Globally the snail is known to carry as many as 35 different species of trematodes and 11 that can infect people. Their presence suggests that there may be even more species of trematodes in local waters. The other five are not likely a risk for people, but are probably also introduced from Asia. Three are known to be infectious to people with two of them being recognized as important yet neglected threats to human health in other areas of the world.

In addition to the snails, the team found they were carrying eight species of trematodes. “The snail wasn’t even really recognized as being present in California, much less being widespread and carrying human-infectious parasites,” said Metz. Still, the team was surprised to see how abundant they were in area waters. The snails are an invasive species probably introduced in the United States through the aquarium trade as much as 60 years ago. Scripps Oceanography biologist and study lead Ryan Hechinger said the work started with an initial serendipitous discovery by graduate student Dan Metz at a reservoir in San Diego where fishing is common. If the fishes are caught and eaten either raw or undercooked, people risk infections with symptoms ranging from abdominal pain to possibly even brain hemorrhage. Some of them then embed in the tissue of fishes. At a certain stage of development, the flatworms send swimming stages from the bodies of the snails.

The scientists also found three species of human-infecting flatworms in the snails in Southern California lakes and reservoirs. Researchers at Scripps Institution of Oceanography at UC San Diego discovered that a snail species carrying human-infecting flatworms known as trematodes is widespread in California.

Flat worm vs snail