Digenetic parasites and their amazing life cycle

Human bathers have relatively little to fear, apart from allergic skin reactions caused by misled parasites which penetrate a human by mistake. By far, most of these parasites are harmless to humans. However, an important exception occurs in the tropics.

In the tropics, many freshwater bodies are infested with the trematodes known as schistosomatids. These trematodes cause one of the nastier human diseases: schistosomiasis or bilharziosis. The larval stage (cercaria) of schistosomatids has a forked tail and swims around vigorously when released from the host snail, until it finds a human host in the water. Then, it will immediately penetrate the skin. Once inside a human, the parasite will wander into the bladder or into the gut, live there for several years as an adult worm, cause trouble, and produce eggs. Since trematode larvae usually occur in large numbers in the water, people might be infected by large numbers of parasites, which can cause serious health problems. Thankfully, medical treatment is possible for those infected with this parasite.

A tip for biology teachers: collect around 50-100 freshwater snails, put each one separately into a small jar (transparent glass) full of water, closed by a lid. The next day, a number of glasses (probably 5-10) will contain tiny white creatures swimming through the water. These are the cercariae which emerged from the snail. They can be picked up with a pippette and watched under a microscope.

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