20 January 2014

Africa: Prawns Show Promise in Schistosomiasis Control

Photo: King James Yiye/RNW
People get infected with schistosomiasis from contact with water containing parasites, which are released by infected snails.

Reintroducing prawns to lakes and rivers in which they have been partially or fully lost may be a sustainable way of controlling the parasitic disease schistosomiasis, which kills more than 200,000 people every year in Sub-Saharan Africa alone, says a study.

Researchers have found some native prawns to be voracious predators of the freshwater snails that transmit schistosomiasis parasites and so could be used as a biological control, they report in a study in press in Acta Tropica.

Field tests are under way in Senegal, and researchers suggest that farming the edible prawns could help local populations cut disease while also providing an additional source of income.

"Prawns may offer a simple and affordable transmission control solution in rural poor communities where few alternatives exist and drug treatment is failing to achieve long-term disease reductions," the study says.

People get infected from contact with water containing schistosomiasis parasites, which are released by infected snails.

Although people who carry schistosomiasis can be treated with the drug praziquantel, reinfection from fresh exposure to infested waters hampers disease control and eradication.

In laboratory experiments, researchers based in the United States set out to measure the rate at which prawns eat uninfected snails. They found that they consume an average of 12 per cent of their body weight in snails each day.

The researchers also found that young prawns that are still growing are more efficient at controlling snail numbers than large, fully-grown prawns. The larger prawns ate more snails but the smaller ones were more efficient as they ate more snails per gram of body weight and fed on snail eggs and hatchlings, too, the study shows.

These results support the idea of the aquaculture of native prawns, and their reintroduction to freshwater bodies where their numbers have fallen, the authors say.

When prawns are too small to be sold at market they are "high-efficiency snail killers", says Susanne H. Sokolow, lead author of the study and a researcher at Stanford University in California. "When they grow and their efficiency declines, we can harvest them."

"It can be a win-win solution. You can have a prawn aquaculture programme that supports public health efforts by reducing snail populations and also boosts the local economy because prawns are more valuable on most markets than fish," Sokolow tells SciDev.Net.

Sokolow and her co-workers tested two native prawn species - one from Cameroon and another from Malaysia - against two snail species that transmit schistosomiasis in Africa and Latin America. They found that both species of prawns fed on these snails.

The researchers have already taken their experiment to field settings to see if the prawns can help curb the disease's transmission from snails to people through water.

They have teamed up with the 20|20 Initiative, a California-based NGO that applies research from the developed world to challenges facing rural Africa, to assess their snail control strategy in the Senegal River in West Africa. The researchers speculate that dam construction may have reduced prawn numbers and increased schistosomiasis infections among local people there.

Over the past few years, they have added prawns to net enclosures built alongside the river in Lampsar village, Senegal, and are monitoring reinfection in local people treated with praziquantel. They plan to expand this field trial to two more villages later this year.

The next step in Sokolow's research is to compare prawns' consumption of infected versus uninfected snails in laboratory experiments.

"It's possible that prawns will preferentially consume infected snails, which would enhance their efficiency as biological control agents," she says.

But Alan Fenwick, director of the Schistosomiasis Control Initiative at Imperial College London, United Kingdom, warns that such "predator-prey relationships normally don't work brilliantly".

He says: "Once they've eaten all the snails, the prawns will die out and the snails will come back."

Sokolow counters that frequent addition of prawns to lakes and rivers could balance any population changes that occur naturally.

"We are proposing to [add] a managed population of prawns via aquaculture and restocking," she says.

Link to abstract in Acta Tropica

References

Acta Tropica doi: 10.1016/j.actatropica.2013.12.013 (2013)

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