27 January 2016

A short story about snails, worms and prawns

When my great colleague and snail expert David Rollinson from the Natural History Museum in London told me « no snails – no schisto », I realized how important it is to include vector control in the overall strategy to eliminate schistosomiasis.

By Dr. Jutta Reinhard-Rupp
Head of Global Health R&D – Merck S.A. Switzerland, an affiliate of Merck KGaA, Darmstadt, Germany

No Snails – No Schisto

When we look into countries that have successfully eliminated this disease, combating the snails was always part of their approach. But finding ecologically viable solutions to kill the snails that are living in a complex habitat of freshwater, is not an easy task to do. Only the chemical compound niclosamide is available as a molluscicide and this has to be used with great care as, although safe for humans, it has the negative effect of killing more than just the snails in the water.

Man with prawn in rice field in Senegal
Credit to Nicolas Jouanard

New biological control approaches are currently under investigation, namely the use of natural predators (in form of river prawns) that eat the snails. One important project was launched in Senegal a few years ago where the team planned to restore natural populations of river prawns into the Senegal river (see www.theupstreamalliance.org). Construction of the Diama Dam had led to a return of schistosomiasis, and infection rates of the local human population were extremely high. The dam, which prevented intrusion of salt water, also blocked the migration of freshwater prawns, voracious predators of the snails that carry the parasite. Without prawns and with an increase in freshwater habitats, the snail populations grew rapidly and high prevalence and intensity of diseases ensued. Hence this study to establish prawn farming and to re- introduce prawns for snail control.

This could be a win-win situation in that in addition to controlling the disease, the prawns could provide a livelihood for the local villagers and some income – the prawns rapidly reach market size and can be sold for more than the local fish. The first positive impact of the project is already measurable: the infection rates in selected villages dropped by half in the initial study.

This new snail control intervention demonstrates and confirms the need for an integrated strategy if, one day, we want to succeed with the ambitious goal to eliminate this debilitating disease.