The ability of freshwater leeches to digest snails may aid in the prevention of illness.

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• schistosomiasis
• larvae
• parasite
• leech
• diseases
• illnesses
• freshwater
• biology

Despite their antiquated appearance, leeches continue to serve a purpose in modern medicine. Researchers report on April 12 in Biology Letters that the freshwater leech Helobdella austinensis may devour its weight in snails every day, implying that leeches could be utilized to reduce freshwater snail populations and the myriad of illnesses they carry.

Diseases like schistosomiasis and fascioliasis are transmitted from snails to humans and cattle. Snail-dwelling flukes are the culprits in both of these diseases. Prevention therapy for the blood fluke that causes schistosomiasis was necessary for about 250 million individuals in 2021, including many youngsters. Snail population control may be more useful for preventing the spread of the parasite.

Although leeches are commonly used as a source of fear in films, snails are the ones who should be truly terrified of H. austinensis. Daniel Shain, an evolutionary biologist at Rutgers University in Camden, N.J., claims, "Everyone who works in the very little leech world internationally, we all know that these specific varieties of leeches adore snails." The leeches won't discriminate towards different species of snails. We simply linked the dots and found that snails are a major health concern throughout the world.

Laboratory leech H. austinensis was employed, along with H. serendipitous and H. modesta, all of which are easily discovered by turning over rocks in nearby New Jersey lakes. Seven species of freshwater snails were used in the laboratory tests, but only H. austinensis showed to be a voracious leech eater. In the presence of sufficient snails, a single leech might subsist on 0.97 milligrams of snail every day.

By simulating predator-prey relationships between leeches and snails, Shain and his colleagues found that leeches might decimate snail populations in as little as six months.

Civitello adds that despite the common belief, H. austinensis is not exclusively a snail eater. He says this is a persistent issue in snail biocontrol. The snails we wish to be eaten by these predators are usually not a specialty of these animals.

Ecologist David Civitello from Emory University in Atlanta, who was not engaged in the study, thinks it's a novel concept. Nonetheless, he believes a great deal more study is required. He observes that after snail numbers dropped, it became difficult for leeches to locate food. He adds that if you have trouble locating your target, it means you won't be able to eradicate it. It's possible that food rivalry may be weakened if people were to randomly kill snails without completely eliminating the snail population. He worries that the spread of snail-borne diseases will actually increase if leeches are allowed to leave low populations of huge, well-fed snails in the wild.

Several varieties of snails are enjoyed by the leeches. Civitello is concerned that the leeches, who are fed both live and dead bug larvae in the lab, will have an unhealthy variety of foods available to them in the wild.

Shain and his coworkers are curious about the viability of leeches as a food for snails, so they want to conduct field testing. They are also on the lookout for local kinds of leech that prey on snails. While H. austinensis may be at home in Texas, Shain argues that reintroducing native species to non-Texas regions would have a lesser impact on local ecosystems.

Shain, in the meantime, is praying that people would be able to get beyond their fear of leeches. Instead of snails, he suggests putting 10,000 leeches in the pond. "The thought of having, you know, 10,000 leeches within the pond maybe isn't very enticing," he says. H. austinensis has the advantage of being extremely small and showing no interest in people. I hope just the snails are cold.

CITATIONS

N. Saglam, D.S. Melissaratos and D.H. Shain. Biocontrol of snail-borne parasites with the glossiphoniid leech, Helobdella austinensis. Biology Letters. Published online April 12, 2023. doi: https://doi.org/10.1098/rsbl.2022.0484.