Trematodes in Danish freshwater systems

PhD student: Yajiao Duan


Digenean trematodes belong to the phylum Platyhelminthes. They have a complex life cycle and require at least two hosts, one of which is usually a mollusc. Other invertebrates and vertebrates may serve as second and/or definitive hosts. The free-living stage (termed cercariae), which is released from the mollusk, to the aquatic system becomes exposed to a range of abiotic and biotic factors. Infected hosts at all levels suffer from diseases of different severity induced by the parasites. Well known are diseases of veterinary and medical importance but organisms in natural resorts and wildlife may be equally severely affected. Additionally, Due to the multi host characteristic of those organisms in the ecosystem, these parasites may serve as bioindicators and used to indicate presence of obligate hosts, environmental conditions and disease risks under various conditions (climate changes, anthropogenic impacts).

Several trematodes species utilize freshwater fish as second intermediate host and penetrate the fish skin on their way to the preferred microhabitat. Especially, eye flukes metacercariae of digenean trematodes, are prevalent in freshwater fish populations. Heavy infestations are associated with weight loss as the infections reduce the visual abilities of the host and thereby its food search capacity. Freshwater fish may harbour the trematodes for years if the infection level is low, but we here present evidence, suggesting that massive invasion is potentially lethal. Those effects could be investigated by field research and experimental study.

The aims of this project

Firstly, to provide an overview of digenean trematode occurrence in Danish freshwater systems.

Secondly, to investigate the trematode invasion in freshwater fish in a freshwater ecosystem and analyze the transmission between snail and fish host.

Thirdly, to explore the effect of eye fluke (Diplostomum pseudospathaceum) invasion on freshwater fish by using zebrafish as a model.


A total of 22 trematode species were recovered from 5657 snail specimens from 21 Danish freshwater lakes, which accounted 12.6% overall trematode prevalence. The genus Plagiorchis was the most frequently recorded in selected lakes. The prevalence of each snail species occurred large variations, of which snails Lymnaea stagnalis and Radix balthica harbored the main part of the trematode species and could be regarded as “key host species” in Danish freshwater systems. Besides, three zoonotic potential species (Isthmiophora melis, Trichobilharzia sp. and Metorchis orientalis) were recorded from five 

freshwater lakes (Skanderborg sø, Bagsværd sø, Tjele langsø, Lyngby sø and Furesø) both in Zealand and Jutland of Denmark.  

Additionally, Heavy invasion of eye fluke were recovered from 77 freshwater fish in two freshwater lakes (Lyngby Sø and Bromme Lillesø). The experimental infection of cercariae of Diplostomum pseudospathaceum might be lethal to zebrafish under high dosage (600 and 1000 cercariae/fish), and mild infection level could cause immunosuppressive effect (only three genes encoding cytokines were upregulated) on zebrafish.

Trematodes transmission study (from snail host to fish host) was shown that species Posthodiplostomum cuticola was shared both in snail and fish in Lyngby Sø, and species Diplostomum mergi and Tylodelphys clavata  were shared both in snail host and fish host in Bromme Lillesø. It suggested that trematode species which could alter host behavior has relatively higher transmission rate.