Effect of permethrin-impregnated nets on exiting behavior, blood feeding success, and time of feeding of malaria mosquitoes (Diptera: Culicidae) in western Kenya.

The impact of permethrin-treated bednets on the feeding and house entering/exiting behavior of malaria vectors was assessed in two studies in western Kenya. In one study, matched pairs of houses were allocated randomly to receive bednets or no bednets. Exiting mosquitoes were collected in Colombian curtains hung around half of each house; indoor resting mosquitoes were collected by pyrethrum spray catches. The number of Anopheles gambiae Giles and An. arabiensis Patton estimated to have entered the houses was unaffected by the presence of bednets; Anopheles funestus Giles was less likely to enter a house if bednets were present. Anopheles gambiae and An. funestus were less likely to obtain a blood meal and significantly more likely to exit houses when bednets were present. No difference was detected in An. arabiensis rates of blood feeding and exiting. In a second experiment, hourly night biting collections were done on 13 nights during the rainy season to assess whether village-wide use of permethrin-treated bednets caused a shift in the time of biting of malaria vectors. A statistically significant shift was detected in the biting times of An. gambiae s.l., although the observed differences were small. No change was observed in the hourly distribution of An. funestus biting. Our study demonstrated that, at least in the short-term, bednets reduced human-vector contact and blood feeding success but did not lead to changes in the biting times of the malaria vectors in western Kenya.

Characteristics of larval anopheline (Diptera: Culicidae) habitats in Western Kenya.

A longitudinal survey of mosquito larval habitats was carried out in Asembo Bay, western Kenya, during the rainy season of 1998. All pools of standing water along a 700-m transect were sampled twice per week. For each habitat, eight environmental variables were recorded and a sample of anopheline larvae was collected for identification. In total, 1,751 Anopheles gambiae s.l. and 2,784 Anopheles funestus Giles were identified. Identification of An. gambiae s.l. by polymerase chain reaction (PCR) indicated that 240 (14.7%) were An. gambiae Giles and 858 (52.4%) were An. arabiensis Patton; PCR failed to identify 539 (32.9%) specimens. Repeated measures logistic regression analysis indicated that An. gambiae and An. arabiensis larvae were associated with small, temporary habitats with algae and little or no aquatic vegetation. Anopheles funestus larvae were associated with larger, semipermanent bodies of water containing aquatic vegetation and algae. Direct comparison of habitat characteristics associated with either An. gambiae or An. arabiensis revealed that algae were associated more commonly with habitats containing An. gambiae; no other differences were detected. Chi-square analysis indicated that these species were collected from the same habitat more frequently than would be expected by chance alone. Together, these results indicate that An. gambiae and An. arabiensis have similar requirements for the larval environment and that, at least in western Kenya, they do not segregate into separate habitats.

A geographic information system applied to a malaria field study in western Kenya.

This paper describes use of the global positioning system (GPS) in differential mode (DGPS) to obtain highly accurate longitudes, latitudes, and altitudes of 1,169 houses, 15 schools, 40 churches, four health care centers, 48 major mosquito breeding sites, 10 borehole wells, seven shopping areas, major roads, streams, the shore of Lake Victoria, and other geographic features of interest associated with a longitudinal study of malaria in 15 villages in western Kenya. The area mapped encompassed approximately 70 km2 and included 42.0 km of roads, 54.3 km of streams, and 15.0 km of lake shore. Location data were entered into a geographic information system for map production and linkage with various databases for spatial analyses. Spatial analyses using parasitologic and entomologic data are presented as examples. Background information on DGPS is presented along with estimates of effort and expense to produce the map information.

PIP: The global positioning satellite (GPS) network system is comprised of 24 satellites orbiting at an altitude of about 10,900 miles. The authors describe how a simple modification of GPS known as differential GPS (DGPS) can be used to produce a highly accurate base map in a tropical area. DGPS circumvents the effects of selective availability (SA) error, an intentional error component added for security purposes at each satellite, to yield a highly accurate position fix. This paper documents the use of DGPS to obtain highly accurate longitudes, latitudes, and altitudes of 1169 houses, 15 schools, 40 churches, 4 health care centers, 48 major mosquito breeding sites, 10 borehole wells, 7 shopping areas, major roads, streams, the shore of Lake Victoria, and other geographic features of interest associated with a longitudinal study of malaria in 15 villages in western Kenya. 70 sq. km were mapped, including 42.0 km of roads, 54.3 km of streams, and 15.0 km of lake shore. Location data were entered into a geographic information system for map production and linkage with various databases for spatial analyses. Spatial analyses using parasitologic and entomologic data are presented as examples. Less than $25,000 was spent upon this project, of which $15,000 was for hardware and software.