| dc.description.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
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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
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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. | en_US |
