Malaria Case Study
Now that the basic disease vectors have been outlined, we shall examine a specific case of a disease on which global warming has had a particularly pronounced effect. Malaria has been called the "most important" of the tropical diseases (WHO, 1990). It leaves a heavy toll of illness and death, and is especially lethal for children. It also poses a risk to business travelers, tourists and immigrants, and imported cases of malaria are increasingly seen in nonendemic areas such as Europe and North America. World populations are becoming increasingly vulnerable to malaria epidemics due to the development of vaccine and pesticide resistant strains, which makes the study of global warming's impact on this disease a crucial necessity. The virulogical background of this disease is similar to that of the general factors explained in the previous section. Malaria parasites are inoculated by the bite of infected female mosquitoes of the genus Anopneles (male mosquitoes do not bite). The parasites quickly multiply in the liver and in infected red blood cells. Vector mosquitoes become infected by feeding on the blood of infected people, and the parasites then undergo another phase of reproduction in the infected mosquito. This process is illustrated in Figure 1.
Over the last few years, malaria incidences have increased atypically or epidemically in several areas, including the Amazon region, Ethiopia, Madagascar, Sri Lanka and the Solomon Islands. Although most figures seem grim, several organizations are achieving considerable reductions in regional malaria diffusion. However, the malaria situation is still worsening in some areas with "large-scale epidemics" and increasing mortality. In some countries, the re-establishment of malaria endemicity is taking place, following large-scale malaria epidemics. (World Bank, 1998) Epidemics are most frequent in rural areas currently experiencing intense economic development (WHO, 1990). "In Central Asia and the Caucasus, re-introduction of malaria transmission has followed massive socio-economic degradation and the collapse of health and social services" (World Bank, 1998).
Figure 2 shows the global distribution of the disease, which is clearly concentrated near the equator. Consequently, these regions are also likely to be the most severely affected by global warming, since the lack of economic infrastructure weakens their defenses against droughts, soil degradation, and severe weather.
The available epidemiological statistics on malaria are crude at best, because accurate information on the global incidence of the disease is difficult to obtain. Reporting is particularly "fragmentary and irregular" in areas known to be highly endemic; for example, countries in tropical Africa, which are estimated to have more than 80% of all clinical cases and more than 90% of all parasite carriers, report only 4-7% of the estimated global number (WHO, 1990).
Since more irrigation is required in areas with increasing temperature, more mosquitoes could breed if adequate drainage is not concurrently supplied. Longer seasons for exposure of the host human to infected vectors could also result. (Longstreth and Wiseman, 1989). Studies also further suggest that epidemics of malaria could sweep over much of Europe and parts of North America. "The predictions are based on five models of the changes in temperature, rainfall and humidity that would result from a doubling of the pre-industrial atmospheric concentration of carbon dioxide. Each of the models suggests that large areas of Britain, Australia, Canada, Russia and the Middle East will be hit by malaria" (New Scientist, 1995).
Treatment and control of the disease have become more difficult with the spread of drug resistant strains of malaria and insecticide resistant strains of the mosquito vectors. In many parts of Africa, where malaria has long been highly endemic, people are infected so frequently that they develop a degree of acquired immunity, and may become "asymptomatic" carriers of the infection. This is reflected in the estimated number of people infected (267 million), compared with the estimated number of clinical cases of malaria (107 million per year). See Figure 2 for a visual representation of areas where significant drug resistance has been reported.
Epidemics of clinical malaria are often associated with numbers of non-immune people moving to highly-endemic areas, such as those in search of work, where they quickly succumb to the infection (WHO, 1990). Global warming is likely to exacerbate this problem, perhaps in the other direction (immune people moving to nonendemic areas to escape the harmful environmental degradation and other anticipated effects of global warming). This could pose a significant threat for the United States and other areas not used to dealing with such a disease.
Malaria was once a significant health problem in the southern and western United States (Faust, 1938). With the realization of the role played by the mosquito in malaria transmission, breeding of the mosquito was controlled by better drainage and ambient spraying (Williams, 1937). Resistance of the vector to pesticides, however, can make present-day control programs more difficult. (Longstreth and Wiseman, 1989) With a large influx of immigrants, sufficiently large amounts of the malaria parasite could be introduced into the United States in the presence of competent vectors. These vectors include An. freeborni in California and the states that border Mexico, as well as A. quadrimaculatus in the entire Southeast during the warmer months. "As the climate becomes warmer, it may be expected that these vectors will increase in their geographic range throughout the entire Southeast during the warmer months, and that An. quadrimaculatus will be present for a larger part of each year" (Longstreth and Wiseman, 1989).
The increased spread of malaria is a very serious and potentially disastrous issue of global concern. The combination of increased drug resistance and migratory pressures is conducive to global epidemics, even in areas where the disease has supposedly been eradicated. Although the citizens of industrialized nations feel confident in their immunity to the diseases of impoverished nations, the likely spread of malaria is a very real threat.