HumBio 153: Parasites and Pestilence
Surveillance Program for the Three Gorges Reservoir: A Long-Term Study on the Effect of the Three Gorges Dam on Schistosoma japonicum Transmission
Andee Nos, Simon Wu
Schistosomiasis has been a prevalent health issue in China for over two thousand years (Zhou et al., 2005), causing health problems such as intestinal damage, anemia, and hepatomegaly (CDC, 2008). Efforts to reduce Schistosoma japonicum transmission along the Yangtze River and in other parts of China have been extremely successful in many areas, but the blood fluke is now reemerging (Liang et al., 2006). The construction of the Three Gorges Dam on the Yangtze River adds a new variable in controlling the spread of schistosomiasis along the Yangtze River as it may contribute to the formation of new potential snail habitats (Sleigh and Jackson, 1998). The area around the dam is currently free of schistosomiasis, but as a "result of large-scale displacement of people, creation of new marshland areas around the perimeter of the dam's reservoir, and the expansion of irrigated farming in the area," there is a strong potential for the non-endemic area to become endemic (Yang, 2005). At the moment, there is no surveillance in the reservoir region (E. Seto, oral communication, May 2008). Therefore, the proposed project will use predictive technology and ground surveillance to provide a baseline and monitor schistosomiasis prevalence and transmission in the new reservoir region upstream of the dam (R. Spear, oral communication, May 2008).
The proposed project will determine how the Three Gorges Dam affects the potential for schistosomiasis upstream in the Three Gorges Reservoir. In other endemic countries, dam construction has increased schistosomiasis prevalence in the reservoir regions (Jobin, 1999), but due to the fact that these studies in other countries looked at different snail and schistosoma species, there's still the potential that schistosomiasis may not spread into the Three Gorges Reservoir. Thus, we need to implement a surveillance plan to definitively provide an answer.
Our proposed surveillance plan will:
The parasitic disease schistosomiasis, caused by the blood fluke Schistosoma japonicum, has been a widespread endemic health problem in China. S. japonicum, in its cercariae form, can burrow through unbroken skin to infect humans. Once inside, the cercariae travel to the veins, develop into mating adults, and eventually lay eggs that are subsequently discharged along with the feces. The eggs hatch while in water to become the parasiteÕs miracidia form, which need to infect aquatic snails, the intermediate host, to develop into the human-infecting form. The snail host for S. japonicum is Oncomelania hupensis, an amphibious snail that can survive in marshlands and irrigated rice fields (Jobin, 1999). Cattle and water buffalo can act as competent reservoirs for the parasite and they both figure heavily in the transmission cycle in China (Zhao et al., 2005). Figure 1 shows a more detailed visual representation of S. japonicumÕs life cycle.
At the height of its disease burden in China, S. japonicum in the 1950s infected more than 11 million people (Zhou et al., 2005). To explain why schistosomiasis was able to become so prevalent, the diseaseÕs risk factors must be considered. Sanitation is often lacking in ChinaÕs rural areas. Frequent water contact puts one at significant risk for schistosomiasis infection, since the cercaria form of S. japonicum can only infect humans in water. Rice is commonly grown in China and needs extensive irrigation. This irrigation system serves as a suitable habitat for O. hupensis. As evidence of this enormous risk factor for schistosomiasis, farmers have both the third highest rates of water contact and the third highest risk of S. japonicum infection in China (Huang and Manderson, 2005).
In response to this high prevalence, the Chinese government launched a control program in the 1950s that emphasized distribution of Praziquantel to existing endemic areas and habitat control of O. hupensis. This public health initiative was successful in reducing the schistosomiasis burden all the way down to 694,788 infections in 2000. The schistosomiasis control program successfully eliminated transmission in 5 out of the 12 originally endemic provinces. However, O. hupensis habitats are still a fixture in China and transmission still occurs regularly in the other 7 provinces. Additionally, the number of cases has recently risen back to 843,011 in 2003 (Zhou et al., 2005). Hypotheses for the reemergence phenomenon include China relaxing its control efforts after the World Bank stopped loaning money to China in 2001 for the control project (Zhou et al., 2005; Chen et al., 2005) and the increased trade of infected cattle into non-endemic areas (R. Spear, oral communication, May 2008).
Two of the provinces that remain endemic are the Sichuan and Hubei provinces. These two provinces are of special interest to schistosomiasis control in China because of the Three Gorges Dam project. As shown in Figure 2, the dam is located in the Hubei province, while the reservoir that the dam creates, named the Three Gorges Reservoir, stretches from the dam for about 600 kilometers west into the Sichuan province. The Three Gorges Dam will be the largest dam ever built (Jackson and Sleigh, 2000). The damÕs purpose is both to provide China with an alternative source of energy and to control flooding in the Yangtze River (Jobin, 1999).
Dams have been known to significantly affect their surrounding water ecology and consequently increase transmission of water-borne diseases. A meta-analysis of water resource development studies found that people in Africa who lived next to a dam reservoir were 2.4-2.6 times more likely to become infected with schistosomiasis than people who didnÕt live near a reservoir (Li et al., 2007). After the Diama Dam in Senegal was completed in 1988, rates of schistosomiasis from S. mansoni skyrocketed from limited transmission in 1986 to 50% in 1990 and then to 90% in 1991 (Jobin, 1999). The damÕs operation stopped previous flooding that prevented snail development, thus leading to the formation of brand-new snail habitats in the reservoir. The lack of flooding also allowed people to increase irrigation for rice farming, further increasing snail habitat formation.
As of 2002, no O. hupensis snails were found in the Three Gorges Reservoir because the water was thought to be flowing too quickly to allow development. However the flora, temperature and rainfall are all suitable for O. hupensis development (Zheng et al., 2002). Additionally, the damÕs operation will invariably slow down the water flow upstream of the dam. Decreased water flow in the reservoir over time will cause siltation and create new marshlands, ideal habitat for O. hupensis (E. Seto, oral communication, May 2008; Jackson and Sleigh, 2000). Schistosomiasis endemic areas and O. hupensis have been found both upstream and downstream of the reservoir and it wouldnÕt be hard for the snail to be accidentally imported into the reservoir (Jackson and Sleigh, 1998; Zheng et al., 2002). Figure 3 shows a map of the Yangtze River and the Three Gorges Reservoir along with the locations of schistosomiasis endemic areas in the region. It is clear then that the Three Gorges Dam could easily cause the reservoir to become endemic in the years ahead, as was the case in Senegal.
There are also demographic, anthropologic, and geographic factors in the Three Gorges Reservoir that are important to consider in the context of schistosomiasis emergence. Farming was common in the reservoir region before the damÕs construction, but the dam flooded 23,800 hectares of farmland and forced 361,500 people from their homes (Zheng et al., 2002). Many of these people surveyed said they were reluctant to move far away from reservoir because it is difficult to grow crops they are not accustomed to (Heming et al., 2001). This means that most of the people whose villages were flooded in the reservoir region will be forced to move higher up into the slopes above the reservoir when they resettle. It is important to keep in mind that this future population will face many conditions putting them at risk for a schistosomiasis outbreak in the event the reservoir becomes endemic. The reservoir will be crowded with about 350 people/km2 and social services like healthcare and sanitation will be lacking (Jackson and Sleigh, 1998). Fifty percent of the farmland flooded by the Three Gorges Dam was previously used for rice growing (Jackson and Sleigh, 2000). If we make the reasonable assumption that the people resettling end up growing the same crops as they did before the flooding, the simultaneous presence of all the risk factors discussed here could realistically enable an outbreak of schistosomiasis, making the Three Gorges Reservoir region a perfect storm waiting to happen.
With all this potential for an outbreak at the reservoir, surveillance of the area is of the highest priority. In terms of surveillance, researchers often use remote sensing for schistosomiasis because it predicts where transmission is likely. Remote sensing helps triage different areas and determine where public health officials should prioritize control efforts. Remote sensing relies on geographical analysis of data sets and satellite imagery of the area of interest. Two main staples of remote sensing methods used in schistosomiasis surveillance in China include Geographic Information Systems (GIS) and LandSat.
GIS is a system of computer mapping that can overlay data sets so that all the different data can be given a geographic reference. GIS relies on the fact that most data sets inherently have a geographical component to them. Layering these data sets onto a map can help researchers detect associations between data that otherwise would be hard to notice. GIS has been used since 1998 in China to predict schistosomiasis transmission with the first application of GIS to the Yangtze River in 2000 (Yang et al., 2005). Researchers use GIS by inputting different environmental parameters such as water flow, temperature, and rainfall in order to judge the geographical limits of O. hupensis and therefore S. japonicum development (Bergquist et al., 2000). Researchers can then create maps with all of these layered data sets and use them to determine where the highest risk for schistosomiasis infection lies along the reservoir.
LandSat is the name for a specific group of satellites, now in its 7th generation, which can provide imagery of Earth at a 30-meter pixel-size resolution (R. Spear, oral communication, May 2008). See Figure 4 for an example of a LandSat image of Sichuan, China. Researchers often use LandSat to monitor land-use changes. This is especially important for a country like China, because it is in the midst of great economic development, and thus has many rapid changes in land use (E. Seto, oral communication, May 2008). Monitoring land use is important for predicting how certain regions can change ecologically over time and become suitable habitat for O. hupensis. For example, land that was previously used for dry farming that farmers have now converted to wet farming (i.e. rice paddies) would now be at risk for becoming endemic.
However, since LandSat and GIS are only predictive in nature, field surveillance should always accompany and corroborate any existing remote sensing data (Li et al., 2007). Field studies in the context of schistosomiasis often include three facets of surveillance: testing for human cases, detecting animal reservoir cases, and monitoring snail habitats (Zhao et al., 2005). To diagnose cases in humans, stool samples are taken and checked for S. japonicum eggs using the Kato-Katz stool smear method. For cattle, researchers use the stool hatching method (Liang et al, 2006). Surveying snails entails monitoring the number of snails and also the frequency of infection in the snails (Zhao et al., 2005). Fieldwork is important because it helps researchers understand transmission patterns since they are studying how the disease interacts with local human behavior and ecology (R. Spear, oral communication, May 2008).
While there have been LandSat-based research and field surveys have already been performed at endemic areas downstream of the dam, no research of any kind has been done yet on the reservoir since the area is currently non-endemic. However, as mentioned previously, many different risk factors for transmission already exist in the reservoir, leading researchers to predict that in as little as 5 to 10 years, new snail habitats may form (E. Seto, oral communication, May 2008). Therefore it is critical that surveillance of the reservoir starts soon—prolonged neglect could prove shortsighted. Ideally, the surveillance would be long-term so that baseline levels of snail populations and S. japonicum cases could be reported and be compared to future data to accurately measure the effect of the Three Gorges Dam on the reservoir. Ultimately, data from both predictive methods and field epidemiology should be combined to Ōnot only show current endemicity, but [to] also provide early warning of areas where transmission could become established, and where surveillance and control activities might need to be stepped upĶ in the reservoir area (Bergquist et al., 2000, pg. 363).
The proposed program has three broad aims that support the goal of determining how the dam affects the transmission of schistosomiasis in the Three Gorges Reservoir. The first aim is to use LandSat satellite imagery of the reservoir region to determine where people have resettled after being displaced from their homes due to flooding (R. Spear, oral communication, May 2008). The LandSat images will provide an overall picture of where the largest populations of people are settled and thus where we should focus our baseline surveillance. We are interested in the transmission of schistosomiasis in humans, and thus want to focus on areas with irrigation systems in place. In these areas, people would be more likely to become infected than people with less water contact. Another benefit of LandSat is that it is relatively cheap and would save time and money in the long run because it can be used in lieu of a census of the reservoir area (R. Spear, oral communication, May 2008). The satellite image allows the team to quickly decide what parts of the reservoir should be surveyed for schistosomiasis.
The second aim of this surveillance program is to gather baseline data for the reservoir about the following three areas: infection rate of humans, prevalence and infection rate of O. hupensis snails, and infection rate of cattle. Though we will be conducting this surveillance program separate from the national surveillance program, techniques of our surveillance system are based on those of the existing successful Chinese system. Findings from this program can then be used to improve the governmentÕs knowledge about the effects of the dam on schistosomiasis transmission in the reservoir.
Our proposed program will randomly survey people from the sites selected after LandSat analysis. These people will undergo Kato-Katz stool smears to test for the presence of S. japonicum eggs in their feces. Those infected will be given Praziquantel, provided by the Chinese national control program (Zhao et al., 2005). Interviewing people about their personal histories is also important because it elucidates whether or not infection was imported or local. Therefore, people will be surveyed both with stool smears and survey questions. The age range of those surveyed will be between 4 and 60 years (Liang et al., 2006).
Along with surveying humans for infection, we need to survey for the presence of Oncomelania snails to see if it is possible for schistosomiasis to be transmitted in this region currently, since snails are the essential intermediate host for transmission of S. japonicum. We are expecting, however, to not find snails in the region when we begin surveillance (R. Spear, oral communication, May 2008). If this turns out to be correct, then any human infection in the reservoir would have been imported from an outside source.
The final part of collecting baseline data is testing any cattle for infection using the stool hatching method. However, it is important to keep in mind the fluke can only be transmitted if snails are present in the reservoir. Since no snails are expected to be present, infected cattle, like humans, would have to be imported from outside sources. Any infected cattle will also be given Praziquantel to limit transmission in the area.
The third aim of this project is long-term surveillance to see how S. japonicum prevalence in the reservoir area changes over time. While some experts predict the reservoir to become endemic within 5 to 10 years, other researchers believe it will take nearly 30 to 40 years (R. Spear, oral communication, 2008; E. Seto, oral communication, 2008). Since we donÕt want to cut our surveillance off too soon and risk missing changes that may occur later, we are going to randomly sample people from the same populations sampled for the baseline data once every 5 years for a total of 40 years. Performing surveillance every year would be very costly. However, we also did not want the time between surveys to be very long because then it would be more difficult to pinpoint how the dam affected the transmission of schistosomiasis. This follow-up surveillance would consist of the same methods described for obtaining the baseline data.
There is little doubt that with a highly trained team of surveyors and sufficient monetary support, this project will be surely feasible since it relies a lot on existing surveillance techniques that have already been used in the long-running Chinese schistosomiasis program (Zhao et al., 2005). One thing to note is that the reservoir is 660 kilometers long (Heming et al., 2001) so it will be necessary to first recruit a sizable team of researchers to collect the necessary data. The data collected from this surveillance project hopefully will be use to supplement existing knowledge about how dams change disease transmission dynamics in their reservoirs. Although it is not known how exactly the Three Gorges Dam will affect transmission of S. japonicum in the Three Gorges Reservoir, it would be foolhardy to perform no surveillance at all in the area.
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