Abstract
The liver fluke Fasciola gigantica is a trematode parasite of ruminants and humans that occurs naturally in Africa and Asia. Cases of human fascioliasis, attributable at least in part to F. gigantica, are significantly increasing in the last decades. The introduced snail species Galba truncatula was already identified to be an important intermediate host for this parasite and the efficient invader Pseudosuccinea columella is another suspect in this case. Therefore, we investigated snails collected in irrigation canals in Fayoum governorate in Egypt for prevalence of trematodes with focus on P. columella and its role for the transmission of F. gigantica. Species were identified morphologically and by partial sequencing of the cytochrome oxidase subunit I gene (COI). Among all 689 snails found at the 21 sampling sites, P. columella was the most abundant snail with 296 individuals (42.96%) and it was also the most dominant species at 10 sites. It was not found at 8 sites. Molecular detection by PCR and sequencing of the ITS1-5.8S-ITS2 region of the ribosomal DNA (rDNA) revealed infections with F. gigantica (3.38%), Echinostoma caproni (2.36%) and another echinostome (7.09%) that could not be identified further according to its sequence. No dependency of snail size and trematode infection was found. Both high abundance of P. columella in the Fayoum irrigation system and common infection with F. gigantica might be a case of parasite spill-back (increased prevalence in local final hosts due to highly susceptible introduced intermediate host species) from the introduced P. columella to the human population, explaining at least partly the observed increase of reported fascioliasis-cases in Egypt. Eichhornia crassipes, the invasive water hyacinth, which covers huge areas of the irrigation canals, offers safe refuges for the amphibious P. columella during molluscicide application. As a consequence, this snail dominates snail communities and efficiently transmits F. gigantica.
Highlights
The liver fluke Fasciola gigantica is a trematode parasite native to Africa and Asia and infects ruminants, and humans as final hosts
The results of the present study show that the invasive snail P. columella is found frequently in irrigation channels in the Fayoum governorate and it even turned out to be the most abundant snail species at most sites
Our findings imply that P. columella probably became one of the major snail intermediate host species for F. gigantica
Summary
The liver fluke Fasciola gigantica is a trematode parasite native to Africa and Asia and infects ruminants, and humans as final hosts. Intermediate hosts are various lymnaeid snails [2], inside of which the parasite proliferates asexually and produces free swimming cercariae that will attach to submerged surfaces, mostly plants These cercariae develop into encysted and durable metacercariae that are transmitted when the final host ingests the metacercariae together with plants, or by consumption of water contaminated with metacercariae [3]. According to estimations of the World Health Organization [8], at least 830,000 people are infected with either the introduced Fasciola hepatica or F. gigantica in the Nile delta Presence of both of these closely related species has been confirmed for Egypt [1,9,10], but they are usually not distinguished diagnostically [11]
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