18.104.22.168 Drought and infectious disease
Countries within the ‘Meningitis Belt’ in semi-arid sub-Saharan Africa experience the highest endemicity and epidemic frequency of meningococcal meningitis in Africa, although other areas in the Rift Valley, the Great Lakes, and southern Africa are also affected. The spatial distribution, intensity and seasonality of meningococcal (epidemic) meningitis appear to be strongly linked to climatic and environmental factors, particularly drought, although the causal mechanism is not clearly understood (Molesworth et al., 2001, 2002a, b, 2003). Climate plays an important part in the interannual variability in transmission, including the timing of the seasonal onset of the disease (Molesworth et al., 2001; Sultan et al., 2005). The geographical distribution of meningitis has expanded in West Africa in recent years, which may be attributable to environmental change driven by both changes in land use and regional climate change (Molesworth et al., 2003).
The transmission of some mosquito-borne diseases is affected by drought events. During droughts, mosquito activity is reduced and, as a consequence, the population of non-immune persons increases. When the drought breaks, there is a much larger proportion of susceptible hosts to become infected, thus potentially increasing transmission (Bouma and Dye, 1997; Woodruff et al., 2002). In other areas, droughts may favour increases in mosquito populations due to reductions in mosquito predators (Chase and Knight, 2003). Other drought-related factors that may result in a short-term increase in the risk for infectious disease outbreaks include stagnation and contamination of drainage canals and small rivers. In the long term, the incidence of mosquito-borne diseases such as malaria decreases because the mosquito vector lacks the necessary humidity and water for breeding. The northern limit of Plasmodium falciparum malaria in Africa is the Sahel, where rainfall is an important limiting factor in disease transmission (Ndiaye et al., 2001). Malaria has decreased in association with long-term decreases in annual rainfall in Senegal and Niger (Mouchet et al., 1996; Julvez et al., 1997). Drought events are also associated with dust storms and respiratory health effects (see Section 8.2.6). Droughts are also associated with water scarcity; the risks of water-washed diseases are addressed in Section 8.2.5.