Water for people.pdf - WHO Thailand Digital Repository

1 1 8 / C H A L L E N G E S T O L I F E A N D W E L L - B E I N GBasic Needs and the Right to HealthBox 5.5: Freshwater cyanobacterial toxins – an emerging dam-related health issueIn tropical, subtropical and arid regions of the world it isinevitable that new dams will become eutrophied(nutrient-enriched) rather quickly, often within the firstfew years of filling and operation. Eutrophication bringswith it problems of excessive aquatic weed growth or‘blooms’ of toxic cyanobacteria (a type of microscopicalgae). Arid zones of the world are particularly at risk,where the artificial impoundment of water in hot climatecreates the perfect ecological environment for the growthof toxic cyanobacteria. Added to this natural climatic effectis the enhanced rate of nutrient input that accompaniesthe growth of towns and the development of agriculturein the catchment around a dam, often with inadequateeffluent collection and treatment facilities.Blooms of freshwater algae and cyanobacteria havealways occurred in eutrophied waterways, but the toxicityof these organisms has only been elucidated in recentyears. There are several types of cyanobacterial toxinsfound throughout the world, all of which are potentiallylethal to humans and animals if consumed in sufficientquantities. Additionally, some cyanobacterial toxins canpromote liver cancer during chronic low-level exposure,and most cyanobacteria can cause a range ofgastrointestinal and allergenic illnesses in humans exposedto toxins in drinking water, food or during swimming. Anorm for drinking water concentrations for the commoncyanobacterial toxin microcystin has recently beendeveloped by WHO.The most severe and well-documented case of humanpoisoning due to cyanobacterial toxins occurred in theBrazilian city of Curaru in 1996. Inadequately treatedwater from a local reservoir was used for patients in alocal kidney dialysis clinic. As a consequence, more thanfifty people died due to direct exposure of thecyanobacterial toxin to their blood stream during dialysis.Elsewhere in South America, in 1988, more than eightydeaths and 2,000 illnesses due to severe gastro-enteritishave also been directly linked with toxic cyanobacteria in anewly constructed dam. In China, a high incidence ofprimary liver cancer has been linked to the presence ofcyanobacterial toxins in drinking water.Source: Chorus and Bartram, 1999.a significant nutritional reliance on traditional productionmethods such as recession agriculture.■ Sensitive management of flood plains and water resources toensure wetland conservation, while at the same timeminimizing excessive propagation of water-borne and waterrelatedvector-borne diseases. As with irrigated agriculturalproduction systems, natural seasonal wetting and drying cycleswill be an important management tool. Conventional irrigationand drainage practices often lead to permanent inundation andwetting of previously ephemeral wetlands. The outcome of thisis both degradation of the wetland and increased health risks.3. Design and management options in irrigation schemes■ Minimizing low flow zones in artificial channel networks toeliminate habitats for the propagation of disease vectors.■ Concrete lining of irrigation canals to reduce seepage andprevent pools of standing water where mosquito vectorspropagate. This also has water-saving benefits for irrigation.■ Management of irrigated cropping systems to maintainwetting and drying cycles (while ensuring efficiency in wateruse), crop diversification and synchronization of croppingpatterns. Regular wetting and drying of flooded rice fieldsprovides an important tool to control water-associated vectorbornediseases such as malaria and Japanese encephalitis. Inparticular, there should be no advocacy of excessive multiplecropping within a single production year, and synchronizationof cropping cycles is recommended.■ Management plans for irrigated areas that minimize long-termsalination and water logging and therefore impact on foodsecurity and scheme viability.The cost-effectiveness of water interventionsIn order to allow informed decision-making on interventionsaimed at disease prevention and control, it is crucial to carry outa sound economic evaluation of the various options available inspecific settings. This will permit the selection of an option orcombination of options ensuring maximum health benefits within