Malaria vector population dynamics in highland and lowland regions of western Kenya.

BACKGROUND & OBJECTIVES: Malaria is the major cause of morbidity and mortality in sub-Saharan Africa. A child below five years dies after every 30 min. Highland areas under land use change impact on malaria transmission by altering the microclimate of the immature stages and adult mosquitoes. Adult vector population dynamics is important because it is an indicator of transmission risk of the disease. This study was to investigate the effects of microclimatic changes on the mosquito indoor-resting behavior. METHODS: The study was conducted at a highland site of Marani and at a lowland site of Kombewa where 30 houses were randomly selected at either site. Outdoor and indoor weather conditions were monitored throughout the study period. Indoor mosquitoes were collected using the pyrethrum spray catch method, gonotrophic stage of the females determined and species identified to species level using rDNA polymerase chain reaction method. ELISA was carried out to determine the Plasmodium sporozoites in mosquitoes. RESULTS: Anopheles gambiae s.s. was more abundant at the highland site whereas An. funestus at the lowland site. Indoor densities were highest in June 2003 at both the sites: An. gambiae at the highland site and An. funestus at the lowland site. There was an association between An. gambiae s.s. abundance and relative humidity at the highland site. Combined entomological inoculation rate (EIR) for both the vector species was 0.4 infected bite per year (ib/yr) at the highland site and 31.1 ib/yr at the lowland site. Prolonged indoor spraying with insecticide decreased vector indoor abundance.

Effect of swamp cultivation on distribution of anopheline larval habitats in Western Kenya.

BACKGROUND & OBJECTIVES: Malaria resurgence in highland regions of East Africa has been on increase. The spatio-temporal distribution of larval habitats of malaria vectors determines the distribution of adult vectors, hence, disease transmission. Vector's ecology is necessary for strategic vector control through effective plan for source reduction. Mapping of the larval habitats is necessary for targeted control measures. The purpose of this study is to assess and compare the spatial and seasonal variations in anopheline larval habitats in Western Kenya. METHODS: A comparative study was conducted on spatial distribution of GPS geo-located anopheline larval habitats in relation to highland and lowland environments. Land use types were categorized and all potential aquatic habitats of malaria vectors were examined in February, May, August and November 2004. Data analyses were performed using SAS JMP software. RESULTS & DISCUSSION: Results showed a higher percentage of Anopheles gambiae s.s. (70.9%) than An. funestus (29.1%) in highland. In the lowland, An. gambiae s.l. comprised 60.1% while An. funestus represented 39.9%. The distribution of larval breeding is confined to the valley bottom in the highland while it was dispersed in the lowland. Land use type influenced the occurrence of positive breeding habitats in the highland. In the lowland, distribution was due to seasonality. We found high proportion of potential and positive breeding sites in cultivated swamps and farmlands at the highland site. These results suggest that swamp cultivation increases the availability and suitability of larval breeding habitats of malaria vectors, thus malaria transmission in the Western Kenya highlands environment.

Land use and land cover changes and spatiotemporal dynamics of anopheline larval habitats during a four-year period in a highland community of Africa.

Spatial and temporal variations in the distribution of anopheline larval habitats and land use and land cover (LULC) changes can influence malaria transmission intensity. This information is important for understanding the environmental determinants of malaria transmission heterogeneity, and it is critical to the study of the effects of environmental changes on malaria transmission. In this study, we investigated the spatial and temporal variations in the distribution of anopheline larval habitats and LULC changes in western Kenya highlands over a 4-year period. Anopheles gambiae complex larvae were mainly confined to valley bottoms during both the dry and wet seasons. Although An. gambiae larvae were located in man-made habitats where riparian forests and natural swamps had been cleared, Anopheles funestus larvae were mainly found in permanent habitats in pastures. The association between land cover type and occurrence of anopheline larvae was statistically significant. The distribution of anopheline positive habitats varied significantly between months, during the survey. In 2004, the mean density of An. gambiae was significantly higher during the month of May, whereas the density of An. funestus peaked significantly in February. Over the study period, major LULC changes occurred mostly in the valley bottoms. Overall, farmland increased by 3.9%, whereas both pastures and natural swamps decreased by 8.9% and 20.9%, respectively. The area under forest cover was decreased by 5.8%. Land-use changes in the study area are favorable to An. gambiae larval development, thereby risking a more widespread distribution of malaria vector habitats and potentially increasing malaria transmission in western Kenya highlands.

Landscape determinants and remote sensing of anopheline mosquito larval habitats in the western Kenya highlands.

BACKGROUND: In the past two decades the east African highlands have experienced several major malaria epidemics. Currently there is a renewed interest in exploring the possibility of anopheline larval control through environmental management or larvicide as an additional means of reducing malaria transmission in Africa. This study examined the landscape determinants of anopheline mosquito larval habitats and usefulness of remote sensing in identifying these habitats in western Kenya highlands. METHODS: Panchromatic aerial photos, Ikonos and Landsat Thematic Mapper 7 satellite images were acquired for a study area in Kakamega, western Kenya. Supervised classification of land-use and land-cover and visual identification of aquatic habitats were conducted. Ground survey of all aquatic habitats was conducted in the dry and rainy seasons in 2003. All habitats positive for anopheline larvae were identified. The retrieved data from the remote sensors were compared to the ground results on aquatic habitats and land-use. The probability of finding aquatic habitats and habitats with Anopheles larvae were modelled based on the digital elevation model and land-use types. RESULTS: The misclassification rate of land-cover types was 10.8% based on Ikonos imagery, 22.6% for panchromatic aerial photos and 39.2% for Landsat TM 7 imagery. The Ikonos image identified 40.6% of aquatic habitats, aerial photos identified 10.6%, and Landsate TM 7 image identified 0%. Computer models based on topographic features and land-cover information obtained from the Ikonos image yielded a misclassification rate of 20.3-22.7% for aquatic habitats, and 18.1-25.1% for anopheline-positive larval habitats. CONCLUSION: One-metre spatial resolution Ikonos images combined with computer modelling based on topographic land-cover features are useful tools for identification of anopheline larval habitats, and they can be used to assist to malaria vector control in western Kenya highlands.

Association between land cover and habitat productivity of malaria vectors in western Kenyan highlands.

We examined the effects of land cover type on survivorship and productivity of Anopheles gambiae in Kakamega in the western Kenyan highlands (elevation = 1,420-1,580 meters above sea level). Under natural conditions, An. gambiae sensu lato adults emerged only from farmland habitats, with an estimated productivity of 1.82 mosquitoes/meter(2)/week, but not from forest and swamp habitats. To determine the effects of intraspecific competition and land cover types, semi-natural larval habitats were created within three land cover types (farmland, forest, and natural swamp), and three different densities of An. gambiae sensu stricto larvae were introduced to the larval habitats. The mosquito pupation rate in farmland habitats was significantly greater than in swamp and forest habitats, and larval-to-pupal development times were significantly shorter. At higher densities, the larvae responded to increased intraspecific competition by extending their development time and emerging as smaller adults, but initial larval density showed no significant effects on pupation rate. Land cover type may affect larval survivorship and adult productivity through its effects on water temperature and nutrients in the aquatic habitats, as shown by the significantly higher water temperature in farmland habitats, enhanced pupation rates and shortened development times from the addition of food to habitats, and a significant negative correlation of the occurrence of An. gambiae larvae with canopy cover and emergent plants in natural habitats. These results suggest that deforestation and cultivation of natural swamps in the western Kenyan highland create conditions favorable for the survival of An. gambiae larvae, and consequently increase the risks of malaria transmission to the human population.

Plasmodium falciparum spatial analysis, western Kenya highlands.

We carried out a population-based study to determine the unbiased, age-specific Plasmodium falciparum prevalence, asexual and sexual parasite density, and spatial distribution to establish rates of infection at a site in western Kenya. Three cross-sectional surveys were carried out in western Kenya highlands. Blood samples were taken from 1,388 persons from 6 months to 75 years of age. Parasite prevalence and densities in the population decreased with age and distance from valley bottoms. Children from 1 to 4 years of age had the highest parasite prevalence (38.8%-62.8%); in adults, prevalence declined to 2.9%-24.1%. Malaria prevalence declined by an average of 19% from July to December 2002 across age groups. These observations suggest that parasite transmission is intense at this altitude. Asexual parasite density indicated clustering near major vector breeding habitats. Variability in seasonal prevalence indicates transmission instability and susceptibility to epidemics.

Spatial distribution of anopheline larval habitats in Western Kenyan highlands: effects of land cover types and topography.

The distributions of anopheline larval habitats were aggregated in valley bottoms in Kenya in both the rainy and dry seasons, although the degree of aggregation was higher in the dry season than in the rainy season. Larvae of the Anopheles gambiae complex larvae were found more frequently in habitats in farmlands and pastures. However, An. funestus larvae were found more frequently in natural swamps and pastures. Canopy cover was the only variable significantly associated with the occurrence of the An. gambiae complex and An. funestus. The average canopy cover was significantly less in the habitats with the An. gambiae complex and An. funestus larvae than those without the anopheline larvae. Thus, land cover types and topographic features showed important effects on the distribution of anopheline larval habitats. These results suggest that clearing riparian forests would improve growing conditions of the An. gambiae complex and An. funestus larvae in Kenyan highlands.