text-only page produced automatically by LIFT Text Transcoder Skip all navigation and go to page contentSkip top navigation and go to directorate navigationSkip top navigation and go to page navigation
National Science Foundation
Discoveries
design element
Discoveries
Search Discoveries
About Discoveries
Discoveries by Research Area
Arctic & Antarctic
Astronomy & Space
Biology
Chemistry & Materials
Computing
Earth & Environment
Education
Engineering
Mathematics
Nanoscience
People & Society
Physics
 

Email this pagePrint this page

Discovery
Snails in the Waters, Disease in the Villages

Treatment for snail-borne schistosomiasis works best over the long haul

Photo of a man examining water for infectious snails in coastal Kenya.

Monitoring for infectious snails in bodies of water near villages in coastal Kenya.
Credit and Larger Version

November 19, 2012

The following is part two in a series on the NSF-NIH Ecology and Evolution of Infectious Diseases (EEID) Program. For part one, please see Cool Cat in a Hot Zone. For part three: Underwater Whodunit: What's Killing Florida's Elkhorn Coral? For part four: 'Defective' Virus Leads to Epidemic of Dengue Fever. For part five: Sick Sea Fans: Undersea "Doctors" to the Rescue. For part six: California's iconic redwoods in danger from fire and infectious disease. For part seven: Tasmanian devils: Will rare infectious cancer lead to their extinction?

Watch where you jump in for a swim or where your bath water comes from, especially if you live in Africa, Asia or South America. Snails that live in tropical fresh water in these locations are intermediaries between disease-causing parasitic worms and humans.

People in developing countries who don't have access to clean water and good sanitation facilities are often exposed to the infected snails. Then they're left open to the parasitic worms.

The worms' infectious larvae emerge from the snails, cruise in shallow water, easily penetrate human skin and mature in internal organs.

The result is schistosomiasis, the second most socioeconomically devastating disease after malaria. As of 2009, 74 developing nations had identified significant rates of schistosomiasis in human populations.

There has been much debate about how best to prevent the disease, says Charles King, a physician and researcher at Case Western Reserve University in Cleveland, Ohio. "Beyond that," he asks, "how long should treatment last once someone has schistosomiasis?"

"Current guidelines focus on suppressing the disease's effects by limiting the infection during childhood," says King. "But that may not be enough to cure it or to prevent reinfection, leaving children still at risk for stunted growth and anemia."

King and colleagues recently published results of a study of long-term treatment of schistosomiasis in the journal PLOS Neglected Tropical Diseases.

The team's work is funded by the National Science Foundation (NSF)-National Institutes of Health (NIH) Evolution and Ecology of Infectious Diseases (EEID) program.

At NSF, the EEID program is supported by the Directorate for Biological Sciences and Directorate for Geosciences. At NIH, it's supported through the Fogarty International Center.

Schistosomiasis is usually treated with a single dose of the oral drug praziquantel.

World Health Organization (WHO) guidelines set forth in 2006 recommend that when a village reports that more than 50 percent of its children have parasite eggs in their urine or stool--a clear sign of schistosomiasis--that everyone in the village should receive treatment.

When 10 to 50 percent of children are affected, say the guidelines, only school-age children should be treated--every two years. With less than 10 percent, mass treatment is not suggested.

But because of the long-term health effects of schistosomiasis, says King, "we now think it's better to provide regular yearly treatment."

He and scientists Xiaoxia Wang, David Gurarie and Peter Mungai of Case Western Reserve University; Eric Muchiri of the Ministry of Public Health and Sanitation in Nairobi, Kenya; and Uriel Kitron of Emory University in Atlanta, Ga., used data collected in 10 villages in southeastern Kenya to run advanced models of village-level schistosomiasis transmission.

They scored the number of years each of the 10 villages would be projected to remain below a 10 percent infection level during a simulated 10 to 20-year treatment program.

All strategies that included an initial four annual treatments reduced community prevalence of the disease to less than 10 percent. Programs with gaps in treatment, however, didn't reach this objective in half the villages.

At typical levels of treatment, the researchers found, current WHO recommendations likely could not achieve full suppression of schistosomiasis.

"With more aggressive annual intervention that lasts at least four years," says King, "some communities might be able to continue without further treatment for 8 to 10 years.

"But in higher-risk villages, repeated annual treatment may be necessary for an indefinite period--until the eco-social factors that foster the disease [such as poor wastewater treatment] are removed."

In high-risk places, ongoing surveillance for the disease and annual drug treatment, the scientists say, need to become the mainstays of control.

In short, these villages require what they call "re-worming after de-worming."

But what happens if townspeople move to a more arid location, one with less fresh water and fewer snails?

In drier landscapes, schistosomiasis is a rare event that happens only during floods. Response to treatment, therefore, may be much better. Unless or until another flood occurs.

Although drier locales carry less risk for the disease, they're by no means free and clear. Even in arid locations, people would likely need to be treated more than once to get rid of the parasites.

"This research demonstrates the value of understanding where disease-causing organisms are in the environment," says Sam Scheiner, NSF program officer for EEID.

"Such knowledge can reduce human diseases much more effectively and at a lower cost than simply focusing on treatment."

The best goal, says King, is complete eradication of schistosomiasis.

To achieve that, scientists need to determine what makes a "wormy village," how often therapy is needed to prevent disease in such locations, and what can be done to change the environment so that a high-risk village becomes a low-risk one.

Related Websites
NSF Special Report: Ecology and Evolution of Infectious Diseases: http://www.nsf.gov/news/special_reports/ecoinf/index.jsp
NSF News Release: Controlling the Spread of Diseases Among Humans, Other Animals and the Environment: http://www.nsf.gov/news/news_summ.jsp?cntn_id=125496

Map of the world showing countries at risk for the snail-borne disease schistosomiasis.
Countries worldwide where people are at risk for the snail-borne disease schistosomiasis.
Credit and Larger Version

Photo of woman working in a rice field.
People who work in rice fields are exposed for many hours to snail-infested waters.
Credit and Larger Version

GIS map showing four communities in Kenya that are at high risk for schistosomiasis.
GIS map showing four communities in Kenya that are at high risk for schistosomiasis.
Credit and Larger Version

Photo of two men analyzing urine samples.
Diagnosis of schistosomiasis is made by detection of parasite eggs in urine samples.
Credit and Larger Version

Bulk containers of cans of pills of the anti-schistosomiasis drug praziquantel.
Bulk containers of cans of pills of the anti-schistosomiasis drug praziquantel.
Credit and Larger Version



Email this pagePrint this page
Back to Top of page