Molecular evidence of the absence of Metagonimus yokogawai (Katsurada, 1912) in Europe: report of Metagonimus sp. in cyprinoid fish from the River Danube in Hungary

Viability of Metagonimus romanicus (Ciurea, 1915) metacercariae after physico-chemical treatments

Digenean (Heterophyidae) trematodes include several zoonotic species such as the genus Metagonimus (Katsurada, 1912). Metagonimus romanicus (Ciurea, 1915) is a species widely distributed in Europe, whose metacercariae can be found on cyprinoids from the River Danube in Hungary. The aim of the study was to measure the viability of Metagonimus romanicus metacercariae by physical (freezing, heating, desiccation) and chemical (acetic acid and NaCl solutions) treatments. The methods were chosen as a model for procedures commonly used in traditional food preservation (such as freezing, salting, pickling and smoking) to measure the survival rate of metacercariae under different conditions. Most physical treatments (freezing of metacercariae at -20 °C, keeping them at 40 °C and 60 °C and desiccation) and chemical treatments (2.5%, 5%, 10% acetic acid solution and 5% and 10% NaCl solution) killed the metacercariae in a relatively short time (from 30 min to 6 days depending on the treatment) so their effects eliminated the risk of zoonotic infection. On the other hand, the metacercariae survived at room temperature and at 4 °C up to one month. Therefore, storing unprepared fish in domestic refrigerators cannot prevent infection with metacercariae in humans.

Small intestinal flukes of the genus Metagonimus (Digenea: Heterophyidae) in Europe and the Middle East: A review of parasites with zoonotic potential

The heterophyid trematode Metagonimus romanicus (Ciurea, 1915) (Digenea) is redescribed on the basis of type material from domestic dogs (Canis familiaris) in Romania, vouchers from experimentally infected cats (Felis catus) and adults recovered from golden hamsters (Mesocricetus auratus) infected with metacercariae from scales of chub (Squalius cephalus) and common nase (Chondrostoma nasus) (Cypriniformes: Leuciscidae) in Hungary. This trematode, endemic to Europe and neighbouring regions (northwestern Türkiye), was previously misidentified as M. yokogawai (Katsurada, 1912), a zoonotic parasite of humans in East Asia. However, the two species differ considerably both genetically and morphologically, e.g., in the position of the ventral sucker, the presence of the prepharynx, the anterior extent of the vitelline follicles and the posterior extent of the uterus. Metagonimus ciureanus (Witenberg, 1929) (syn. Dexiogonimus ciureanus Witenberg, 1929), described from domestic cats and dogs in Israel, is a valid species distributed in the Middle East and Transcaucasia, which is also confirmed by molecular data. It differs from all Metagonimus species, including M. romanicus, in having symmetrical testes instead of the oblique testes of the other congeners. The zoonotic significance of M. romanicus and M. ciureanus is unclear, but appears to be low in Europe, mainly because raw or undercooked, whole fish with scales are generally not consumed. Accidental infection of fishermen by metacercariae in the scales when cleaning fish is more likely, but has never been reported. Remains of cyprinoids with scales infected with metacercariae of Metagonimus spp. can be an important natural source of infection for dogs, cats, and other carnivores, which can serve as a reservoir for these parasites.

Epidemiology and Geographical Distribution of Human Trematode Infections

Digenetic trematodes infecting humans are more than 109 species that belong to 49 genera all over the world. According to their habitat in the definitive hosts, they are classified as 6 blood flukes (Schistosoma japonicum. S. mekongi, S. malayensis, S. mansoni, S. intercalatum, and S. haematobium), 15 liver flukes (Fasciola hepatica, F. gigantica, Clonorchis sinensis, Opisthorchis viverrini, O. felineus, Dicrocoelium dendriticum, D. hospes, Metorchis bilis, M. conjunctus, M. orientalis, Amphimerus sp., A. noverca, A. pseudofelineus, Pseudamphistomum truncatum, and P. aethiopicum), nine lung flukes (Paragonimus westermani, P. heterotremus, P. skrjabini, P. skrjabini miyazakii, P. kellicotti, P. mexicanus, P. africanus, P. uterobilateralis, and P. gondwanensis), 30 heterophyid intestinal flukes (Metagonimus yokogawai, M. takahashii, M. miyatai, M. suifunensis, M. katsuradai, M. pusillus, M. minutus, Heterophyes heterophyes, H. nocens, H. dispar, Haplorchis taichui, H. pumilio, H. yokogawai, H. vanissinus, Centrocestus formosanus, C. armatus, C. cuspidatus, C. kurokawai, Procerovum calderoni, P. varium, Pygidiopsis genata, P. summa, Stictodora fuscata, S. lari, Stellantchasmus falcatus, Heterophyopsis continua, Acanthotrema felis, Apophallus donicus, Ascocotyle longa, and Cryptocotyle lingua), 24 echinostome intestinal flukes (Echinostoma revolutum, E. cinetorchis, E. mekongi, E. paraensei, E. ilocanum, E. lindoense, E. macrorchis, E. angustitestis, E. aegyptica, Isthmiophora hortensis, I. melis, Echinochasmus japonicus, E. perfoliatus, E. lilliputanus, E. caninus, E. jiufoensis, E. fujianensis, Artyfechinostomum malayanum, A. sufrartyfex, A. oraoni, Acanthoparyphium tyosenense, Echinoparymphium recurvatum, Himasthla muehlensi, and Hypoderaeum conoideum), 23 miscellaneous intestinal flukes (Brachylaima cribbi, Caprimolgorchis molenkampi, Phaneropsolus bonnei, P. spinicirrus, Cotylurus japonicus, Fasciolopsis buski, Gastrodiscoides hominis, Fischoederius elongatus, Watsonius watsoni, Gymnophalloides seoi, Gynaecotyla squatarolae, Microphallus brevicaeca, Isoparorchis hypselobagri, Nanophyetus salmincola, N. schikobalowi, Neodiplostomum seoulense, Fibricola cratera, Plagiorchis muris, P. vespertilionis, P. harinasutai, P. javensis, P. philippinensis, and Prohemistomum vivax), one throat fluke (Clinostomum complanatum), and one pancreatic fluke (Eurytrema pancreaticum). The mode of transmission to humans includes contact with cercariae contaminated in water (schistosomes) or ingestion of raw or improperly cooked food, including fish (liver flukes, heterophyid flukes, echinostomes, and throat flukes), snails (echinostomes, brachylaimids, and gymnophallid flukes), amphibia, reptiles (neodiplostomes), aquatic vegetables (fasciolids and amphistomes), and insect larvae or adults (lecithodendriids, plagiorchiids, and pancreatic flukes). Praziquantel has been proven to be highly effective against almost all kinds of trematode infections except Fasciola spp. Epidemiological surveys and detection of human infections are required for a better understanding of the prevalence, intensity of infection, and geographical distribution of each trematode species.