Scientists suspect that an invasive snail species Pseudosuccinea columella in Egypt is spreading fascioliasis or liver rot, a parasitic disease that affects animals and humans.
Fascioliasis is caused by two closely related parasitic flatworms: Fasciola hepatica, commonly known as the liver fluke, and Fasciola gigantica. They are normally spread by freshwater snails—the intermediate hosts—that release larvae into the water; the larvae swim and latch onto nearby leaves and stems of plants growing in the water, like watercress for example. Once attached, the larvae can become encysted or enclosed, allowing them to survive for a long time. Humans can then get infected by ingesting the raw plants containing the infectious larvae or drinking contaminated water.
The emerging disease is found in all continents except Antarctica, though it is mostly restricted to developing countries, especially those with large numbers of livestock such as cattle and sheep. Curiously, over the past decades, cases have been growing worldwide, including in Europe and the Americas. Egypt has been gripping with a surge in cases among animals and humans for decades: up to 17 percent of the population is infected in some areas.
Since P. columella was already shown to be a host of F. hepatica, researchers postulated that F. gigantica might also be taking advantage of this invasive snail to expand its distribution range and increase its prevalence in humans and animals—its final hosts; hence the snail may be responsible for the surge in fascioliasis cases in Egypt—in what is called the parasite spill-back hypothesis.
To find out if this was the case, they collected snails at 21 irrigation canal sites in Fayoum Egypt, where fascioliasis is widespread, and determined their infection rates with F. gigantica. They identified snail species using both morphological and molecular techniques. And using DNA sequencing of ribosomal DNA, they determined the presence of F. gigantica in the snails.
Surprisingly, P. columella turned out to be the most abundant snail species as it comprised almost half of all the snails collected (from a total of 689), confirming its successful invasion in the area. It was the dominant species in 10 sites though it was absent in 8 sites. One of the reasons the team think it is so abundant is that during molluscicide (pesticide against snails) treatment it may seek refuge in the water hyacinth Eichhornia crassipes, another invasive species that has gained foothold in the canal. Another factor is their ability to endure extremely dry periods by burrowing into moist mud.
Among P. columella they detected less than 5 percent were infected with F. gigantica. This is the first study to show molecular evidence that F. gigantica can colonize and maintain its life cycle in P. columella, allowing it to gain access to and proliferate in humans. Indeed, the authors think that this colonization is at least partly responsible for the rise in fascioliasis cases in Egypt. This study demonstrates how local parasites can adapt to and flourish in invasive species.
Grabner DS, Mohamed FAMM, Nachev M, Méabed EMH, Sabry AHA, et al. (2014) Invasion Biology Meets Parasitology: A Case Study of Parasite Spill-Back with Egyptian Fasciola gigantica in the Invasive Snail Pseudosuccinea columella. PLoS ONE 9(2): e88537. doi:10.1371/journal.pone.0088537