Low larval vector survival explains unstable malaria in the western Kenya highlands

Several highland areas in eastern Africa have recently suffered from serious malaria epidemics. Some models predict that, in the short term, these areas will experience more epidemics as a result of global warming. However, the various processes underlying these changes are poorly understood. We therefore investigated malaria prevalence, malaria vector densities and malaria vector survival in a highland area in western Kenya, ranging from approximately 1550-1650 m altitude. Although only five adult malaria vectors were collected during 180 light traps and 180 resting collections over a 23-month study period, malaria was prevalent among school children (average parasite prevalence: 10%). During an extensive survey of potential larval habitats, we identified only seven habitats containing Anopheles gambiae Giles s.l. larvae. Their limited number and low larval densities suggested that their contribution to the adult vector population was small. Experiments on adult and larval survival showed that at this altitude, adult mosquitoes survived inside local houses, but that larval development was severely retarded: only 2 of 500 A. gambiae s.l. larvae developed to the pupal stage, whereas all other larvae died prior to pupation. At present, high vector densities are unlikely because of unfavourable abiotic conditions in the area. However, temporary favourable conditions, such as during El Niño years, may increase larval vector survival and may lead to malaria epidemics.