Morphologic description and molecular characterization of Trichobilharzia species (Digenea: Schistosomatidae) infection in field-collected Radix auricularia (Linnaeus, 1758) in North of Iran

In the quiet waters of rice paddies and slow-moving streams, a hidden cycle of life unfolds involving tiny snails and microscopic parasites. These parasites belong to a group known as bird schistosomes. While their adult forms live inside waterfowl like ducks and geese, their young stages must pass through a specific type of freshwater snail to survive and grow. If these young parasites, called cercariae, accidentally penetrate human skin while people are swimming or working in the water, they cause a painful, itchy rash known as swimmer's itch. This condition is a common nuisance in many parts of the world, but understanding exactly which snails carry the parasites and where they are found is essential for predicting and preventing outbreaks. Scientists in northern Iran set out to map this hidden network, focusing on a specific snail species that lives in the region's wetlands and the parasites it might be harboring.

The researchers focused their investigation on the Guilan Province, a lush area in northern Iran known for its heavy rainfall and extensive rice cultivation. This environment creates perfect habitats for freshwater snails, including the great pond snail, a species known scientifically as Radix auricularia. The team collected nearly 8,000 of these snails from various locations, including canals, rivers, and rice fields, to see if they were infected. They used a simple but effective method to check for infection: they placed individual snails in containers of water under a warm light. This warmth encourages the snails to release any parasites living inside them. The researchers then examined the water under a microscope to look for the tiny, fork-tailed larvae of the bird schistosomes.

The results revealed that while the vast majority of the snails were healthy, a small number were indeed carrying the parasites. Out of the 7,359 great pond snails examined, 74 were found to be infected, representing an infection rate of about one percent. The team identified two specific types of bird schistosomes within these snails: Trichobilharzia franki and Trichobilharzia regenti. The first type, T. franki, was the more common of the two, found in nearly 90 percent of the infected snails, while the second type, T. regenti, was found in the remaining infected snails. Interestingly, the researchers also collected thousands of other snail species from the same waters, including the invasive New Zealand mud snail and another common pond snail, but they found no evidence of these bird schistosomes in those other species. This suggests that in this specific region, the great pond snail is the primary host for these parasites.

To confirm exactly what they had found, the scientists moved beyond the microscope and looked at the genetic code of the parasites. They extracted DNA from the tiny larvae and the soft tissues of the snails to read specific genetic markers. This molecular analysis confirmed the presence of the two parasite species and allowed the researchers to compare their genetic makeup with samples from around the world. They discovered that the parasites found in Iran were genetically very similar to those found in Europe, including in countries like France, Switzerland, and the Czech Republic. The genetic data showed that these parasites have not evolved into a unique local strain but are part of a broader population that moves across continents.

The study also provided a detailed look at the physical characteristics of the parasites found in Iran. The researchers measured the length and width of the larvae and counted the tiny structures inside them, such as the flame cells that help the parasite filter waste. They found that the parasites from Iran were slightly smaller than some previously recorded specimens from other countries, a difference likely caused by environmental factors or the specific conditions of the snail hosts. Despite these size variations, the genetic evidence remained clear: the parasites in the Iranian snails are the same species that infect birds across Europe.

This connection between the parasites in Iran and those in Europe points to a clear mechanism for how these organisms travel. Migratory waterfowl, such as wild ducks and geese, fly between Europe and northern Iran during their seasonal journeys. These birds carry the adult parasites, which release eggs into the water. The eggs hatch into larvae that infect the local snails, which then release the infective stage back into the water, completing the cycle. The presence of identical genetic strains in both regions suggests that these birds are the primary transporters, carrying the parasites across borders and maintaining the infection cycle in the rice fields of Guilan.

The findings highlight the role of the great pond snail as a crucial link in this transmission chain. Because these snails thrive in the same shallow waters where people farm rice and raise domestic ducks, the risk of human exposure to the parasites is real. The study concludes that while the infection rate in the snails is relatively low, the sheer number of snails and their ability to release thousands of larvae daily means they can still infect many birds and potentially cause swimmer's itch in humans. The researchers recommend that future efforts focus on understanding the movement of migratory birds and implementing preventive measures for local residents to reduce the risk of infection. By identifying the specific snail host and confirming the genetic identity of the parasites, this work provides a clearer picture of how these microscopic organisms persist and spread in the wetlands of northern Iran.