The Three Gorges Dam: Does it accelerate or delay the progress towards eliminating transmission of schistosomiasis in China?
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The Three Gorges Dam, located in the largest endemic area of schistosomiasis in China, is one of the world’s largest hydroelectric projects to date. Some large-scale hydro projects have resulted in schistosomiasis emergence or re-emergence. Therefore, the dam’s potential impact on the transmission of Schistosoma japonicum has raised concerns from medical researchers worldwide. A systematic literature review, coupled with an analysis of data on the water level and snail density in the Yangtze River was conducted to assess the impact of the dam on schistosomiasis transmission after more than 10 years of operation. The dam has significantly altered the water levels in the Yangtze River according to different seasons. These changes directly impact the ecology of the schistosome snail host. Due to the dam, there has been a reduction in the density of Oncomelania snails and/or changes in the distribution of snails. The prevalence of infection with S. japonicum has decreased in the downstream areas of the dam, including in the Dongting and Poyang Lakes. The prevalence of infection with S. japonicum in humans has decreased from 6.80 % in 2002 (before the dam began operating) to 0.50 % in 2012, and the number of people infected with S. japonicum have decreased from 94 208 in 2002 to 59 200 in 2011 in the Poyang Lake region. The presence of the dam does not seem to affect snail breeding or the prevalence of schistosomiasis in the Three Gorges Reservoir. Overall, the Three Gorges Dam has significantly contributed to changes in hydrology after more than 10 years of the dam operating. The changes caused by the dam, together with integrated control of schistosomiasis, might be accelerating the progress towards eliminating the transmission of S. japonicum in the middle and lower reaches of the Yangtze River. Despite the positive effect the dam is having in controlling S. japonicum transmission, continued surveillance is required to monitor the future ecological impacts of the dam over the long term.
KeywordsThree Gorges Dam Schistosomiasis Schistosoma japonicum Oncomelania hupensis hupensis Elimination China
Please see Additional file 1 for translations of the abstract into six official working languages of the United Nations.
The filling of the Three Gorges Reservoir began in June 2003, and its water level rose to 135 metres by the end of 2003, to 156 metres in 2006 and 172 metres in 2008. Since 2009, the water level has been maintained at 175 metres throughout November and December, and is lower in the other months.
It has previously been reported that some large-scale hydro projects (e.g., the Sudanese Gezira-Managil Dam, the Egyptian Aswan High Dam and the Ethiopian Melkasadi Dam) have resulted in schistosomiasis emergence or re-emergence [4, 5, 6, 7, 8, 9]. Hence, the potential impact of the Three Gorges Dam on the transmission of Schistosoma japonicum in the Yangtze River basin has raised concerns from researchers worldwide [10, 11, 12, 13, 14, 15, 16, 17, 18]. Over the past decades, numerous studies have been carried out to forecast or assess the impact of the dam on the distribution of O. h. hupensis snails and the transmission of S. japonicum [19, 20].
Through a systematic review and an analysis of data on the water level and snail density, we assessed the dam’s impact on S. japonicum transmission after more than 10 years of operation. In this paper, we also discuss the potential implications for national strategies to control and eliminate schistosomiasis.
We conducted a systematic literature review by searching all relevant articles, published up to September 2015, which examined the impacts of the Three Gorges Dam on the transmission of schistosomiasis. Relevant studies were identified from the following electronic databases: PubMed, Science Citation Index Expanded™, China National Knowledge Infrastructure, Wanfang Data, China Science and Technology Journal Database and SINOMED. The following keywords and any combinations thereof were used: “Three Gorges Dam” in combination with “schistosomiasis”, “Schistosoma”, “snail” and “Oncomelania hupensis”. No language restrictions were applied.
Two reviewers independently checked the titles and abstracts of all identified articles for inclusion eligibility. We excluded the following: (1) review articles, (2) dissertations, (3) conference abstracts and presentations, and (4) non peer-reviewed reports. For eligible publications, full papers were retrieved and reviewed by the same two reviewers. They were then grouped into two categories: field simulation studies or prediction studies that were carried out before the dam started operating and therefore did not include data on the impact of the dam on schistosomiasis transmission in the study period; and observational or monitoring studies that were implemented after the dam started operating and were able to include data on the impact of the dam on schistosomiasis transmission in the study period.
Data on changes in the water level and snail density
Daily data on the water level (above sea level, 8:00 AM) at the Chenglingji Hydrological Station (located at the junction of the Yangtze River and Dongting Lake) were collected from 1995 to 2013. In 2005, seven snail surveillance sites (villages) along the Yangtze River were set up to monitor the changes in the density of snails in the bottomland areas of the middle and lower reaches of the Yangtze River (see Fig. 1). In these snail surveillance sites, molluscicides for snail control were either not used or used very sparingly. These bottomland areas were surveyed using the traditional Chinese method of systematic sampling (20 m × 20 m) done annually in the spring . Data on snail density (snail/0.11 m2) were collected from the seven snail surveillance sites from 2005 to 2013.
Calculating the time of how long snail habitats are inundated with water
O. h. hupensis snails are distributed mainly in marshlands that are 25–28 metres in elevation in the Dongting Lake area close to the Chenglingji Hydrological Station . Therefore, we calculated the time of 25 metre, 26 metre, 27 metre and 28 metre elevated marshes being inundated with water. If the water level in a day was higher than the elevation of the marsh, we regarded that marsh to be inundated with water. We estimated the link relative ratio of the days the marshes were inundated with water. The link relative ratio refers to the comparison between the days the marshes were inundated with water in a year and the days they were inundated with water in the previous year. For example, the days a marsh was inundated in 1996 are divided by the days it was inundated in 1995, thus achieving the link relative ratio of 1996.
Predicted impact of the dam on schistosomiasis transmission
Predicted impact of the Three Gorges Dam on the distribution of Oncomelania snails and transmission of S. japonicum
Segment of Yangtze River
Water level rising
Water level decreasing
Hubei (Jianghan Plain)
Jan – May
New habitats for Oncomelania snails would appear.
The probability of humans and livestock becoming infected would increase.
Hunan (Dongting Lake)
Jan – May
The distribution of snails would not be significantly affected, however, the reproduction of snails would be effectively curbed.
The probability of humans and livestock becoming infected would increase and schistosomiasis epidemics would worsen.
Jiangxi (Poyang Lake)
Jan – Mar
The reproduction and distribution of snails would be unaffected or the density of snails would increase, or the density of snails would decrease in the autumn.
The epidemiology of schistosomiasis would be very limitedly affected.
Jan – Apr
Oct – Dec
The distribution of snails would not be significantly affected, however, some new habitats for snails would appear.
The probability of humans and livestock becoming infected would increase.
Feb – Apr
The distribution of snails would be not affected and the density of snails would decrease.
The incidences of schistosome infections in people and livestock would increase.
Before the dam started operating, the Three Gorges Reservoir (see Fig. 1) was free for Oncomelania snails and S. japonicum for Yangtze water in the region was rapid following and collides with the cliffs, sandbars and precipices that flank the river . Some studies reported that great environmental changes (e.g., sedimentation and formation of marshlands) resulting from the dam could form potential habitats for Oncomelania snails [27, 28, 29], whereas some simulated studies reported that Oncomelania snails could survive and breed in the Three Gorges Reservoir [30, 31, 32, 33, 34]. Some researchers reported that both Oncomelania snails and transmission sources of schistosomiasis might have been introduced into the Three Gorges region due to the dam [35, 36, 37]. However, other studies reported that the Three Gorges Reservoir would not be an ideal location for the reproduction of Oncomelania snails, as the dam’s ‘winter-water, summer-land’ operation cycle contradicts with the snails’ ‘winter-land, summer-water’ breeding cycle [38, 39].
Dam’s impact on the water level
Dam’s impact on schistosomiasis transmission
The construction of the Three Gorges Dam commenced in 1994 after decades of research and fierce debate. Benefits of the dam, such as flood control and power generation, are indisputable and the dam has lived up to these expectations. However, this study found that the dam’s impact on the transmission of S. japonicum is different to previous forecasts (see Table 1). Overall, this study found that the changes in hydrology caused by the Three Gorges Dam might be favourable for the control of S. japonicum transmission, based on the results from the observational and monitoring studies [19, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50].
There might be several reasons for this. Firstly, the dam has effectively controlled floods and deterred the dispersal of Oncomelania snails, and thus curtailed the infection to humans and animals [52, 53]. For example, some studies reported that the number of patients infected with S. japonicum reduced significantly, and especially that acute S. japonicum cases were rarely seen in the middle and lower reaches of Yangtze River after more than 10 years of the dam’s operation [40, 41, 45]. Secondly, during the first few months of the year, the presence of the dam leads to the production of an early flooded bottomland , and this not only can result in the death of adult Oncomelania snails or reduce their survival rate and oviposition, but is also not a favourable environment for embryo development of Oncomelania eggs . Moreover, after the dam started operating, it has been observed that in the summer, many snail-harbouring marshlands are either not flooded or only flooded for a very short period of time. This is also not favourable for the development of juvenile snails, as Oncomelania snails have to live in water during the early stages of their development . Some studies, together with our monitoring results, showed that the density of Oncomelania snails decreased significantly in many snail-harbouring marshes in the downstream areas of the dam [40, 45, 46, 47, 48]. Thirdly, earlier water recessions in the autumn caused by the dam have advanced the exposure time of snail-harbouring marshes, thus they may be suitable for early planting of crops such as wheat and rapeseed. Currently, some of these marshlands have begun to be planted and this has resulted in a significant decrease in the density of Oncomelania snails . Although some farmers spend a longer period of time working on the marshlands and therefore have a higher chance of coming into contact with cercariae-infected water, other people and livestock such as bovines, the primary infection source of S. japonicum transmission in China, are kept away from these marshlands, meaning that the probability of most humans and livestock becoming infected with S. japonicum decreases. In addition, many studies predicted that advanced water recessions in the autumn would lengthen the amount of time humans and livestock spend on the marshlands and increase the incidence of schistosome infections for both humans and livestock [17, 20]. However, an integrated control programme, aiming to reduce the roles of bovines and humans as infection sources, has been implemented in China since 2005 . The control programme consists of agricultural mechanisation, fencing bovines, chemotherapy for humans and bovines, health education, provision of clean water and improved sanitation . Hence, the programme has led to a reduction in the amount of time that humans and livestock spend on marshlands. Up until now, there have been no reports of increased incidence of schistosome infections due to earlier water recessions in the autumn. Fourthly, the dam operates in a so-called ‘winter-water, summer-land’ cycle, which contradicts with the Oncomelania snails’ ‘winter-land, summer-water’ breeding stimulation. In particular, Oncomelania snails struggle to survive in high water levels in the winter, which is the situation due to the dam [38, 50, 51]. Although both Oncomelania snails and human transmission sources for schistosomiasis may have been introduced from schistosomiasis endemic areas into the Three Gorges Dam region , no local sites of Oncomelania snails or locally infected people have been identified so far .
The changes in hydrology caused by the Three Gorges Dam may have an important implication for the Chinese national control strategies that aim to eliminate schistosomiasis. For example, tractors should plant crops such as wheat and rapeseed in the snail-harbouring marshlands that are suitable for early planting of crops due to early water recessions in the autumn. This can reduce the density of Oncomelania snails or even eliminate snails, as well as increase the income of farmers. Of course, farmers should be educated about how to avoid contact with cercariae-infected water when planting on these snail-harbouring marshlands.
Our study had some limitations. Although a large number of articles were found in the course of the systematic review, most of these were excluded due to duplication or irrelevance, and only 55 papers reporting the relationship between the Three Gorges Dam and the transmission of schistosomiasis were included in the study. Most of the included articles were simulation or forecast studies, or used only routine monitoring data to assess the dam’s impact on the transmission of schistosomiasis. Hence, it was difficult for us to systematically and quantitatively analyse the impacts of the Three Gorges Dam on the transmission of schistosomiasis. In view of this limitation, the relationship between the dam and the transmission of schistosomiasis outlined in this study should be interpreted with caution.
There have been significant changes in hydrology since the Three Gorges Dam started operating in 2003. These changes, together with the integrated control programme currently implemented in China, might be accelerating the progress towards the elimination of S. japonicum transmission in the middle and lower reaches of the Yangtze River. Continued surveillance is needed to monitor the longer-term ecological impacts of the dam on the transmission of schistosomiasis.
This work was supported by the National S & T Major Program (Grant Nos. 2012ZX10004-220 and 2008ZX10004-011).
Availability of data and materials
The data supporting the findings in our paper can be found in Center for Tropical Disease Research, Fudan University.
YBZ and SL contributed to the literature search, YBZ and QWJ conducted the monitoring studies, YBZ and QWJ analysed the data, YBZ, SL, YC and QWJ contributed to the writing of the paper. All authors approved publication.
The authors declare that they have no competing interests.
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