Restoring river prawns fights disease

In 1986, people finished building an 18-meter-high dam on the Senegal River in West Africa. Soon afterward, an “unprecedented, massive, and persistent” epidemic of the disease schistosomiasis broke out in nearby villages, researchers write in Proceedings of the National Academy of Sciences. The dam had probably created a breeding ground for snails that harbor the parasite responsible for the disease.

The dam also blocked native crustaceans called river prawns from migrating up and down the river. Normally, the prawns eat the snails that spread schistosomiasis. Without these natural predators, the snails — and the parasites they carried — multiplied.

Now researchers have found that by restoring prawns to the ecosystem, they can reduce the number of schistosomiasis infections in people. Combined with medication, this inexpensive control strategy could stamp out schistosomiasis in places where the disease stubbornly lingers.

About 220 to 240 million people contract schistosomiasis every year. The disease causes anemia, infertility, and stunted growth, and it can lead to cancer and liver failure. The pathogen responsible is a parasitic worm, which infects snails. The snails then release larvae that infect humans. Infected people shed parasite eggs in their urine and feces, and those eggs hatch into larvae that infect more snails — and the cycle goes on and on.

The researchers performed an experiment at two villages in the Senegal River Basin. The team set up a 10- by 20-meter net containing prawns in the river by the downstream village, then monitored the number of snails at both sites for a year and a half. They found that at the village with the added prawns, the number of snails was roughly half the number at the village where no prawns had been added. And the number of infected snails was about 80 percent lower.

The team also treated people at both villages with medication to cure the disease, then tested the participants for re-infection. The prevalence of schistosomiasis was 18 percent lower in the village with the added prawns, and the average number of eggs in people’s urine was about half.

In addition to boosting public health, the prawns could provide a source of food and income for villagers. “[R]iver prawn restoration might become a win-win-win-win: for disease control, biodiversity restoration, poverty alleviation, and improved nutrition,” the authors conclude. Roberta Kwok | 23 July 2015

Source: Solokow, S.H. et al. 2015. Reduced transmission of human schistosomiasis after restoration of a native river prawn that preys on the snail intermediate host. Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1502651112.

Image © Susanne Sokolow

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