Vector-borne diseases are significant causes of mortality and morbidity in Tropical Asia. Climate change may cause a change in the distribution of malaria, dengue, bilharzia, leishmaniasis, and schistosomiasis. The region also is particularly vulnerable to extreme weather events such as tropical cyclones, storms, and floods. The vulnerability of the population to these events is significantly increased by high population densities and poor sanitation; thus, waterborne and food-borne infectious diseases also are major problems. For example, waterborne and waterrelated diseases account for about 70% of the epidemic emergencies in India. The additional impacts of climate change, superimposed on existing conditions, could seriously exacerbate health problems.
Malaria is the most serious and widespread tropical disease in the world today. In most areas of the Asia-Pacific region where malaria now occurs, its distribution was considerably reduced or eliminated during the 1960s and 1970s. In recent years, however, the situation has been worsening in frontier regions of economic development (Porter, 1994). The emergence and rapid spread of drug-resistant malaria have become major health concerns to all malaria-affected countries in the region (WHO, 1996a).
Climate has a direct influence on malaria mosquitoes because each species has
a range of climate conditions suitable for its development (see Figure
11-4). Climate also has an indirect effect on malaria through its influence
on suitable vegetation and vector breeding sites. Precipitation is important
because mosquitoes require water to lay their eggs, as well as for the subsequent
development of larvae.
|Figure 11-4: Critical temperatures in malaria epidemiology (°C) (adapted from WHO, 1996b).|
Several estimates of the potential distribution of malaria have been produced, using climate change scenarios from a number of GCMs. These projections indicate that, in the future, malaria is most likely to extend its range into the fringes of established endemic areas (Martens et al., 1995). In southeast Asia, this would include new areas of Indonesia-notably Sumatra and Irian Jaya-and Papua New Guinea (Porter, 1994). Martin and Lefebvre (1995) report that the intensity and extent of malaria's potential transmission would change significantly under scenarios generated by the five GCMs they used. In all cases, an increase in seasonal malaria at the expense of perennial malaria results; such an increase is most likely to lead to epidemics among unprepared or nonimmune populations. Similarly, Martens et al. (1995) conclude that newly affected populations would initially experience high case-fatality rates because of their lack of naturally acquired immunity.
Increased incidence of malaria associated with climate change may significantly strain the economies of many countries of the region. Picard and Mills (1992) estimated losses of five working days for infected persons, during the period from infection to recovery, in two districts in Nepal. Mills (1994) found that the economic consequences of malaria could be high in the areas without malaria-control programs. The high cost of alternative drugs has put enormous pressure on the scarce economic resources of the countries in Asia (WHO, 1996a). ISPAN (1992) estimated that a complete course of treatment in rural Bangladesh may cost one month's wage for an agricultural worker (about US$40), or roughly one-fifth of overall per capita income. Hammer (1993) discusses the economics of malaria treatment and cites examples from Nepal, Thailand, and Indonesia-where the economic return of malaria treatment was found to be substantial.
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