Pathogens often are found in coastal waters; transmission occurs though shellfish consumption or bathing. Coastal waters in developed and developing countries frequently are contaminated with untreated sewage. Higher temperatures encourage microorganism proliferation. The presence of Vibrio spp. (some of which are pathogens that cause diarrhea) has been associated with higher sea-surface temperature (SST) (Lipp and Rose, 1997). Vibrio vulnificus is a naturally occurring estuarine bacterium that may be more often transmitted to humans under conditions of higher SST (Patz et al., 2000).
Acute poisoning can occur following consumption of fish and shellfish contaminated
with biotoxins (WHO, 1984). Phytoplankton organisms respond rapidly to changes
in environmental conditions and therefore are sensitive biological indicators
of the combined influences of climate change and environmental change (Harvell
et al., 1999). Algal blooms are associated with several environmental factors,
including sunlight, pH, ocean currents, winds, SSTs, and runoff (which affects
nutrient levels) (Epstein et al., 1993; NRC, 1999). Algal blooms can be harmful
to fish and other aquatic life, often causing severe economic damage, and are
reported to have increased globally in the past several decades (Hallegraeff,
1993; Sournia, 1995), although some of the observed increase is attributed to
changes in monitoring, effluent, and land use.
There is no straightforward relationship between the presence of an algal bloom
and an outbreak of poisoning. Human poisoning can occur in the absence of a
bloom. Two main types of biotoxin poisoning are associated with temperate climates
and colder coastal waters: paralytic shellfish poisoning and diarrheic shellfish
poisoning. If water temperatures rise as a result of climate change, shifts
in the distribution of these diseases could follow. Biotoxins associated with
warmer waters, such as ciguatera in tropical waters, could extend their range
to higher latitudes (Tester, 1994). An association has been found between ciguatera
(fish poisoning) and SST in some Pacific islands (Hales et al., 1999a).
Recent evidence suggests that species of copepod zooplankton provide a marine
reservoir for the cholera pathogen and facilitate its long-term persistence
in certain regions, such as the estuaries of the Ganges and Bramaputra in Bangladesh
(Colwell, 1996). The seasonality of cholera epidemics may be linked to the seasonality
of plankton (algal blooms) and the marine food chain. Studies using remote-sensing
data have shown a correlation between cholera cases and SST in the Bay of Bengal
(Lobitz et al., 2000). Interannual variability in cholera incidence in Bangladesh
also is linked to ENSO and regional temperature anomalies (Pascual et al., 2000).
Epidemiological evidence further suggests a widespread environmental cause of
the 1991 epidemic in Peru, rather than point-source contamination (Seas et al.,
2001). There is some evidence for a link between warmer sea surfaces and cholera
risk in the Bay of Bengal, but it is not possible to extrapolate such findings
to cholera incidence inland or in other regions. The potential impact of long-term
climate warming on cholera incidence or risk of epidemics remains uncertain.
Climate-related ecological changes may enhance primary and secondary transmission of cholera in developing countries, particularly among populations settled in low-lying coastal areas in the tropics. However, the causal link between sea temperature, plankton blooms, and human disease requires further elucidation and confirmation.
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