A comparative study of ecological factors that influence potential transmission of malaria in the Highlands and lowlands of western Kenya

Abstract

Vectored malaria, a major cause of morbidity and mortality in Africa South of the Sahara presently still prevails in highland areas. Malaria was eradicated from highland areas in the late 1950s using DDT against mosquitoes. In the 1980s malaria resurgence was recorded in highland regions where it spread from 3 districts to 13 in Western Kenya. The objectives of this study were to elucidate the ecological factors that attribute to increased malaria transmission in highland regions of Western Kenya and to test the hypotheses that have been proposed to be the cause of increased malaria epidemic. Survey of larval breeding sites was done to determine spatial distribution of breeding habitats of immature vectors. Adult mosquito vector distribution was done using the spray catch method at highland (Marani) and lowland (Kombewa) areas in Western Kenya. Larval breeding habitats were characterized. The following variables were determined; land cover types, habitat type, presence of larvae, surface area (rrr'), height above sea level (m), canopy coverage (%), algae coverage (%), grass coverage (%) and debris coverage (%). Vector productivity in natural and semi natural environments was determined using emergent traps and a comparison made between the two sites. Survivorship of aquatic stages of An. gambiae in the two habitat types was determined. Cohorts of two-hours old Instar one larvae of F1 An. gambiae were put on plastic basins and placed under cultivated and natural swamp conditions. They were observed through emergence and the dead scored daily. Survivorship, reproductive rates, fecundity, generation time and intrinsic growth rate of adult An. gambiae at both lowland and highland sites were determined. Adult F1 females and males were put in a cage and suspended in four randomly selected houses at Marani and Kombewa. Mosquitoes were VI counted daily, the number dead noted and eggs laid counted and recorded. The results showed that An. gambiae s. s. larval populations were higher during both dry and rainy (12.0% and 84.7% respectively) seasons than for An. funestus (1.1% and 2.2%) at the highland site. The study also established that at the high topography, a 100m distance from the nearest river, cultivatable land and land plant cover change determine occurrence of 87.3% breeding habitats of the malaria vectors. About 86% of the positive breeding sites were located at altitude below 1600 meters above sea level. Altitude significantly influenced occurrence of anopheline larval breeding habitats (X2 = 4.2, df= 1, P < 0.05). Of the land cover types surveyed, cultivated swamps and farmland comprised the highest percentage of positive breeding habitats (41.0% and 33.8% respectively) while natural swamps, pasture and river had 8.2%, 10.1% and 4.2% respectively. Forest and road land use had the least proportions (1.9% and 0.9% respectively) of positive breeding habitats. At the low topography, land use type and distance from the nearest river had no significant influence on occurrence of mosquito breeding habitats (X2 = 5.4, df= 7, P > 0.05) during the rainy season. Whereas in the dry season, elevation. and distance of breeding sites from the nearest river had effect on occurrence of anopheline larvae (X2 = 9.4, df= 3, P < 0.05). The study demonstrated that Anopheles gambiae s. s., the principal malaria vector also breeds in semi-permanent open sunlit water pools. Immature stage development time was significantly longer at the highland site. Swamp cultivation improved food availability and microclimate thus increased survivorship of immature stages of the malaria vectors. Survival rate of pupa in cultivated swamp was 35% higher than in natural swamp. Swamp drainage also had effect VII of reducing the development time of the vectors by 1.7 days. The mean minimum and mean maximum temperature of cultivated swamp varied by O.SOCand 2.0°C respectively between Kombewa and Marani. Vector productivity in the natural swamp environment was very low. It was evident that during the long rain season mosquitoes survived longer thus increased risk of malaria transmission. Associations were established between occurrence of anopheline mosquitoes and grass coverage, ammonium phosphate ion, pH, surface area of breeding sites and elevation. In conclusion, the study has demonstrated that land use change (cultivation of natural swamp) increases water temperatures and improves habitat quality accelerating development of aquatic stages of An. gambiae. Development time of the mosquito is shortened and survivorship increased that can likely cause localised epidemic. It is highly recommended that conversion of natural to cultivated swamps due to food production be accompanied by chemical, biological and integrated control of aquatic stages of mosquitoes. Constarit indoor residual spraying against mosquitoes be encouraged in epidemic prone highland regions. Information generated can be used to forecast malaria out break in heterogeneous environments and for formulating sound environmental management policies.