Name: Bøgh, Claus Home Country: Denmark Research Country: Gambia Project period: 1996-1999 Title Variation in Malaria Transmission in Rural Gambia Abstract Introduction After more than a century of intense work on malaria and malaria transmission, it is clear that the relationships between mosquito density, malaria transmission, parasite prevalence, immune response and clinical disease are very complex issues that are still far from being fully described and understood. One of the key puzzles in the equation of malaria is the complex relationship between the level of malaria transmission and the severity of disease experienced by the human population. To be able to shed light on this complex relationship it is necessary to have a clear understanding of the level of transmission experienced by the human population. Two main factors affect the level of transmission: the geographical variation in transmission intensity within the area and the level of personal protection enjoyed by the individuals. Geographical variation in transmission intensity can exist at many levels. There can be variation between houses within a village, between neighbouring villages, between regions and between countries (Trape et al., 1992; Smith et al., 1995; Thomson et al., 1996; Snow et al., 1997). In addition to the geographical variation, individuals living in a given area can benefit from different types of personal protection against mosquito bites. These include good housing construction, zooprophylactic protection by the presence of domestic animals, screening against mosquitoes, the use of bed nets, insecticide spraying, and the use of local repellents or mosquito coils (Bradley et al., 1986; Service, 1991; Adiarnah et al., 1993; Coosemans & Camevale, 1995). To give an accurate estimate of the risk of malaria transmission experienced by the human population in a given area, it is necessary to study a whole line of factors that may affect malaria transmission. A series of studies was organised in The Gambia to investigate geographical variation in malaria transmission intensity and identify the key factors affecting the transmission level as experienced by individuals. A parallel project was done in the same villages, investigating the causes of variation in human malaria and risk factors associated with disease. The overall aim of the combined projects was to shed light on the complex relationships between mosquito breeding habitats, local transmission risk and the level and severity of disease in the human population. The study site selected for the projects was in a mesoendemic area of The Gambia, West Africa. Previous studies had shown that there were differences in mosquito density within the area (Lindsay et al., 1993; Thomson et al., 1994; Thomson et al., 1996) as well as evidence of geographical variation in the parasite prevalence in the human population (Bradley et al., 1986; Alonso et al., 1993; Thomas & Lindsay, submitted). Previous studies had also suggested that people living in the area enjoyed some degree of protection by sleeping under bed nets (Bradley et al., 1986; Lindsay et al., 1989; Aikins et al., 1993). l The PhD thesis presented here describes the findings of the geographical variation in malaria transmission intensity and illustrates some of the key factors affecting the level of malaria transmission experienced by individuals living in the local communities. The study area The area selected for this study was located around Farafenni town in the central part of The Gambia, West Africa. The area is open Sudan savannah with rural economy and with no major differences in altitude. The highest natural point in the area is about 30 m above sea level and dense forest is only seen in a few sites within the study area. The villages in the area are discrete clusters of houses and not farms scattered in the area, as seen in some other parts of Africa. The key factors affecting the variation in vector density and transmission fisk were expected to be localised breeding of the mosquitoes and distance from mosquito breeding sites. h1 contrast to the studies of spatial distribution of vectors and transmission intensity described from urban and mountainous areas, this study aimed to investigate the situation in a rural setting dominated by open savannah or farm land which is the common situation for much of sub-Saharan Africa. Basic study designs Entomological surveys carried out in a 2500 km2 area throughout the rainy seasons of 1996 and 1997 represented the core of the project. These surveys were categorised into two major studies. One was the investigation of the spatial distribution of mosquito breeding sites in the area and the other was a detailed study of the spatial distribution of adult vector mosquitoes and transmission intensity in a random sample of the villages in the area. In addition to this work, a series of studies was conducted in the same area to describe some of the confounding factors affecting transmission risk experienced by the human population. Since farm animals are very common in The Gambia, a cohort study was organised to investigate the effect of passive zooprophylaxis on transmission intensity and parasite prevalence in the human population. Bed net use is also very common in the area, for which reason an investigation was done to measure the impact of untreated and impregnated bed nets on the prevalence and intensity of infection in the human population. Logistics The project involved a very extensive logistic set-up since a large number of villages and field sites had to be surveyed throughout two rainy seasons. The study was done in collaboration with The Medical Research Council's Laboratories, which provided laboratory facilities at the Farafenni field station, including excellent logistic backup and housing. The number of persons involved in the conduct of the study was substantial. The entomological field team consisted of three technicians and a driver, the entomological laboratory team consisted of four technicians plus the principal investigator. The zooprophylaxis survey team consisted of two entomology technicians, two parasitology technicians, an interviewer, a nurse and the principal investigator. In order to accomplish this project, we made use of a Landrover and an off road motor cycle. Content of the thesis The thesis is divided into an introduction and six papers ready for submission, each describing specific topics related to the risk of malaria transmission in The Gambia. Chapters one and two describe the spatial and temporal variation in mosquito breeding, dispersal of adult mosquitoes and geographical variation in transmission intensity. Chapters three and four describe the effect of passive zooprophylaxis on malaria transmission and parasite prevalence and intensity. Chapters five and six are based on the results from the parasitological surveys and interviews done by Siân Clarke and describe factors affecting bed net use and other anti-mosquito measures, as well as the effect of bed nets on the prevalence of parasites and clinical attacks Expected Future Investigations This PhD thesis consists of the primary research findings from the investigations done in The Gambia in 1996 and 1997. It is expected that a line of publications will follow in the next couple of years, based on the vast amount of, data collected within these two field seasons. Below is a description of what is expected from future investigations. Maps of larval breeding habitats based on remotely sensed data The results presented on larval breeding in this thesis are the basic findings on larval breeding and a description of the main habitats. The next level of analysis that will be conducted on the basis of these findings will be based on the association found between certain vegetation and land cover types and the breeding of the vector mosquitoes. This information, combined with a high resolution satellite image, will form the basis of a detailed map of larval breeding in the area that is hoped to be much more accurate than the alluvial deposits used in the present paper. With accurate maps of the breeding habitats, it should be possible to make much more accurate estimates of the productivity of adult mosquitoes and thus transmission risk in the associated upland areas. It is hoped that this type of information can be extended for use in areas outside our study area. Risk maps of malaria transmission in The Gambia The results on the geographical variation in malaria transmission intensity presented in this thesis illustrate how great variation can be within a limited area of flat bush land. However, the results presented are averages of results from several villages, for which reason a lot of detail is lost in the analysis of spatial variation. The next level of analysis of variation will be done using Geographical Information Systems (GIS) to describe the detailed variation in transmission intensity seen within the study area. This level of analysis will be particularly successful if it is possible to derive larval breeding maps from the high resolution satellite image of the area. The final aim of these analyses is to develop a high resolution risk map of malaria transmission in The Gambia. The relationship between transmission intensity and malaria in the human population As previously mentioned, a parallel study of the geographical variation in the parasite prevalence in the human population was conducted in the same villages in both 1996 and 1997. Many article s are expected to be published as a result of the combined entomologica1 and parasitological investigations. It is expected that we will be able to shed light on the complex relationship between malaria transmission level and parasite prevalence in the human population. The final level of analysis will be to predict the general risk of disease in all villages within the study area. Concluding remarks It is hoped that these investigations will inspire additional research projects on malaria and other vector borne diseases and will eventually be used for the protection of human lives and the enhancement of the quality of life in rural Africa. Involved research institutions Institute for Health Research and Development Department of Population Biology, Zoological Institute, University of Copenhagen Supervisor(s) Steven Lindsay, Henry Madsen, Chris Thomas, DBL Correspondence