14.2.5.2. Vector-Borne Diseases
Arthropod vector organisms for vector-borne diseases (VBDs) are sensitive to
climatic and hydrometeorological conditions (especially temperature and humidity,
stagnant water pools, and ponds), as are life-cycle stages of the infecting
parasite within the vector (Bradley, 1993; Haines et al., 1993; Curto
de Casas et al., 1994; Ando et al., 1998). Hence, the geographic
range of potential transmission of VBDs may change under conditions of climate
change (Leaf, 1989; Shope, 1991; Carcavallo and Curto de Casas, 1996; McMichael
et al., 1996; Patz et al., 1996; WHO, 1998). Several studies have
concluded that temperature affects the major components of vectorial capacity
(Carcavallo et al., 1998; Carcavallo, 1999; Moreno and Carcavallo, 1999).
Mosquitoes, in particular, are highly sensitive to climatic factors (Curto
de Casas and Carcavallo, 1995). Anopheline spp. and Aedes aegypti
mosquitoes have established temperature thresholds for survival, and there are
temperature-dependent incubation periods for the parasites and viruses within
them (the extrinsic incubation period) (Curto de Casas and Carcavallo, 1995;
de Garín and Bejarán, 1998a,b; Epstein et al., 1998; de
Garín et al., 2000). Climate change may influence the population
dynamics of vectors for Chagas' disease (Burgos et al., 1994), as
well as the number of blood-feedings of mosquitoes and, therefore, the possibilities
of infective direct contacts (Catalá, 1991; Catalá et al.,
1992).
Of relevance to infectious disease distribution, minimum temperatures are now
increasing at a disproportionate rate compared to average and maximum temperature
(Karl et al., 1995). Such conditions may allow dengue and other climate-sensitive
VBDs to extend into regions that previously have been free of disease or exacerbate
transmission in endemic parts of the world. Temperature is one of the factors
that can influence the seasonal transmission of malaria. Near the equator in
Iquitos, Peru, seasonality in transmission is driven by small temperature fluctuation
(1-2°C) (Patz et al., 1998). The current and projected expansion
of the range of vector species into the subtropics and to higher elevations
warrant heightened entomological and epidemiological surveillance and control
in highland areas and for populations living on the fringes of regions that
now are affected (Epstein et al., 1998).
Reemergence of dengue fever in Colombia followed reinvasion of the country
by the principal mosquito vector (Aedes aegypti), and the disease hit
with large upsurges following periods of heavy rain (Epstein et al.,
1995). The mosquito vector for dengue and yellow fever has been reported at
an elevation of 2,200 m in Colombia (Suárez and Nelson, 1981).
Climate variability, environmental change, and lack of control of vector reproduction
already have affected the distribution of VBDs. In Honduras, a sustained increase
in ambient temperature makes the southern part too hot for anopheline mosquitoes,
and reported cases of malaria have dropped off. Large areas of northeast tropical
rainforest have been cleared, and migrants concentrated there tend not to be
immune to malaria (Almendares et al., 1993).
14.2.5.3. Water-Borne Diseases
Extremes of the hydrological cycle, such as water shortages and flooding, could
worsen the diarrhea disease problem. In developing countries, water shortages
cause diarrhea through poor hygiene. On the other extreme, flooding can contaminate
drinking water from watershed runoff or sewage overflow (Patz, 1998). Depending
on the disease agent and its transmission maintenance cycle, the effect may
be an increase or a decrease in the incidence of infectious diseases (Gubler,
1998).
Between the first case of the current cholera outbreak—reported in Peru
in 1991—and December 1996, cholera spread to more than 21 countries, resulting
in almost 200,000 cases and more than 11,700 reported deaths (OPS, 1998). Colwell
and Huq (1994) have collected data in Bangladesh and Peru suggesting that cholera
has a complex route of transmission that is influenced by climate—in particular,
SST and sea-level variations (Lobitz et al., 2000). It has been suggested
that the spread of Vibrio cholerae may be related to the development
of various algae and zooplankton. Extensive studies during the past 25 years
confirming the hypothesis that V. cholerae is autochthonous to the aquatic
environment and is a commensal of zooplankton (i.e., copepods), combined with
the findings of satellite data analyses, provide strong evidence that cholera
epidemics are climate-linked (Lobitz et al., 2000). Increased coastal
algae blooms (which are sensitive to changes in climatic conditions) therefore
may amplify V. cholerae and enhance transmission (Epstein, 2000). Furthermore,
V. cholerae follows a salinity gradient, which might bring the disease to
new shores if sea level rises (WHO, 1998).
In 1998, Ecuador's vulnerability to cholera increased as a result of climatic
phenomena (OPS, 1999). In 1997-1998 in Peru, the same areas affected by
climatic phenomena showed an increase in cholera cases, probably as a result
of floods, problems with drainage, and food contamination in shelters (OPS,
1999). In Peru, persistence in transmission of diarrheal diseases such as Salmonella
typhi and cholera was related to changes in environmental, climatic, and
sanitary conditions (Carrillo, 1991a,b).
Floods foster fungal growth and provide new breeding sites for mosquitoes,
whereas droughts concentrate microorganisms and encourage aphids, locusts, and
whiteflies and—when interrupted by sudden rains—may spur explosions
of rodent populations (Epstein and Chikwenhere, 1994).
The first recorded outbreak of Weil's disease (leptospirosis) in Colombia
occurred mainly in children from poor neighborhoods. Symptoms of leptospirosis
are similar to those of dengue, and the former can be fatal rapidly in patients
not receiving proper treatment. The probable agents and disease seem to be linked
with rodents escaping from floods (Epstein et al., 1995).
In Cuba, acute diarrheal diseases occur more often during the warm and rainy
period, when ecological conditions are favorable for reproduction of bacteria,
viruses, and protozoa. Acute respiratory infection reports diminish after climatic
conditions become warmer, more humid, and thermally less contrasting (Ortiz
et al., 1998). In Mexico, some rain in semi-arid zones has caused bubonic
plague outbreaks (Parmenter et al., 1999).
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