Malaria in Africa: Vector Species' Niche Models and Relative Risk Maps

A central theoretical goal of epidemiology is the construction of spatial models of disease prevalence and risk, including maps for the potential spread of infectious disease. We provide three continent-wide maps representing the relative risk of malaria in Africa based on ecological niche models of vector species and risk analysis at a spatial resolution of 1 arc-minute (9 185 275 cells of approximately 4 sq km). Using a maximum entropy method we construct niche models for 10 malaria vector species based on species occurrence records since 1980, 19 climatic variables, altitude, and land cover data (in 14 classes). For seven vectors (Anopheles coustani, A. funestus, A. melas, A. merus, A. moucheti, A. nili, and A. paludis) these are the first published niche models. We predict that Central Africa has poor habitat for both A. arabiensis and A. gambiae, and that A. quadriannulatus and A. arabiensis have restricted habitats in Southern Africa as claimed by field experts in criticism of previous models. The results of the niche models are incorporated into three relative risk models which assume different ecological interactions between vector species. The “additive” model assumes no interaction; the “minimax” model assumes maximum relative risk due to any vector in a cell; and the “competitive exclusion” model assumes the relative risk that arises from the most suitable vector for a cell. All models include variable anthrophilicity of vectors and spatial variation in human population density. Relative risk maps are produced from these models. All models predict that human population density is the critical factor determining malaria risk. Our method of constructing relative risk maps is equally general. We discuss the limits of the relative risk maps reported here, and the additional data that are required for their improvement. The protocol developed here can be used for any other vector-borne disease

Zoophily of Anopheles arabiensis and An. gambiae in Madagascar demonstrated by odour-baited entry traps

International audienceIn Madagascar we used odour-baited entry traps (OBETs) for host choice tests of wild female anopheline mosquitoes (Diptera: Culicidae) at representative localities on the East and West sides of the island (villages Fenoarivo and Tsararano, respectively) and at the southern margin of the central plateau (Zazafotsy village, 800 m altitude). No insecticide house-spraying operations have been undertaken at these villages. Odours from a man and a calf of similar mass, concealed in different tents, were drawn by fans into separate OBETs set side by side. Traps were alternated to compensate for position effects, and different pairs of individual baits were employed for successive replicates. Totals of 266 An. funestus Giles sensu stricto and 362 An. gambiae Giles sensu lato were collected in 48 trap nights during March–June 1999. For each mosquito species the ‘index of anthropophily’ was defined as the proportion of females caught in the human-baited trap. For An. funestus this index was found to be consistently greater than 0.5 (value for random choice between traps/hosts), indicating that this species ‘preferred’ human to calf odour (index = 0.83). Conversely, the index of anthropophily for An. gambiae s.l. indicated they ‘chose’ calf in preference to human odour (index = 0.26). No significant differences of relative preference for calf or man were detected between villages; geographical variance accounted for  97% An. gambiae Giles sensu stricto at Fenoarivo, in accordance with prior knowledge of the differential distributions of these sibling species on the island. Predominant zoophily (i.e. intrinsic ‘preference’ for cattle odours) by both An. arabiensis and An. gambiae s.s. in Madagascar contrasts with their greater anthropophily in continental Africa

Tsiperifery, the wild pepper from Madagascar, emerging on the international spice market whose exploitation is unchecked: current knowledge and future prospects

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