Transcript of "Dams and Disease Triggers on the Lower Mekong River"
ViewpointsDams and Disease Triggers on the Lower Mekong RiverAlan D. Ziegler1*, Trevor N. Petney2, Carl Grundy-Warr1, Ross H. Andrews3,4, Ian G. Baird5,Robert J. Wasson1, Paiboon Sithithaworn31 Geography Department, National University of Singapore, Singapore, 2 Department of Ecology and Parasitology, Zoology Institute, Karlsruhe Institute of Technology,Karlsruhe, Germany, 3 Department of Parasitology, Liver Fluke and Cholangiocarcinoma Research Center, Medical Faculty, Khon Kaen University, Khon Kaen, Thailand,4 Faculty of Medicine, Imperial College London, United Kingdom, 5 Department of Geography, University of Wisconsin, Madison, Wisconsin, United States of AmericaOngoing and proposed construction ofseveral large hydropower dams along themainstream Mekong River and varioustributaries has created a number ofunanswered environmental and societalquestions for governments and communi-ties in Cambodia, China, Lao PDR,Myanmar, Thailand, and Vietnam [1–3].Most concern over the controversial dam-building projects focuses on the extent towhich river health and food security willbe affected negatively. Foremost, the 85 ormore proposed dams threaten to reducethe diversity and abundance of freshwaterfish, the major animal protein source formany of the 67 million inhabitants of theMekong River basin [4–7].The Xayaburi Dam is the first of 12dams proposed for the Lower MekongRiver (Figure 1). Delayed in mid-2012 inresponse to international and regionalpressure to reevaluate potential environ-mental consequences, the project hasrecently been approved again. It is pro-jected to affect the livelihoods of hundredsof thousands of people through detrimen-tal impacts on river ecology—fishing inparticular. This is just one dam. Theimpact of the entire network could bemuch more devastating. A recent studysimulated catastrophic losses to fish pro-ductivity and diversity as a result ofconstruction of 85 dams on the mainchannel and tributaries .From a hydro-geomorphological stand-point, the cascade of dams would funda-mentally change the river flow regime thatmaintains fish habitat and sediment trans-port. Not only would seasonal flows beaffected, the magnitude and timing ofhigh-flow events would lessen, greatlyaffecting aquatic environments . Sea-sonally inundated floodplains adjacent tothe river channels are productive feeding,spawning, and nursing habitats for impor-tant Mekong fishes. Perhaps more impor-tant, deep pools in some sections of themain channel serve as fish refuge habitatsduring the dry season . Furthermore,sediments that are naturally transported inthe river are vital to maintaining aquatichabitats such as pools and sand bars inriver channels, as well as downstreamdeltas. Aquatic life also depends onsediment-associated nutrients, detritus,and organic debris.Habitat destruction caused by a lack offlushing during large flood events andreductions of sediment could result in aloss of fish diversity and abundance,further adding to impacts caused bydisruption of migration patterns. Theimplementation of fish ladders or otherfish passage systems—such as those pro-posed for the Xayaburi dam—are antici-pated to have a limited effect for Mekongriver fish . Fish would still have tonavigate through the turbines to passdownstream. Furthermore, fish passeswould provide no relief for habitat loss,nor would they allow the downstream driftof fish larvae.While the linkage between dam build-ing, fish ecology, and food security isobvious, limited attention has been givento the potential threat Mekong dams willpose to public health via disease ecologyand food safety . The rapidly changingMekong River basin harbors a widediversity of water- and food-associatedpathogens. These range from mosquitovectors carrying malaria and dengue feverto diarrheal disease–related protozoa (e.g.,Cryptosporidium, Giardia lamblia, and Ent-amoeba species) and schistosomes that infecthumans contacting water containing in-fectious cercariae . Food-borne trem-atodes such as liver, lung, and intestinalflukes also contribute to great morbidityand mortality, largely because of thepredilection of many Mekong inhabitantsto eat partially cooked or raw fish, some ofwhich are infected with carcinogenicflukes, such as Opisthorchis viverrini[12,13,14] (Figure 2).Dam building could potentially triggeran increase in the incidence of many ofthese neglected tropical diseases, becausetheir epidemiology is inherently linked towetland ecology and surface water man-agement. For example, dam building mayincrease the habitat required for thesurvival and/or reproduction of malariavectors, such as Aedes, Anopheles, and Culexspp. . Elsewhere, emergences or re-emergences of schistosomiasis have result-ed from large-scale hydropower projects: e.g., Gezira-Managil Dam (Sudan),Aswan Dam (Egypt), Melkasadi Dam(Ethiopia), and the Danling and HuangshiDams (China). Similarly, changes in waterlevel and downstream sediment depositionresulting from the building of the ThreeGorges Dam in China are expected toincrease the schistosomiasis transmissionseason within the marshlands along themiddle and lower reaches of China’sYangtze River [11,15]. Schistosomiasisnaturally occurs in the mainstream Me-kong in southern Laos and northeasternCambodia, and dam building could in-crease its prevalence.There is already an example in theLower Mekong Basin linking infectiousdisease ecology and anthropogenic chang-es in surface water. Development projectsCitation: Ziegler AD, Petney TN, Grundy-Warr C, Andrews RH, Baird IG, et al. (2013) Dams and Disease Triggerson the Lower Mekong River. PLoS Negl Trop Dis 7(6): e2166. doi:10.1371/journal.pntd.0002166Editor: Xiao-Nong Zhou, National Institute of Parasitic Diseases, China CDC, ChinaPublished June 13, 2013Copyright: ß 2013 Ziegler et al. This is an open-access article distributed under the terms of the CreativeCommons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,provided the original author and source are credited.Funding: This work was funded by grants from the National University of Singapore (R-109-000-092-133, R-109-000-090-112). Travel funding for international cooperation was provided by the Deutsche Forschungsge-meinschaft (PE 1611/3-1), the National Research Council of Thailand, ASEAN-EU, and Karlsruhe Institute ofTechnology International Affairs. The funders had no role in study design, data collection and analysis, decisionto publish, or preparation of the manuscript.Competing Interests: The authors have declared that no competing interests exist.* E-mail: firstname.lastname@example.orgPLOS Neglected Tropical Diseases | www.plosntds.org 1 June 2013 | Volume 7 | Issue 6 | e2166
in Lao PDR aimed at road infrastructureimprovement and flood reduction havefacilitated the excavation of small aqua-culture ponds within villages, which nowprovide anthropogenic microhabitats inwhich the full life cycle of Opisthorchisviverrini can occur . The threat of anincrease in the prevalence of O. viverrinifurther escalates because there are noprecautions to prevent the stocking ofaquiculture systems with fish that areinfected with trematodes, including O.viverrini, in nurseries and hatcheries, aswas found in the Lao study and anothersite in northeast Thailand [16,17].The linkage between opisthorciasis andhome garden ponds in Lao PDR demon-strates the potential for dam building onthe Mekong to both directly and indirectlycreate new habitats where the entire lifecycle of O. viverrini can be completed. Forexample, it is likely that aquaculture insmall-scale ponds as well as in reservoirsbehind dams, will increase to help offsetthe reduction in wild-captured fish proteincaused by dam building—an estimated340,000 tons, representing the world’smost important inland fishery . Thethreat of the disease magnifies in suchconfined environments, because free-swimming cercariae have a greater prob-ability of contacting and infecting nativefish hosts. Additionally, water releasedfrom dams for irrigation and duringhigh-capacity periods can act as vectoraccumulation sinks, providing new habi-tats for infected snail and fish intermediatehosts .We recognize a number of uncertaintiesin our assessment of the linkages betweendam building and disease triggers. Forexample, the risk of disease incidence willprobably always vary greatly throughoutthe Mekong basin. In the case of O.viverrini, incidence is in part determinedby food-related elements of the culture,namely the predilection of some groups ofpeople to eat insufficiently cooked fishdishes . In areas where infection fromaquaculture fish are of concern, introduc-ing exotic fish that are not known hosts oftrematode parasites could reduce the riskof increased human infection, but suchspecies might not be accepted in particulartypes of local dishes because of tastepreferences [12,14].At a larger scale, the proposed cascadeof dams may offset increases in thetransition of several types of water-borneand vector-borne communicable diseasesthat often occurs following large, protract-ed floods. However, such a positiveoutcome is uncertain, as floods of thisscale are related to unpredictable climaticevents, as well as to reservoir storage andrelease management decisions. With theFigure 1. Proposed and existing dams on the man stem of the Mekong river. More than 85 dams are now proposed to be built on the mainchannel and tributaries of the Mekong River in Southeast Asia (only those on the main stem are shown). The Xayaburi Dam is the first of several damsproposed for the Lower Mekong mainstream. Three dams are already built on the main branch of the upper Mekong River in China (Manwan,Dachaoshan, Jinghong); two more are under construction (Xiaowan, Nuozhadu) and three more are planned.doi:10.1371/journal.pntd.0002166.g001PLOS Neglected Tropical Diseases | www.plosntds.org 2 June 2013 | Volume 7 | Issue 6 | e2166
major focus of Mekong dams being powergeneration, high water levels are quitelikely to be maintained throughout the latepart of the monsoon season when tropicalstorms strike the Mekong basin .Recent floods on the Chao Phraya Riverin Thailand and the Sesan River inVietnam were arguably exacerbated bythe difficulties of managing reservoirswhen catchments were wet, reservoirswere full, and tropical storms struck[18,19].Uncertainty aside, political and ad-ministrative measures are needed tominimize the impact of disease vectorsthat benefit from dam building andlandscape modifications in the basin.Environmental and social impact assess-ments should address the negative effectson aquatic biodiversity and food produc-tion, as well as changes in infectiousdisease ecology related to the modifica-tion of hydrological patterns and wetlandenvironments. Moreover greater atten-tion should be given to the impacts ofdams built on Mekong tributaries, whichhave received less attention but wouldalso be affected negatively.While some may believe that buildingcascades of dams on the Mekong River isan appropriate solution to growing energydemands in the Mekong region, thepotential threat to biodiversity, regionalfood security, food safety, and publichealth are potentially unmatchable. Webelieve the long-term impacts likely coun-ter the economic gains of energy export,which will be short-term because of thelimited lifetimes of reservoirs (estimated at50–100 years). More carefully designeddams could reduce or mitigate someimpacts, but so far no changes have beenFigure 2. The life cycle of the O. viverrini parasite. The life cycle of the O. viverrini parasite involves intermediate snail (Bithynia sp.) and nativefish (cyprinid sp.) hosts, which are commonly found in wetland environments within the Mekong Basin, as well as in definitive human and carnivorereservoir hosts. Alarmingly, O. viverrini currently infects more than 10 million people in Thailand and Laos PDR . The cholangiocarcinoma causedby the O. viverrini liver fluke will likely kill hundreds of thousands of people in the coming decades . The wide variability of incidence across theregion is largely related to culture and eating behavior . There are also insufficient data to make accurate assessments in countries such as Laosand Cambodia. Humans become infected by ingesting metacercariae in uncooked fish . The ingested metacercariae excyst in the duodenum andenter the bile duct, where they develop into sexually mature adult worms. The eggs produced by the adults are discharged with bile fluid into theintestine, and out of the body with the feces. When viable eggs from improperly treated waste reach a body of freshwater and are ingested by anappropriate snail, miracidia hatch and develop into sporocysts and rediae. The rediae gave rise to free-swimming cercariae and, when exposed toappropriate cyprinid species of fish (the second intermediate hosts), the cercariae penetrate into the tissues or skin of freshwater fish and becomefully infective metacercariae. When humans then eat these fish raw or undercooked, the life cycle is completed.doi:10.1371/journal.pntd.0002166.g002PLOS Neglected Tropical Diseases | www.plosntds.org 3 June 2013 | Volume 7 | Issue 6 | e2166
proposed to reduce impacts on food safety,disease, and longevity of reservoirs.An alternative approach for supplyingenergy locally with fewer negative impactscould involve working with the region’smonsoon climate regime, which creates adistinct seasonality with respect to naturalresources that could be used for energygeneration. Integrated systems could bedeveloped to maximize the generation ofsolar energy in the relatively cloud-free dryseason, pico- or small-scale hydropower inthe uplands during the wet season, andwind and geothermal energy in areas and/or seasons that are appropriate. Addition-ally, green toilet systems could be used forthe dual purpose of creating energy andimproving the sanitation of human wastethat is in part responsible for the presenceof some water-borne parasites in wetlandsystems.AcknowledgmentsThe manuscript benefitted from insights fromAndrew Campbell, Carl Middleton, SussaneFach, Stefan Norra, Steve Van Beek.References1. Grumbine RE, Xu JC (2011) Mekong hydropow-er development. Science 332: 178–179.2. Stone R (2011) Mayhem on the Mekong. Science12: 814–818.3. Vaidyanathan G (2012) Remaking the Mekong.Nature 478: 305–307.4. Dugan PJ, Barlow C, Agostinho AA, Baran E,Cada GF, et al. (2010) Fish Migration, Dams, andLoss of Ecosystem Services in the Mekong Basin.Ambio 39: 344–348.5. Baird IG (2011) The Don Sahong Dam: PotentialImpacts on regional fish migrations, livelihoods,and human health. Critical Asian Studies 43:211–235.6. Orr S, Pittock J, Chapagain A, Dumaresq D(2012) Dams on the Mekong River: Lost fishprotein and the implications for land and waterresources. Global Environ. Change. 22: 925–932.7. Ziv G, Baran E, Nam S, Rodrı´guez-Iturbe I,Levina SA (2012) Trading-off fish biodiversity,food security, and hydropower in the MekongRiver Basin. Proceed Nat Acad Sci 109: 5609–5614.8. Ra¨sa¨nen TA, Koponen J, Lauri H, Kummu M(2012) Downstream Hydrological impacts ofhydropower development in the upper Mekongbasin. Water Res Manage 26: 3496–3513.9. Baird IG (2006) Strength in diversity: Fishsanctuaries and deep-water pools in Laos. Fish,Manage Ecol 13: 1–8.10. Guerrier G, Paul R, Sananikhom P, Kaul S,Luthi R (2011) Strategic Success for Hydropowerin Laos. Science 334: 38.11. Petney TN, Taraschewski H (2011) Water-borneparasitic diseases: hydrology, regional develop-ment and control. In: Frimmel FH, editor. WaterChemistry and Microbiology, vol. 3, A Treatiseon Water Science. Elsevier. pp. 303–366.12. Grundy-Warr C, Andrews RH Sithithaworn P,Petney TN, Sripa B, et al. (2012) Raw Attitudes,Wetland Cultures, Life-Cycles: Socio-culturaldynamics relating to Opisthorchis viverrini inthe Mekong Basin. Parasitol Int 61: 65–70.13. Sripa B, Kaewkes S, Sithithaworn P, Mairiang E,Laha T, et al. (2007) Liver Fluke InducesCholangiocarcinoma. PLoS Med 4(7): e201.doi:10.1371/journal.pmed.004020114. Ziegler AD, Andrews RH, Grundy-Warr C,Sithithaworn P, Petney TN. (2011) Fightingliverflukes with food safety education. Science331: 282–283.15. McManus DP, Gray DJ, Li Y, Feng Z, WilliamsGM, et al. (2010) Schistosomiasis in the People’sRepublic of China: the Era of the Three GorgesDam. Clin Microbiol Rev 23: 442–466.16. Sithithaworn P, Ziegler AD, Grundy-Warr C,Andrews RH, Petney T (2012) Alterations to thelifecycle of liverflukes: dams, roads, and ponds.Lancet Infect Dis 12: 588.17. Piraksakulrat O. Sithithaworn P, Laoporn N,Laha T, Petney TN, et al. (2013) A Cross-sectional study on the potential transmission ofthe carcinogenic liver fluke Opisthorchis viverrini andother fishborne zoontic trematodes by aquiculturefish. Foodborne Pathog Dis 10. DOI: 10.1089/fpd.2012.125318. Ziegler AD, Lim HS, Tantasarin C, Jachowski N,Wasson RJ (2012) Floods, False Hope, and theFuture. 2012. Hydrol Process 26: 1748–1750.19. Wyatt AB, Baird IG (2007) Transboundaryimpact assessment in the Sesan River Basin: thecase of Yali Falls Dam. International Journal ofWater Resources Development 23: 427–442.PLOS Neglected Tropical Diseases | www.plosntds.org 4 June 2013 | Volume 7 | Issue 6 | e2166