In one of Alterra’s climate rooms are six trays with pond snails that were still living in a lake in Emmeloord until a few days ago. Some of these snails are carrying Trichobilharzia, a parasite that causes swimmer’s itch. Ecologist Marieke de Lange will be figuring out what makes us humans so attractive for the larvae (cercariae) of this parasite, which uses the snail as a host.
Photo Guy Ackermans. Some of the hundreds of pond snails that ecologist Marieke de Lange has captured are carrying the parasite Trichobilharzia.
In the spring, once the water is warm enough, the cercariae swarm off en masse in search of water fowls. When the cercariae mature into flatworms, they lay eggs in the birds. These eggs end up in the water via the birds’ excrement, develop into tiny larvae (miracidia) and look for snails to be their host again. The cercariae also see humans as a host, but this is a mistake because they are not able to get into our bloodstream. But they do leave us with nasty bumps and itches.
According to De Lange, we may be attracting cercariae because we excrete linoleic acid. ‘That at any rate is the case for the tropical variant of bilharzia.’ She will be testing whether that also applies to the Dutch cercariae. She will also be testing cholesterol and ceramide as bait.
If one of these baits works, this will point to a way of trapping the larvae. ‘The idea is to drench ropes in linoleic acid and attach them to buoys around the swimming area,’ explains De Lange. She wants to cleanse the water with a kind of curtain of ropes.
De Lange’s research was commissioned by six water boards and two recreational businesses. Water boards regularly get complaints about swimmer’s itch. De Lange: ‘They feel that the complaints have been increasing over the past five to ten years.’