation, semi-variograms, and trend surface analysis. c) Visual and statistical detection of associations between malaria and environmental determinants, by means of proportional circle maps and multiple regression analysis, in the search for predictive models. Residuals of final models have been mapped to investigate possible patterns in over- and underestimation. A general increase in disease incidence and transmission levels during the study period, as well as areas of high risk have been identified. Investigation of first order variation highlighted these high risk areas. Semi-variograms revealed a high degree of spatial variability, with little or no spatial dependence between observations. No clear second order variation could therefore be identified, nor any reasonable first or second order spatial variation model be deduced. Relative humidity and temperature measures appear visually to be associated with malaria occurrence, and were statistically confirmed to account for a substantial percentage of disease variance through regression modeling, together with distance and vegetation density measures. However, the small proportion of variance explained by some models, together with the irregular spatial distribution and high levels of residuals, made reliable prediction impossible. It is therefore clear that the relationships between malaria morbidity or transmission and the environmental factors are complex and often highly site specific, possibly requiring higher order polynomial functions and a wide spectrum of determinants, still to be identified and included in subsequent studies.