FIGURE 1-4 ENSO teleconnections and risk map for malaria. SOURCE: Reprinted from Kovats et al. (2003) with permission from Elsevier.


not occur in the southern African region and some areas received above average rainfall.

Despite the limitations of available data, a number of robust associations have been described at various sites throughout the world, particularly with epidemic malaria in countries such as Colombia, Venezuela, and Guyana (Kovats et al., 2003). In highland areas, such as northern Pakistan, higher temperatures associated with El Niño may increase the transmission of malaria. In many desert fringe regions, such as the Punjab and the Thar Desert, increased rainfall during the post-El Niño and La Niña years has historically increased epidemic risk. Conversely, El Niño-related droughts have been associated with malaria outbreaks in Sri Lanka, Colombia, and Venezuela, possibly as a result of reduced river flows, which in turn permitted increased breeding of mosquitoes.

The yearly incidence of dengue has also been associated with the El Niño year in Indonesia. Monthly cases of dengue on some but not all Pacific Islands have been associated with the Southern Oscillation index. In Australia, epidemic years for the Ross River virus disease over a 70-year period up to 1998 showed some association with the Southern Oscillation index, but two other studies did not show a relationship between yearly notifications of this disease and the index. Associations of health outcomes with climatic factors may be localized and may not be detectable with aggregated data.

The association between cholera and ENSO has been established in Dhaka, Bangladesh, and the relationship has become apparent in recent decades (Rodo et al., 2002).

The annual incidence of visceral leishmaniasis in Bahia State, northeastern Brazil, was significantly related to sea surface temperature in the Pacific Ocean (Franke et al., 2002) probably as a result of the association between the ENSO cycle and drought in northeast Brazil.

The El Niño event of 1993 resulted in increased rainfall in the southern United States, which was then followed by a drought that facilitated the emergence of hantavirus pulmonary syndrome. There was also a fivefold increase in reported cases associated with the 1997-1998 El Niño. Although there is a plausible mechanism for ways in which climatic factors can lead to substantial increases in the rodent population and thus hantavirus transmission, a consistent association with ENSO has not been established (Engelthaler et al., 1999). Likewise in the case of Rift Valley fever in East Africa, there is good evidence that epidemics in the dry grasslands are triggered by heavy rainfall events, but there is no association between this disease in Kenya and the ENSO index (Linthicum et al., 1999).

Extreme Climate Events and Infectious Diseases

Case reports suggest that extreme climatic events such as floods, droughts, and storms can be associated with outbreaks of infectious disease, but the associa-


tion is not always seen and is likely to depend on socioeconomic factors, damage to infrastructure, and the extent of population displacement.

There are a number of mechanisms by which the risk of flooding is likely to increase as a result of climate change. These include the melting of glaciers, increased frequency of episodes of heavy precipitation, and sea level rise. However, an extensive review of the effects of floods in Europe (Hajat et al., 2003) showed that infectious disease outbreaks were rarely a major public health problem. A more consistent finding seems to be increased prevalence of common mental disorders (i.e., anxiety, depression) following exposure to floods. This is likely to be due to loss of familiar possessions, forced evacuation, loss of livelihood, and increased poverty.

However, following floods, increases in diarrheal and respiratory diseases have been reported in both high- and low-income countries. Crowding of displaced populations may contribute to transmission. In low-income countries in particular, there may be outbreaks of leptospirosis, hepatitis, and vector-borne diseases, including malaria and Rift Valley fever (Ahern et al., 2005). On occasions, such as in Tanzania in 1997-1998, flooding had the opposite effect and washed away vector breeding sites.

Populations in low-lying areas are vulnerable to the effects of sea level rise, particularly in low-income countries where flood defenses may not be upgraded. For example, up to 57 percent of people in unprotected dry land areas in Bangladesh could be inundated if there was a 4oC temperature increase and, as a consequence, a 100 cm increase in sea level together with increases in monsoon precipitation and discharge into major rivers (BCAS/RA/Approtech, 1994). Deltaic regions are particularly vulnerable to the effects of sea level rise as illustrated in Figure 1-5. Salinization of freshwater aquifers may occur as a result of incursion of seawater. Even in industrialized countries, densely populated urban areas are at risk from sea level rise, as shown by the impact of Hurricane Katrina on New Orleans. After Hurricane Katrina there were increases in diarrheal disease incidence due to fecal contamination of drinking water (Manuel, 2006).

The potential health effects of drought in developing countries are likely to be wide ranging (see Figure 1-6). It is important to note, however, that domestic water consumption represents only 2 percent of the global total, and a flow sufficient to meet the domestic water requirements of around 1,000 people would be sufficient to irrigate only 1 hectare of land, which is capable of feeding a couple of families (Shiklomanov, 2000). It has also been pointed out that the weight of evidence from studies in low-income countries suggests that ready access to water that results in increased quantities used for hygiene is likely to be more important than water quality improvements by themselves in determining benefits for health (Cairncross, 2003). This is likely to be due to the fact that most endemic diar-rheal disease is transmitted person-to-person by hands, food, and other fomites because of poor hygiene.

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