Zoonotic liver flukes found in roach in the Gulf of Finland
Larval forms of flukes were found in roach caught in the Gulf of Finland in a joint study of the Finnish Food Safety Authority Evira and the University of Helsinki Faculty of Veterinary Medicine. This parasite may infect humans who eat raw fish through its secondary intermediate host, the roach. Liver fluke infection may cause symptoms such as abdominal pain, jaundice, fever, and nausea. The study was carried out on roach caught in the eastern part of the Gulf of Finland.
According to the preliminary results of the studied samples, the prevalence of fluke infections, which occur in humans through eating raw fish, is 30-50 per cent. However, the study is still ongoing.
"A wide range of further analysis is still needed on flukes. More detailed information is needed on the extent of their prevalence in Finland as, based on literature, other fish species of the carp family can also serve as intermediate hosts for flukes that are capable of infecting humans. It is also suspected that other species of fish may serve as intermediate hosts. Further information is also needed about, for example, the cold and heat resistance of flukes," says DVM (PhD) Antti Oksanen of Evira.
Increasing prevalence of flukes observed
An increase in the number of fluke parasites that infect warm-blooded animals, and possibly humans, through fish has been observed in Finland.
The prevalence of flukes in the skin and muscles of roach caught in the Gulf of Finland has been examined in a joint study of the University of Helsinki Faculty of Veterinary Medicine and the Finnish Food Safety Authority Evira, launched in 2016. In the study, larval forms of flukes were found in almost every roach. The species of flukes found in the fish include Pseudamphistomum, Metorchis and Posthodiplostomum, in addition to other species that have not been identified yet.
"For instance, an increase has been observed in the occurrence of black spot disease in fish in the Gulf of Finland, caused by the digenean flatworm Posthodiplostomum cuticola. In addition to roach, the larval stages of this parasite are also found in perch. The definitive hosts include certain heron birds, such as the grey heron. These flukes of herons do not infect humans", says Oksanen.
In previous research carried out by Evira, two new species of flukes, liver worms Metorchis bilis and Pseudamphistomum truncatum, have been found in foxes in Finland in recent years. Liver worms are parasites of fish-eating mammals and birds. They can infect dogs, cats and humans that eat raw fish. Freshwater snails of the Bithyniidae family serve as definitive hosts of such liver worms, while roach serve as their secondary intermediate hosts.
How to avoid parasite infections when cooking fish
"When cooking freshly caught fish at home without heating it, the fish should be frozen for at least 24 hours in -20 degrees Celsius or below. The law requires that any ready-to-eat fish products sold in stores and served in restaurants go through a similar freezing process. This also applies to roe. The freezing process does not apply to Baltic herring and sprat," says Senior Inspector Carmela Hellsten.
When curing or cold-smoking purchased or self-caught fish, fish should be put in the freezer either unprepared or as the finished product. Evira recommends that roach are frozen for a minimum of seven days.
The risk of parasite infection in rainbow trout farmed in Finland and salmon farmed in Norway is very low, so these fish do not need to be frozen even if they are to be cooked without heating.
For further information, please contact:
Antti Oksanen, DVM (PhD) Tel. +358 44 561 6491 (parasites)
Anna Maria Eriksson-Kallio, Researcher, Tel. +358 50 439 2788 (fish diseases)
Carmela Hellsten, Senior Inspector, Tel. +358 50 433 6643 (microbiological food safety)