Freshwater Snails: Helpful Carriers of Harmful Parasites

Freshwater snails, also known as aquatic snails, are part of a huge range of snail species — we're talking about 4,000 species — that live in bodies of fresh water.

Technically, these creatures are gastropod mollusks; you can also them freshwater gastropods if you want to sound especially smart. They're found all over the world in a wide range of habitats, and they come in many shapes, colors and sizes.

Many species feed on algae, but some feed on dead plant matter and animal remains. Like worms, these freshwater snail species are known as "dentritivores." Others are filter feeders, like krill and some whales.

Several species have gills, although others — such as pouch snails — breathe air. A few species can do both.

Despite their unassuming appearance and widespread habitat, these diminutive invertebrates are among the most harmful animals to human health. Although their damage is indirect, their presence in certain bodies of fresh water contributes to the death of an estimated 200,000 people a year, many of them children.

This isn't due to the snails themselves, but to the parasites they are host to. These parasites hitch a ride in the snails and then migrate into the bodies of humans who are wading, washing or swimming nearby. Once they've entered the host, they can cause all kinds of health problems.

Not every species of freshwater snail is a serious threat to human life. As with many adverse health outcomes, the worst effects are felt in the areas that are poorest, with the least access to good medical care, hygiene and nutrition.

Freshwater snails carry different types of parasites, which lead to different diseases, and one of the most dangerous is schistosomiasis. This disease is caused by a parasitic worm that can enter the human body through the skin; wading in contaminated water is enough to get an infection.

The worms that cause schistosomiasis use freshwater snails as an intermediate host. This means they hang out in the snails on their way to other creatures, such as human beings. The life cycle is complete when the eggs they lay are excreted out, where the larvae can then be passed along to new hosts.

Schistosomiasis is officially classified as a "neglected tropical disease." It's likely you've never heard of it, even though in many places it's second only to malaria in terms of the impact it has on human societies, especially those in South America, Africa and Asia.

It's most prevalent and damaging in the tropics, and in poor areas where people might bathe, launder clothes, fish or even drink water that's contaminated with the parasites.

The parasite takes up residence in different parts of the body, laying eggs and causing tissue and organ damage. It's a nasty affliction that can cause all kinds of health problems, and children often bear the brunt of the harm.

Freshwater snails aren't just a problem for humans. Many species are also considered invasive, meaning they pose a problem to entire ecosystems.

Take the apple snail (Pomacea maculata), for instance. Apple snails are native to South America, but they're an invasive species in Europe and North America, causing major problems on native habitats. Sometimes, they even threaten other, native species of aquatic snails.

If you come across some tiny snails in your local pond, you should probably just leave them alone. They might be local pond snails, and you won't have to worry much about them. But if you're really interested, take some pictures and contact your local wildlife center; you might be helpful in pest control.

Interestingly, some of the most effective measures taken to control freshwater snails involve natural methods, such as the introduction of predators that will consume the snails.

This, combined with public health initiatives (such as education and the targeted use of certain medications) have shown significant results in reducing schistosomiasis, as well as other negative effects of freshwater snail populations.

On the other hand, when snail control is attempted through heavy-handed applications of pesticide, it tends to be less effective. The snails usually manage to hang on, and other plants and animals are negatively affected.

Besides their role in natural ecosystems (for better and for worse), aquatic snails are also a common addition to aquariums.

For example, consider the two species of mystery snails and nerite snails. These are frequently used to help spruce up the underwater space, as well as a novel way to keep the tank clean.

Assassin snails (Anentome helena), with their distinctive alternating yellow and black banded shell, are commonly found in aquariums that have problems with other freshwater snail species. As the name suggests, the assassin snails will attack and eat these other unwanted snails.