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It is projected that climate-related mortality will increase in the large cities of north Africa (IPCC, 1996), from direct effects as well as from indirect impacts of climate change. These impacts will include potential increases in the incidence of VBDs such as malaria, yellow fever, dengue fever, onchocerciasis, and trypanosomiasis arising from elevated temperatures and altered rainfall. High-elevation locations such as Nairobi or Harare may become vulnerable to malaria epidemics because the malaria parasite may be able to survive in the possibly warmer conditions (IPCC, 1996) at higher elevations than the current limits.
Minimum temperatures (Tmin or nighttime and winter temperatures) are crucial parameters for vector survival and affect the latitude and elevation of distribution, as well as the length of season permissive to transmission. Tmin is known to play an important role in limiting the distribution of malaria vector populations at a given locality where summer conditions are suitable for transmission (Craig and le Sueur, in preparation; Leeson, 1931). Thus, meteorological variables can create conditions conducive to disease spread or even to clusters of outbreaks (in the case of flooding or drought) (Epstein et al., 1993). A drop in water level in dams and rivers also would affect the quality of household and industrial fresh water because reduced water volume would increase the concentration of sewage and other effluent in rivers-resulting in outbreaks of diseases such as diarrhea, dysentery, and cholera. During 1992 and 1993, cholera affected almost every country in the SADC region, claiming hundreds of lives. In many drought-affected areas in Zambia, Zimbabwe, Botswana, and South Africa, streams and rivers dried up. Villagers (mainly women) had to walk long distances-only to collect polluted water, which they shared with wild animals and livestock. SARDC (1994) notes that when a major cholera outbreak occurred in several countries in southern Africa in the mid-1980s, the region had just come out of another drought. Reduced water flow during these droughts reduced the capacity of rivers, streams, and swamps to dilute agrochemicals and process fertilizers in fields, adversely affecting soil ecosystems. These drought-related problems are likely to increase under projected climate change. Vulnerabilities and control measures will affect the impact, however.
The growing resistance of insects to insecticides and of microorganisms to antimicrobials, as well as the toxicity of pesticides to helpful (predator) insects and larger animals (including humans), will limit the effectiveness of these control measures. Thus, nurturing environmental conditions that reduce vulnerability to VBD spread (e.g., extensive land clearing and monocultures), and those that shore up generalized defenses (trees around plots and settlements to harbor birds that consume bugs) becomes increasingly important (Epstein, 1993, 1995).
In west Africa, the population of blackflies-which carry onchocerciases (river blindness)-may increase by as much as 25%, according to precipitation projections from the GISS GCM. Schistosomiasis and malaria, both of which depend on water availability, also are likely to increase as a result of projected expansion of irrigation in hot climates. African trypanosomiasis (sleeping sickness), which is carried by the tsetse fly, also could proliferate because higher temperature would increase the range of the vector in areas prone to this infection. The distribution of tsetse flies may increase in east Africa (Rogers and Packer, 1993). Other minor health effects could include higher incidences of skin disorders or cancers, cataracts and similar forms of eye damage, and suppression of immune systems as a consequence of stratospheric ozone depletion-which leads to greater ultraviolet radiation (IPCC, 1996).
Nutritional status also is likely to be severely affected by droughts and associated crop failures, as in southern Africa during the droughts of the 1980s to early 1990s. This factor will further reduce the natural persistence of African communities and increase exposure to disease.
There is increasing evidence that many emergent and resurgent diseases may be related to ecosystem instability (Epstein, 1995). In many cases, this resurgence may be related not to climatic change but to other human-induced changes in the environment (e.g., lyme disease, dengue, hantavirus) (Levins et al., 1994; Wenzel, 1994; Epstein, 1995). Other diseases with clear links to climate and climatic change include malaria (Bouma et al., 1994; Loevinsohn, 1994) and cholera (Epstein, 1992, 1995; Epstein et al., 1993). Nonclimatic environmental modifications often disrupt natural habitats through processes such as deforestation or afforestation, resulting in the provision of new habitats for vectors or pathogens. Climatically induced change, however, can include short-term impacts as a result of interannual variation or long-term change associated with factors such as global warming. The effects of such changes are likely to be most noticeable in areas that constitute the fringes of distribution of a particular disease. Changes in distribution often will cause susceptible populations (those with little or no immunity) to be exposed to diseases not previously encountered-and result in severe morbidity and mortality. In addition, areas of traditionally low risk may become more vulnerable. The implications of such changes, especially in populations that lack immunity, have severe consequences (individual, social, and economic) for exposed individuals and health authorities.
Rogers (1996) modeled the effect of projected future climate changes on the distribution of three important disease vectors-mosquitoes, tsetse flies, and ticks-in southern Africa. The human diseases relating to these vectors are malaria (mosquitoes) and human African trypanosomiasis (tsetse flies). Climatic change may alter not only the physiological constraints placed on the vector but also the ability of the parasite to survive within the vector (Molineaux, 1988).
Within Africa, little evidence exists of causal changes in disease transmission and climate. This lack of evidence does not mean that these changes do not exist; rather, it may reflect the lack of available epidemiological data as a result of poor or absent surveillance and health information systems. Within Africa, 71.3% of the burden of disease is attributed to infectious diseases; malaria is the single greatest contributor (10.8%). All other vector-borne, helminthic, and environmentally related diseases that are affected by climate contribute about 2% of the total burden of disease. With regard to environmentally related diseases in Africa, malaria contributes more than 80% of the cause of lost disability adjusted life years (World Bank, 1993; WHO, 1996b). These estimates exclude diarrhea but include cholera.
Malaria contributes the highest percentage (>80%) of the climate-related disease burden in Africa. The physiological relationships among climate, vectors, and pathogens are only partially understood. Malaria provides a good example of how potential climate change may affect environmental and vector-borne diseases. Surveillance systems and epidemiological data on malaria exist in some of the regions most susceptible to climate change, allowing future monitoring to move from speculative to causal relationships.
One of the deficient micronutrients in malnourished Africans is iron. Woman and children are disproportionately affected by anemia, and pregnant women are especially at risk. In addition, one of the major causes of death in acute and complicated malaria is anemia. Thus, the potential exists for exacerbated morbidity and mortality in regions in which climate change may decrease nutritional status and increase malaria transmission.
Climate-related impacts on health status are likely to increase most within Africa, largely as a result of the high burden of disease within the continent and its vulnerability in terms of mitigation and nutritional status. In contrast to other regions of the world, a significant proportion of the burden of disease in Africa is related to climatically affected infectious diseases. Typical of many of these diseases in Africa is the fact that their severity focuses within the tropics, and infection rates approach saturation. Thus, the marginal regions of distributions, where severity of transmission is restricted climatologically, are most likely to be affected by climatic change. It is important to distinguish between restrictions related to rainfall and those related to temperature. In cases where rainfall restricts distribution, increases in temperature may result in a decrease in disease incidence and distribution. Conversely, in areas where low temperature currently limits distribution, increased temperature may increase regional severity and cause distributional extension.
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