Remote sensing helps tackle tropical disease schistosomiasis in southern China

Dec 18 2008

Scientists are looking to outer space for help in their attempt to prevent new outbreaks of the tropical disease schistosomiasis in southern China.

Once the Three Gorges Dam is fully operational, researchers plan to use satellite data from space to determine whether changing water conditions in Poyang Lake, China's largest freshwater lake, create favorable conditions for the snails associated with the chronic disease that can damage internal organs and impair growth and cognitive development in children.

Adult and juvenile snails living in stagnant tropical freshwater lakes and ponds serve as hosts during part of the life cycle of schistosomes, the family of parasite species that cause schistosomiasis infections in warm climates around the world.

The scientists hope to combine data on changing water levels with biological information about the snails' behavior. This should help inform public health officials on ways to reduce schistosomiasis infection in the Poyang Lake region between the Three Gorges Dam and Shanghai.

"Really, the only prevention is to not touch the water,” said Motomu Ibaraki, an associate professor of earth sciences at Ohio State University and the leader of the research project. “What we hope we can do is point to the most dangerous areas for schistosomiasis transmission based on predictions we can calculate about the snail habitat.”

Ibaraki described the research at the American Geophysical Union annual meeting in San Francisco. The project represents an emerging field, combining the expertise of specialists in hydrology and infectious diseases to tackle public health problems, he said.

Schistosomiasis is the second-most prevalent tropical disease after malaria, affecting 200 million people worldwide, according to the National Institutes of Health. People are infected through contact with water contaminated with the parasite. The parasite burrows into human skin within seconds, eventually matures to adulthood and settles in various areas of the body.

Over time, the infection can lead to a variety of health complications, such as bladder cancer, kidney and liver damage, and blood infections. No vaccine is available to prevent the illness, and medications to treat it generally are not effective at breaking the disease cycle, especially in poor, developing areas of the world.

After infecting humans, the adult worms lay eggs, which are released into water sources through human waste, perpetuating the cycle of contamination. Tiny worms hatch from the eggs in the water, and use snails as an intermediate host until they're large enough to float freely in the water.

The researchers traveled to the Poyang Lake region during the last two years to collect field data that they fed into a geographic information system documenting water levels, vegetation characteristics and air temperatures that, when combined, identify areas favorable for the host snails to thrive.

For future measurements, all Poyang Lake water level data will come from the European Space Agency's ENVISAT satellite, which measures the water level of the lake every 35 days. The radar readings have been calibrated to compare to baseline levels recorded in the field data.

The satellite work is led by C.K. Shum, a professor of earth sciences at Ohio State who specializes in the use of radar altimetry to measure water levels by satellite. The measurements are made by bouncing radio signals off of surfaces and measuring how long the signals take to return.

Others on the research team led by Daniel Janies, associate professor of biomedical informatics, will conduct DNA analyses of the snail species involved to better understand their behavior, their origin and their preferred habitat.

Trying to kill the snails with molluscicides has been considered, but isn't practical in a body of water the size of Poyang Lake, Ibaraki said. The lake's surface area ranges from about 386 square miles during dry seasons to more than four times that -- 1,700 square miles -- during rainy periods. Adult snails number in the billions, and are about the size of a pinky fingertip.

Water levels and vegetation in Poyang Lake are expected to change after the Three Gorges Dam project is completed upstream, but whether these changes will create better or worse conditions for the snails remains unknown, Ibaraki said. The Three Gorges Dam on the Yangtze River is the largest hydro-electric project in the world, intended to combine the generation of clean power with downstream flood control, and enable shipping in China's interior.

Those most in danger of schistosomiasis infection in the Poyang Lake region tend to be fishermen, Ibaraki said. The lake is a major source for the fish industry in the area, which is rural and very poor. In addition, a high infection rate of schistosomiasis is reported in the region.

“There are huge signs there that tell people not to go into the lake. People know about the dangers of infection, but they have to touch the water to some extent. It's part of their lifestyle,” Ibaraki said.

The researchers plan to share the maps and numerical analyses of snail habitat trends with public health officials in China. They also hope to eventually apply the data-fusion techniques to other areas of the world affected by schistosomiasis. The disease is also a public health problem in southern and sub-Saharan Africa, parts of South America, several Caribbean counties, and portions of the Middle East and Southeast Asia.

Coauthors of the AGU presentation are Melanie McCandless, an earth sciences graduate student; C.K. Shum; Hyongki Lee, an earth sciences postdoctoral research fellow; and Song Liang, assistant professor of public health, all at Ohio State.

Editor's note: From Dec. 18-20, the best way to reach Ibaraki is via e-mail.