The effect of water turbidity on the near-surface water temperature of larval habitats of the malaria mosquito Anopheles gambiae

Water temperature is an important determinant in many aquatic biological processes, including the growth and development of malaria mosquito (Anopheles arabiensis and A. gambiae) immatures. Water turbidity affects water temperature, as suspended particles in a water column absorb and scatter sunlight and hence determine the extinction of solar radiation. To get a better understanding of the relationship between water turbidity and water temperature, a series of semi-natural larval habitats (diameter 0.32 m, water depth 0.16 m) with increasing water turbidity was created. Here we show that at midday (1300 hours) the upper water layer (thickness of 10 mm) of the water pool with the highest turbidity was on average 2.8 degrees C warmer than the same layer of the clearest water pool. Suspended soil particles increase the water temperature and furthermore change the temperature dynamics of small water collections during daytime, exposing malaria mosquito larvae, which live in the top water layer, longer to higher temperatures

Variation in malaria transmission dynamics in three different sites in Western Kenya

The main objective was to investigate malaria transmission dynamics in three different sites, two highland villages (Fort Ternan and Lunyerere) and a lowland peri-urban area (Nyalenda) of Kisumu city. Adult mosquitoes were collected using PSC and CDC light trap while malaria parasite incidence data was collected from a cohort of children on monthly basis. Rainfall, humidity and temperature data were collected by automated weather stations. Negative binomial and Poisson generalized additive models were used to examine the risk of being infected, as well as the association with the weather variables. Anopheles gambiae s.s. was most abundant in Lunyerere, An. arabiensis in Nyalenda and An. funestus in Fort Ternan. The CDC light traps caught a higher proportion of mosquitoes (52.3%) than PSC (47.7%), although not significantly different (P = 0.689). The EIR's were 0, 61.79 and 6.91 bites/person/year for Fort Ternan, Lunyerere and Nyalenda. Site, month and core body temperature were all associated with the risk of having malaria parasites (P <0.0001). Rainfall was found to be significantly associated with the occurrence of P. falciparum malaria parasites, but not relative humidity and air temperature. The presence of malaria parasite-infected children in all the study sites provides evidence of local malaria transmission

The behaviour and ecology of malaria vectors and malaria transmission in Kisumu District of western Kenya

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Entomological correlates of epidemiological impacts: how do we know it is working?

Assuming that transgenesis of malaria vector populations is feasible, it will be necessary to carry out entomological, morbidity and mortality trials to measure the degree of efficacy of this new intervention strategy. Taking into consideration the necessity to maintain reproductive fitness of a transgenic mosquito, the best strategy would be to target the sporogonic cycle and/or reduce the anthropophilic feeding behaviour of the vectors. The former would impact on the entomological inoculation rate (EIR) whereas the latter would impact on vectorial capacity. A three-phase trial would be carried out to test safety, efficacy and impact on morbidity and mortality. After confirmation of safety and efficacy, phases two and three would involve largescale multiple site trial

Progress towards understanding the ecology and epidemiology of malaria in the western Kenya highlands: Opportunities and challenges for control under climate change risk

Following severe malaria epidemics in the western Kenya highlands after the late 1980s it became imperative to undertake eco-epidemiological assessments of the disease and determine its drivers, spatial-temporal distribution and control strategies. Extensive research has indicated that the major biophysical drivers of the disease are climate change and variability, terrain, topography, hydrology and immunity. Vector distribution is focalized at valley bottoms and abundance is closely related with drainage efficiency, habitat availability, stability and productivity of the ecosystems. Early epidemic prediction models have been developed and they can be used to assess climate risks that warrant extra interventions with a lead time of 2–4 months. Targeted integrated vector management strategies can significantly reduce the cost on the indoor residual spraying by targeting the foci of transmission in transmission hotspots. Malaria control in the highlands has reduced vector population by 90%, infections by 50–90% in humans and in some cases transmission has been interrupted. Insecticide resistance is increasing and as transmission decreases so will immunity. Active surveillance will be required to monitor and contain emerging threats. More studies on eco-stratification of the disease, based on its major drivers, are required so that interventions are tailored for specific ecosystems. New and innovative control interventions such as house modification with a one-application strategy may reduce the threat from insecticide resistance and low compliance associated with the use of ITNs

The effects of rainfall and evapotranspiration on the temporal dynamics of Anopheles gambiae s.s. and Anopheles arabiensis in a Kenyan village

The population dynamics of the larval and adult life stages of the malaria vector Anopheles gambiae Giles were studied in Miwani. western Kenya, in relation to meteorological conditions. Larval density within a habitat, the number of larval habitats and sibling species composition were investigated as determinants of larval population dynamics. Female vector densities inside local houses and sibling species composition were investigated as determinants of adult population dynamics. Larval densities were estimated using a modified area-sampling method. Within the habitats, all instars showed a highly aggregated distribution, with the exception of second instars. A longitudinal study on the larval populations of A. gambiae s.1. in two different types of habitat (dirt track and ditch) was carried out, using a novel sampling procedure. A. gambiae s.s. and Anopheles arabiensis, the two sibling species occurring sympatrically in the study area, showed some spatial segregation between the two types of habitat. Rainfall was significantly correlated with the number of A. gambiae s.1. larval habitats during the first 6 weeks of study taking I week time lag into account, while over the entire 5-month study period correlations were less clear. With 1 week time lag, rainfall was also significantly correlated with the number of female A. gambiae s.1. collected from CDC-light traps in the Study houses. Both larval and adult populations showed a significant increase in the proportion of A. gambiae s.s. within the mixed population of A. gambiae s.s. and A. arabiensis over time. Although not significantly correlated, the ratio of rainfall over precipitation/potential evapotranspiration (P/PE), indicative of the humidity conditions in the area, was probably the driving force of this increase. (C) 2003 Elsevier B.V. All rights reserved

Competitive interactions between larvae of the malaria mosquitoes Anopheles arabiensis and Anopheles gambiae under semi-field conditions in western Kenya

The present paper reports the occurrence of competition between larvae of the malaria mosquito sibling species Anopheles arabiensis and An. gambiae under ambient conditions in western Kenya. Larvae of both species were reared at the same density and under the same food conditions outdoors in single-species and mixed-species populations (species ratio 1:1) in transparent cups that floated in small and large semi-natural pools, which experienced different diurnal variations in water temperature. In a second experiment, both species were reared at similar densities and under the same food conditions in trays in either single-species or mixed-species populations at different proportions (species ratio 1:1, 1:3 or 3:1). Competition affected the development rate of both species in an opposite way: the development time of larvae of An. arabiensis increased whereas the development time of larvae of An. gambiae decreased in the presence of its sibling species. In small pools larvae developing in mixed-species populations experienced a higher mortality than larvae reared in single-species populations, whereas no such effect was observed in the large pools. In both species the time to pupation was longer and emerging females were larger in the small pools. Larval mortality of An. arabiensis was lower in the small pools compared to the large pools, whereas An. gambiae showed the opposite trend. Overall An. arabiensis showed reduced development rates, higher mortality rates and emerged with a larger body size compared to An. gambiae. The implication of these competitive interactions between larvae of An. arabiensis and An. gambiae under semi-filed conditions needs to be considered in the design and implementation of programmes that aim to reduce malaria transmission as competition may alter the species composition in the field