Linking field-based ecological data with remotely sensed data using a geographic information system in two malaria endemic urban areas of Kenya

BACKGROUND: Remote sensing technology provides detailed spectral and thermal images of the earth's surface from which surrogate ecological indicators of complex processes can be measured. METHODS: Remote sensing data were overlaid onto georeferenced entomological and human ecological data randomly sampled during April and May 2001 in the cities of Kisumu (population ≈ 320,000) and Malindi (population ≈ 81,000), Kenya. Grid cells of 270 meters × 270 meters were used to generate spatial sampling units for each city for the collection of entomological and human ecological field-based data. Multispectral Thermal Imager (MTI) satellite data in the visible spectrum at five meter resolution were acquired for Kisumu and Malindi during February and March 2001, respectively. The MTI data were fit and aggregated to the 270 meter × 270 meter grid cells used in field-based sampling using a geographic information system. The normalized difference vegetation index (NDVI) was calculated and scaled from MTI data for selected grid cells. Regression analysis was used to assess associations between NDVI values and entomological and human ecological variables at the grid cell level. RESULTS: Multivariate linear regression showed that as household density increased, mean grid cell NDVI decreased (global F-test = 9.81, df 3,72, P-value = <0.01; adjusted R(2 )= 0.26). Given household density, the number of potential anopheline larval habitats per grid cell also increased with increasing values of mean grid cell NDVI (global F-test = 14.29, df 3,36, P-value = <0.01; adjusted R(2 )= 0.51). CONCLUSIONS: NDVI values obtained from MTI data were successfully overlaid onto georeferenced entomological and human ecological data spatially sampled at a scale of 270 meters × 270 meters. Results demonstrate that NDVI at such a scale was sufficient to describe variations in entomological and human ecological parameters across both cities

Low recovery rates stabilize malaria endemicity in areas of low transmission in coastal Kenya

The prevalence ofPlasmodium falciparum malaria in African communities can be high and stable even in areas of relatively low transmission where people expose to only a few infectious bites per year. We show in this field study conducted in 30 sites along the coastal Kenya that prevalence in school children was consistently high, although there were many sites where transmission intensity measured by exposure to infectious bites was less than 10 per year. Statistical analyses revealed that prevalence was significantly correlated with the infectious exposure occurring 10–11 months previously, suggesting that long-lived infections were commonplace and one of the major contributors for the stability of malaria in these sites. Using mechanistic models of malaria transmission, we found that the association of high prevalence and low transmission could be due to low recovery rates. Therefore, significant reductions of malaria prevalence and burden require substantial reductions of the duration of acquired infections, even in areas that have quite low transmission intensities by the standards of sub-Saharan Africa. Infection control featured by active detection and drug treatment as well as vector control is critical to combat malaria in areas of relatively low transmission intensity

A geographic sampling strategy for studying relationships between human activity and malaria vectors in urban Africa

This paper describes a geographic sampling strategy for ecologic studies and describes the relationship between human activities and anopheline larval ecology in urban areas. Kisumu and Malindi, Kenya were mapped using global positioning systems, and a geographic information system was used to overlay a measured grid, which served as a sampling frame. Grid cells were stratified and randomly selected according to levels of planning and drainage. A cross-sectional survey was conducted in April and May 2001 to collect entomologic and human ecologic data. Multivariate regression analysis was used to test the relationship between the abundance of potential larval habitats, and house density, socioeconomic status, and planning and drainage. In Kisumu, 98 aquatic habitats were identified, 65% of which were human made and 39% were positive for anopheline larvae. In Malindi, 91 aquatic habitats were identified, of which, 93% were human made and 65% were harboring anopheline larvae. The regression model explains 82% of the variance associated with the abundance of potential larval habitats in Kisumu. In Malindi, 59% of the variance was explained. As the number of households increased, the number of larval habitats increased correspondingly to a point. Beyond a critical threshold, the density of households appeared to suppress the development of aquatic habitats. The proportion of high-income households and the planning and drainage variables tested insignificant in both locations. The integration of social and biologic sciences will allow local mosquito and malaria control groups an opportunity to assess the risk of encountering potentially infectious mosquitoes in a given area, and concentrate resources accordingly

Spatial Distribution of Anopheles gambiae and Anopheles funestus and Malaria Transmission in Suba District, Western Kenya

The study reported here evaluated the distribution, relative abundance, and malaria transmission potential of Anopheles mosquitoes at 30 sites representing different ecological strata in western Kenya. Seasonal variation in anopheline densities and transmission potential, as expressed by Entomological inoculation rates (EIR), was investigated. Of the 6491 indoor resting anopheline mosquitoes collected at the 30 sites, 91.3 % (n = 5926) were An. gambiae s.l. and 8.7 % (n = 565) were An. funestus with an average house density of 6.58 and 0.63, respectively. Analysis of the data indicated significant variation in mosquito densities between study sites, species and season. High densities of both An. gambiae and An. funestus were recorded in the northern and northeastern parts of the district, while generally low densities were recorded in the south. Anopheles gambiae s.s. and An. arabiensis comprised 60.3 % (n = 3573) and 39.7 % (n = 2352) of the total number of An. gambiae s.l. mosquitoes collected, respectively. The composition of the An. gambiae s.l. sibling species showed temporal and spatial variation. Entomologic inoculation rates were estimated at 1.55 and 0.12 infective bites per person per month for An. gambiae s.l. and An. funestus, respectively. This study reveals considerable seasonal and site-specific variation in vector distribution, composition and transmission potential. Application of control interventions must therefore consider seasonal variations since the vectorial system changes quite rapidly over a short period of time. L'étude présentée ici évalue la distribution, l'abondance relative, et le potentiel de transmission du paludisme par des moustiques anophèles dans 30 localités représentatives des différentes conditions écologiques de l'Ouest du Kenya. La variation saisonnière des densités d'anophèles et du potentiel de transmission, exprimées par les taux d'inoculations Entomologiques (EIR), est examinée. Sur les 6491 anophèles récoltées au repos à l'intérieur des maisons dans les 30 localités, 91,3% (n=5926) sont des An. gambiae s.l. et 8,7% sont des An. funeslus avec une densité moyenne par maison de 6,58 et 0,63 respectivement. L'analyse des données indique une variation significative des densités de moustiques entre les sites étudiés, l'espèce et la saison. Des densités élevées à la fois d'An, gambiae s.l. et An. funestus ont été observées dans le nord et le nord est du district, alors que des densités généralement faibles ont été observées dans le sud. An. gambiae s.S. et An. arabiensis représentent respectivement 60,3% (n=3573) et 39,7% (n=2352) du nombre total de An. gambiae s.l récolté. La composition des 2 espèces d'An, gambiae s.l. montre une variation temporelle et spatiale. Les taux d'inoculations entomologiques sont estimés respectivement à 1,55 et 0,12 piqûres infestantes par personne et par mois pour'An. gambiae s.l. et An. funestus. Cette étude révèle une variation considérable de la distribution, de la composition et du potentiel de transmission des vecteurs selon la saison et la localité. La réalisation de campagnes de lutte doit par conséquent prendre en compte les variations saisonnières puisque le complexe vectoriel change très rapidement en un temps très court