11.3.7.2. Dengue and Schistosomiasis
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.
11.3.7.3. Other Diseases
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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