Freshwater snails, gastropod mollusks, inhabit diverse aquatic environments globally, from ephemeral pools to vast lakes and rivers. While often appearing harmless, certain aspects of their biology and environmental interactions can present risks.
Transmission of Human Parasites
Freshwater snails serve as intermediate hosts for parasitic flatworms called trematodes. One significant human health threat is schistosomiasis, also known as snail fever or bilharzia, caused by schistosome worms that rely on specific freshwater snail species to complete their life cycle.
The life cycle begins when schistosome eggs are released into fresh water from an infected person. These eggs hatch into miracidia, which penetrate freshwater snails. Inside the snail, the miracidia reproduce, developing into cercariae. These infective larvae are then released from the snail into the water.
Humans become infected when these free-swimming cercariae penetrate their skin upon contact with contaminated water, often during activities like swimming, bathing, or washing clothes. Inside the human body, the cercariae mature into adult worms, which migrate to blood vessels in organs like the intestines or bladder, depending on the schistosome species. Adult worms can live for years, continuously laying eggs that can become trapped in body tissues, leading to inflammation and organ damage.
Symptoms of schistosomiasis can initially be mild or absent. However, acute symptoms like fever, abdominal pain, bloody diarrhea or urine, cough, and headaches may appear within one to two months. Without treatment, the parasite can remain in the body for years, causing chronic issues such as liver and spleen enlargement, intestinal infections, bladder and urinary tract scarring, and an increased risk of bladder cancer. Schistosomiasis affects over 264 million people globally and causes up to 200,000 deaths annually.
Beyond schistosomiasis, freshwater snails can transmit foodborne fluke infections affecting human organs like the liver, lungs, and intestines. For instance, liver flukes such as Fasciola hepatica infect humans who consume contaminated aquatic plants like watercress. Clonorchis sinensis, or the Chinese liver fluke, uses snails as a first intermediate host, followed by freshwater fish as a second. Humans contract Clonorchis sinensis by eating raw or undercooked freshwater fish containing the infective stage.
Ecological Consequences
Freshwater snails contribute to aquatic habitats by grazing on algae and detritus, serving as a food source, and aiding nutrient cycling. However, some species, particularly those introduced to non-native environments, can disrupt ecosystems. Invasive freshwater snails can rapidly alter the ecological balance of wetlands and other aquatic systems.
Invasive species often outcompete native snails and other organisms for food and habitat resources. For instance, the New Zealand mudsnail outcompetes native snails and insects, and its hard operculum makes it less digestible for fish, altering the food chain. Apple snails (genus Pomacea) are disruptive due to their extensive consumption of aquatic vegetation.
Excessive grazing by invasive snails on aquatic macrophytes and algae can change habitat structure. This reduces native vegetation, impacting species that rely on these plants for shelter and food. Their presence can also alter water quality by increasing nutrient levels, such as carbon, nitrogen, phosphorus, and dissolved solids, through consumption and excretion. Such disruptions compromise wetland ecosystem functions, affecting biodiversity and health.
Threats to Animal Health and Infrastructure
Freshwater snails act as intermediate hosts for various parasites affecting animal health, including livestock, pets, and wildlife. Liver flukes, such as Fasciola hepatica, commonly infect ruminants like cattle, sheep, and goats. Animals become infected by ingesting vegetation contaminated with the fluke’s larval stages. Once ingested, immature flukes migrate through the liver, causing tissue damage and scarring, which can lead to chronic diseases.
Beyond liver flukes, other trematodes can affect different animal species. For example, some lungworms in pets involve snails in their life cycle. The presence of infected snails in pastures or water sources poses a continuous risk to grazing animals and wildlife, potentially impacting agricultural productivity and wildlife populations.
In addition to health risks, freshwater snails can cause physical and economic damage to infrastructure and aquatic agriculture. Large populations of certain snail species, particularly invasive ones, can clog water pipes and irrigation systems. This obstruction can hinder water flow, reduce efficiency, and necessitate costly maintenance or repairs. Invasive apple snails, for instance, infest rice paddies and crawfish ponds, where their consumption of aquatic vegetation can damage crops and impact aquaculture industries. Their rapid reproduction and ability to consume seedling rice contribute to economic losses for farmers.