China's Environmental Black Box

Justin Remais looks at climate change to understand how snail fever spreads from village to village

By Kay Torrance


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Small and slow, snails are paid little heed in the developed world. In poorer countries, the tiny creatures create big havoc—they are the middlemen in schistosomiasis transmission, the most prevalent tropical disease in the world after malaria. Despite their role in carrying the disease, snails have been largely overlooked in international efforts to control schistosomiasis. Environmental health researcher Justin Remais wants to give snails their place in the sun.

Remais, who heads the RSPH's Global Environmental Health Program, is researching how environmental phenomena such as climate change can allow schistosomiasis to gain new ground. He wants to unlock the "black box" of schistosomiasis: How does the parasite move from village to village, from province to province? The perfect place to look is in Sichuan, China.

Schistosomiasis is nicknamed "snail fever" for good reason. Snails host the parasite that causes the disease. When the snails shed the parasite into fresh water, it penetrates the skin of farm animals and people, forms worms, and lays eggs that cause fever, abdominal pain, diarrhea, and enlargement of the liver and spleen. Children may experience stunted growth and cognitive development. Severe cases can lead to bladder cancer, liver disease, and death.

For years, efforts to control schistosomiasis have centered on a once-a-year oral medication, praziquantel. While praziquantel is inexpensive, a more sustainable approach is to target the parasite's environment.

"Treating people is absolutely necessary, but treatment programs are difficult and expensive to sustain," says Remais. "The lack of sanitation in many of the affected areas allows the disease to kick-start again. We have to treat the disease as an environmental problem."

Schistosomiasis has plagued China for centuries, and while the disease cannot be eradicated, it can be controlled. Prior to 1950, more than 10 million people were believed to be infected. In the late 1950s, Chinese leader Mao Tse-tung sent teams of people armed with sharpened chopsticks to spear snails. The effort resulted in several million fewer cases. Today, less than 1 million Chinese are infected.

In the 1990s, a World Bank program in a number of China's provinces that focused on pharmaceutical treatment made substantial progress. The decline in disease even led to economic development in the affected areas. In recent years, however, the disease has re-emerged, including in the Sichuan province. Because of climate change, poor sanitation, and other factors, communities that had achieved control through drug treatment were ripe for reinfection.

Sichuan is an ideal "environmental soup" in which to study schistosomiasis, Remais says. The province is mostly poor and rural. Some infrastructure improvements have been made, such as lining irrigation ditches with concrete (thus eliminating the wet soil in which snails thrive) and installing plumbing for running water. These projects have yet to spread widely. Moreover, little coordination occurs between local government departments such as public works and public health. As a result, some areas in the province have seen schistosomiasis outbreaks while others have not.

This long history of control programs has left Sichuan's citizens "intervention-fatigued," says Remais. Many times, entire villages were given praziquantel when only a few cases emerged. The drug can cause side effects: drowsiness, stomach cramps, sweating, and diarrhea.

While farmers could wear boots and long gloves to avoid infection, many eschew them because of the subtropical climate. So they continue to walk unheeded in the infected water that fills their rice fields.

How to conquer snails

Remais and a team of graduate students make the trip to Sichuan every summer, digging up snails, characterizing the environment, interviewing villagers, and bringing local government departments together. They've dotted thousands of snail shells with nail polish to determine how far the snails travel and when their numbers increase. Dissecting the snails also extracts genetic information about where they have lived and under what conditions.

This past summer, Ian Spain 10MPH evaluated bio-gas systems, which destroy parasite ova. The systems, popular throughout China, use animal and human waste to produce methane gas, which, in turn, is used to cook and heat homes. Anaerobic bacteria do all the work in these systems, breaking down the waste and emitting methane gas which is then piped off to the house. Since the eggs that cause schistosomiasis are destroyed in system, the infection chain is halted.

Because methane is a greenhouse gas, Spain seeks to determine whether the systems are leaking significant amounts.

"To date, no one has done a complete leak evaluation of these systems," Spain says. "The Chinese government has been promoting the systems' construction but lacks the funding to offer the systems to every family, and many families cannot afford them on their own. With my project, I am hoping to draw attention to this funding gap and to possibly promote the systems as viable carbon offset investments by quantifying their greenhouse gas emissions."

Caitlin Worrell 10MPH is developing a new method for detecting parasites in water. The current method, the mouse bioassay, requires exposing mice to contaminated water and then examining them for the parasite. The new method involves filtering water through a silk membrane, intended to trap the parasite. Remais hopes to obviate the need for mouse bioassays and examining water samples under the microscope, both of which involve great expense and time.

All of these efforts yield data that Remais melds into a mathematical model fine-tuned to predict outcomes. Key environmental factors are plugged in, such as temperature, water flow, vegetation, and topography, to determine schistosomiasis risk across the region.

He hopes that his work on schistosomiasis will allow him and other public health leaders to track how the parasite moves from water source to water source, from village to village, to better predict and ultimately prevent the spread of disease, now and in a future certain to include major changes to China's environment.

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