Working Group II: Impacts, Adaptation and Vulnerability

Other reports in this collection 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). 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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