Dengue and dengue hemorrhagic fever (DHF) are widespread in many countries of Tropical Asia. Although the transmission of DHF is said to have resulted from rapid urbanization, the disease vector Aedes aegypti exists in remote areas (WHO, 1996a). In Tropical Asia, children are particularly affected by dengue/DHF; these diseases cause many hospitalizations and deaths. According to WHO (1996a), dengue/DHF will continue to persist in Indonesia, Myanmar, and Thailand, where they are prevalent in endemic form. In recent years, sporadic cases and outbreaks also have been reported in Bangladesh, India, and Sri Lanka.
Studies suggest that climate change would likely affect the distribution, life cycle, and population dynamics of dengue (WHO, 1996b). According to Koopman et al. (1991), an increase of 3-4°C in average temperature may double the reproduction rate of the dengue virus. Results from a simple mathematical model developed in Indonesia suggest that, under the best-estimate climate change scenario, incidences of dengue may increase threefold in Indonesia (ADB, 1994b).
Schistosomiasis is a water-based infectious disease caused by five species of the trematode Schistosoma. The spread of this disease is largely attributed to the expansion of irrigation in tropical regions. According to Martens et al. (1995), climate change-related temperature increases would influence snail reproduction and growth, schistome mortality, infectivity and development in the snail, and human-water contact. Climate change impacts on the spread of schistosomiasis also may be indirect (WHO, 1996b); for example, expansion of irrigation to new areas may introduce schistosomiasis where endemic foci already exist.
A recent assessment of the sensitivity of malaria, dengue, and schistosomiasis (the world's most prevalent vector-borne diseases) to global warming suggests an increase in the extent of geographical areas susceptible to transmission of malarial plasmodium parasites, dengue flavivirus, and schistosoma worms. Martens et al. (1997) show that the transmission potential of the three associated vector-borne diseases is highly sensitive to climate changes on the periphery of the currently endemic areas and at higher elevations within such areas, including Tropical Asia. With reference to present endemic areas, their findings show that the potential increase in epidemic transmission of malaria and dengue may be estimated at 12-27% and 31-47%, respectively; in contrast, schistosomiasis transmission potential may be expected to exhibit a 11-17% decrease.
In recent years, incidence of visceral leishmaniasis has increased in some countries of Tropical Asia. During 1987-1990, visceral leishmaniasis reached epidemic form in the Indian state of Bihar and spread rapidly to surrounding areas. WHO (1996a) estimated that about 110 million people were at risk from visceral leishmaniasis. Major endemic foci are reported in border areas between India (states of Bihar and West Bengal), Bangladesh, and Nepal. In Bangladesh, visceral leishmaniasis already has reached epidemic form; the most vulnerable populations are poor and rural cattlekeepers. Reported cases appear to cluster close to flood-control embankments; there appears to be a significant risk that visceral leishmaniasis prevalence in some localities will increase as a result of flood-control and drainage projects (ISPAN, 1992). In a warmer climate, the incidence of visceral leishmaniasis also may increase (IPCC 1996, WG II, Section 18.3; WHO, 1996b).
Other waterborne and food-borne infectious diseases pose a great threat to public health in the tropical monsoon region. In 1995, in eight countries of the region (Bangladesh, Bhutan, India, Indonesia, Myanmar, Nepal, Sri Lanka, and Thailand), the total death toll from diarrhea was estimated to be 1.03 million; 71% and 12% of the deaths occurred in India and Bangladesh, respectively. Children under age 5 accounted for about 25% of these deaths (WHO, 1996a).
According to Colwell (1996), the major rivers of the Indian subcontinent that discharge into the Bay of Bengal carry huge amounts of agricultural and industrial waste, providing nutrients sufficient to convert coastal waters to eutrophic conditions. Brackish water extends some distance upriver for all rivers. Salinities between 5 ppm and 30 ppm, which were detected in inland coastal areas of Bangladesh, as well as in seawater (Huq et al., 1984), are favorable for the growth of V. cholerae.
In the Bay of Bengal, evidence has been found-by synthesizing satellite remote-sensing, in-situ hydrographic and meteorological data sets, and cholera cases in Bangladesh-that cholera cases occur with a rise in ocean temperature (Colwell, 1996). Two peak periods for cholera outbreaks have been identified: from early April to mid-May and from early September to the end of November. During these periods, high sea-surface temperature, salinity, and concentrations of nutrients probably favor the growth of V. cholerae. Outbreaks during the monsoon have been substantially smaller. Colwell (1996) also indicated a possible link between outbreaks of cholera in Peru and neighboring countries and a warming ENSO event. Although the ENSO phenomenon has a substantial effect on the Indian summer monsoon, its linkage with cholera has not been investigated. Future increases in sea-surface temperature, as well as increased concentrations of pollutants in river flows under climate change scenarios, may create a more favorable environment for the growth of V. cholerae throughout the year in the coastal area of Bangladesh.
Essential steps in recognizing and mitigating the emergence of malaria, dengue, and bilharzia, as well as many other infectious diseases, include enhanced surveillance and response (Martens et al., 1997). Dowlatabadi (1997) argues that because public health measures, case management, and land use play more significant roles in determining the prevalence of malaria than does climate, the most appropriate policy is to introduce or improve simple public health measures in the region; this approach would ensure that, even with climate change, developing countries would be far less susceptible to marginal increases in the potential prevalence of diseases such as malaria, schistosomiasis, dengue, and cholera.
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