Climatic systems display complex interactions of interconnected components, including the atmosphere, hydrosphere, cryosphere, bioshphere and geosphere. Global climate change will interfere with these interactions and alter the hydrologic cycle not only by altering mean meteorological measures but by increasing the frequency of extreme events such as excessive precipitation, storm surges, floods and droughts. These extreme weather-related events can affect water availability, quality, or access, posing a threat to human populations. Water-borne pathogens often act in concert through two major exposure pathways: drinking water and recreational water use. Therefore, we have not discussed water-borne diseases by pathogen but rather by pathway.
Recreational water use
Theoretical, simulation, and empirical data corroborate that increased water vapour, due to higher mean temperatures, triggers more intense precipitation events, even if the precipitation quantity remains constant. Such events can result in run-off and loading of coastal waters with pathogens, nutrients and toxic chemicals that may adversely affect aquatic life and public health. This is particularly true in coastal watersheds where human development and population increases have led to urbanisation of coastal areas. Storm surges greatly increase the risk and the amount of pollutants entering recreational coastal waters. Climate change events can negatively impact public health: exposure to southern California coastal waters during an El Niño winter compared to a La Niña winter doubles the risk of symptoms related to infectious agents. Risk of gastroenteritis and respiratory infections due to recreational water use are much higher during the rainy season rather than the dry season. Precipitation is projected to increase in northern Europe, but no similar studies have been published for Europe so far. Conversely, extended periods of hot weather can increase the mean temperature of water bodies which can be favourable for microorganism reproduction cycles and algal blooms. For example, Vibrio spp bacteria (including Vibrio vulnificus and Vibrio cholerae non-O1 and non-O139), indigenous to the Baltic and the North Sea, have displayed increased growth rates during unusually hot summers (eg,. 2006) and infected open wounds that can necrotise and cause severe sepsis.
Water-borne outbreaks have the potential to be rather large and of mixed etiology but the actual disease burden in Europe is difficult to approximate and most likely underestimated. In 2006, merely 17 water-borne outbreaks were reported by five countries, clearly significantly under-reported. They involved a total of 3952 patients, of which 181 were hospitalised, afflicted by a number of causative agents including campylobacter, calicivirus, giardia, and cryptosporidium.
Erratic and extreme precipitation events can overwhelm water treatment plants and lead to cryptosporidium outbreaks due to oocysts infiltrating drinking-water reservoirs from springs and lakes and persisting in the water distribution system. A study from England and Wales found that 20% of water-borne outbreaks in the past century were associated with a sustained period of low rainfall, compared with 10% associated with heavy rainfall. Droughts or extended dry spells can reduce the volume of river flow possibly increasing the concentration of effluent pathogens posing a problem for the clearance capacity of treatment plants.
In Europe, flooding has rarely been associated with an increased risk of water-borne disease outbreaks, but a few exceptions exist in the UK, Finland, the Czech Republic, and Sweden. Cholera, caused by Vibrio cholerae, is an imported disease in Europe, with only 11 confirmed cases in 2006. However, internationally, cholera outbreaks during the warmer months display a seasonal pattern in higher absolute latitudes and climate change might influence the strength, duration, or appearance of such a seasonal pattern.
Aging water treatment and distribution systems are particularly susceptible to weather extremes posing a significant vulnerability of the drinking water supply. Environmental pollutants can synergistically interact with climatic conditions and exacerbate exposure of human populations. Infrastructure improvements and environmental protection can attenuate potential negative consequences of climate change from water-borne diseases.
Source: Semenza JC, Menne B. Climate Change and Infectious Diseases in Europe. Lancet ID. 2009;9:365-75.
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