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Perspective from Peter J. Hotez, MD, PhD
July 06, 2017
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Three Gorges Dam decreased rate of schistosomiasis around Chinese lake

Perspective from Peter J. Hotez, MD, PhD
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Ecological changes in one of China’s largest freshwater lakes that were caused by the Three Gorges Dam project have led to a significant decrease the rate of schistosomiasis infections in the area around the lake, researchers said.

According to a study published today in PLoS Neglected Tropical Diseases, the decrease in cases of schistosomiasis coincides with dwindling populations of snails in Dongting Lake, a 1,600-square-mile body of water in Hunan province that lies on the Yangtze River downstream from the huge hydroelectric dam.

Image of Three Gorges Dam
The construction of Three Gorges Dam in central China has led to a significant decrease in schistosomiasis infections around a large lake downstream from the dam.
Source: Shutterstock.com

The lake is home to 48% of the snails in China, including the Oncomelania hupensis snails that host Schistosoma japonicum parasites, according to the researchers. The snails live in the lake’s vast marshlands, but their populations have plummeted since Three Gorges Dam — the world’s largest power plant — began impounding water and discharging sediment in 2003, the researchers said.

In turn, between 2003 and 2015, the area saw a reduction in schistosomiasis of 86.98%, according to the researchers.

They said the decrease might be due in part to schistosomiasis control efforts undertaken by the Chinese government — including extensive use of molluscicides, routine praziquantel treatment and health education programs — but that ecological changes disturb the distribution of snails and also affect the prevalence of schistosomiasis.

“Large-scale hydro-projects not only affect the natural environment, but may also alter regional climate, and further lead to changes in the epidemic distribution of some infectious and endemic diseases,” they wrote. “Previous cases have shown that emergence or re-emergence of schistosomiasis [was] often caused by newly built hydro-projects in endemic areas, such as Aswan Dam in Egypt, Tigay Dam in Ethiopia, Gezira-Managil Dam in Sudan, Manantali Dam in Mali and Danling Dam in China. Hence, the potential influence of [Three Gorges Dam] on the transmission of schistosomiasis in downstream Yangtze River basin aroused heated discussion worldwide.”

Image of a freshwater snail
A freshwater snail, which can host parasites that cause schistosomiasis.
Source: Susanna Sokolow/PLOS.

Impact second only to malaria

According to the CDC, more than 200 million people are infected worldwide every year by the parasites that cause schistosomiasis — an impact that the agency says is second only to malaria in terms of parasitic diseases.

Schistosoma parasites live in snails and can penetrate the skin of people in contaminated freshwater where the infected snails live, according to the agency. The parasites mature into adult worms and mate in human blood vessels, with females producing eggs that may travel to the bladder or intestine, where they pass into the urine or stool.

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Infected humans can contaminate fresh water with their urine or stool, leaving behind eggs that hatch parasites which can then infect snails, the CDC says. Symptoms of infection may take days to develop and are produced by the eggs, not the worms. Early on, they may include a rash or itchy skin; after 1 to 2 months, symptoms may include fever, chills, cough and muscle aches.

According to the CDC, children who are infected can develop anemia, malnutrition and learning difficulties. Years of infection may cause damage to the liver, intestine, lungs and bladder. In rare cases, eggs in the brain or spinal cord can cause seizures, paralysis or spinal cord inflammation.

Study results

For their study, the researchers monitored 12 marshland sites in the Dongting Lake area where snails are distributed. They used hydrological information to analyze the impact of Three Gorges Dam on the Dongting Lake area and collected data related to volumes of runoff and sediments, water level and topsoil moisture level of the lake.

They also collected data on the number of living snails and infected snails each April and May, and used an annual report by the Hunan Institute of Schistosomiasis Control to retrieve data on human infections in patients between ages 6 and 65 years in 41 counties in Hunan province.

According to their results, after construction of the dam in 2003, the volume of annual runoff that drained into Dongting Lake from the Yangtze River declined by 20.85%. The researchers also observed large decreases in the annual sediment volume discharged into the lake and the mean lake sedimentation rate.

As a result, between 2003 and 2015, the mean density of living snails decreased by 94.35% and human rates of schistosomiasis infection in one of the most significant endemic regions in China decreased from 3.38% to 0.44%, the researchers said.

Among their interpretations, they said the dam has affected the variation between the highest and lowest water levels of the river and lake, making the microenvironment of the snails unsuitable and having an effect on human infections.

“Given that the impact of [Three Gorges Dam] on snail distribution and schistosomiasis prevalence in Dongting Lake area is much more complex, prolonged and in-depth studies are needed to address these issues for the effective control of snails in Dongting Lake area and eventually achieving the elimination of schistosomiasis,” the researchers concluded. – by Gerard Gallagher

Reference: Li F, et al. PLoS Negl Trop Dis. 2017;doi:10.1371/journal.pntd.0005661.

Disclosure: The researchers report no relevant financial disclosures.