Weather, water and malaria mosquito larvae : the impact of meteorological factors on water temperature and larvae of the Afro-tropical malaria vector Anopheles gambiae Giles

Malaria control using drug treatment and insecticide-impregnated nets or the treatment of mosquito resting sites with long-lasting insecticides are increasingly failing, as drug-resistant parasites emerge and mosquito vectors develop resistance against insecticides. Because malaria is one of the most important infectious diseases in the world, affecting over 350 million and killing over one million people annually, alternative control strategies have been introduced or are under development. Malaria control methods that aim to reduce adult vector populations by targeting their aquatic immature stages are gaining ground. These aquatic stages determine the abundance, dynamics and fitness of mosquito adults and, hence, affect the transmission of malaria. A better fundamental understanding of the biology and ecology of these essential stages could contribute to the implementation of current control methods and to the development of novel strategies. A better understanding of the factors that affect the life-history characteristics of mosquito larvae could furthermore improve current models that assess malaria risk but often do not incorporate the immature stages of the malaria vectors. The aim of this thesis was to study the effects of physical and biological factors on the biology and ecology of immature stages of the malaria vectors Anopheles arabiensis and An. gambiae, the most broadly distributed and most efficient vectors of malaria in sub-Saharan Africa. This thesis shows how meteorological variables can affect the life-history characteristics of larvae of An. arabiensis and An. gambiae and the temperature of their aquatic environment. Meteorological variables and water temperature in differently-sized, semi-natural habitats, which were exposed to the ambient environment, were studied in western Kenya, as well as the growth, development and survival of the immature stages of the malaria vectors in identical habitats. As temperature is an important determinant in the growth, development and survival of malaria mosquito larvae, more accurate measurements are needed of the temperature fluctuations to which these larvae are exposed on a daily basis. Such measurements in clear, semi-natural water bodies are shown in Chapter 2. The diurnal water temperature dynamics in the smallest habitat differed on both a spatial and a temporal scale from that in larger water bodies. Combining these water temperature data with local meteorological data, a model was developed that accurately predicts the daily fluctuations of the water temperature (Chapter 2). Water temperature dynamics were furthermore affected by habitat turbidity: Turbid water bodies had a higher near-surface water temperature during daytime than clear water bodies of identical dimensions (Chapter 3). In addition to the physical experiments, biological experiments were conducted in identical water bodies. Larvae were reared in similar transparent plastic cups that floated in habitats of different sizes, to assess the impact of diurnal water temperature variations on the life-history characteristics of the larvae (Chapter 4, 5). The larval development rate of both species was positively correlated with the size of a habitat: larvae developed more quickly into adults when amplitude of the daily fluctuations of the water temperature decreased (Chapter 4). When the larvae occupied turbid water, their growth and development rates decreased compared to those in clear water. An. gambiae furthermore showed a reduction in larval survivorship in turbid water. As both species frequently co-exist in larval habitats, the effects of competition between both species were assessed. When larvae of An. arabiensis and An. gambiae shared a habitat, their development rate was significantly affected, but in opposite ways: The larval development rate of An. arabiensis decreased whereas the larval development rate of An. gambiae increased. Moreover, in the smallest water bodies, the mortality of both species seemed to increase when they shared a habitat (Chapter 5). Finally, the direct effect of rainfall on the survival of An. gambiae larvae was examined. There were high additional losses of larvae during nights with rainfall compared to dry nights (Chapter 6). The findings described in this thesis demonstrate that biotic and abiotic factors can significantly affect the growth, development and survival of the aquatic stages of malaria mosquitoes in an important but complex way. Future studies on malaria epidemiology and malaria control should consider these factors in order to improve malaria risk predictions and to successfully incorporate the immature stages of the mosquitoes in malaria intervention strategies. As species-specific interactions occur, which may give cause to differential development, detailed knowledge at species level is required before interventions are implemented.