Environmental influence on abundance and infection patterns of snail intermediate hosts of liver and intestinal flukes in North and Central Vietnam

Liver and intestinal flukes (LIF) are important groups of foodborne zoonotic trematodes (FZTs) in Southeast Asia, including Vietnam. Their complex life cycles require specific freshwater snail species as the obligatory first intermediate hosts. In 2019, we conducted a longitudinal study in Yen Bai and Thanh Hoa provinces in North and Central Vietnam, respectively, to investigate the diversity of LIF and their infection prevalence in relation to snail host abundance and environmental factors. Using a combination of morphological and molecular identification techniques, we identified 10 LIF species infecting 11 snail host species. We observed significant seasonal variation in the mean abundance of several snail host species, with the majority of snails collected during the spring. We also detected seasonal changes in LIF species composition, with the highest species richness reported in the spring. Clonorchis sinensis and Fasciola gigantica, two medically important human liver flukes in Asia, were found only in the spring in Yen Bai. Our study revealed that not all snail host species have the same probability of becoming infected, and we recorded seasonal variations in the prevalence of LIF infection in different snail species in relation to water parameters.

Insights on foodborne zoonotic trematodes in freshwater snails in North and Central Vietnam

Foodborne zoonotic trematode (FZT) infections are common neglected tropical diseases in Southeast Asia. Their complicated life cycles involve freshwater snails as intermediate hosts. A cross-sectional study was conducted in Yen Bai and Thanh Hoa provinces in North and Central Vietnam, to investigate the diversity of cercariae of potential FZT and to construct the phylogenetic relationship of trematode cercariae based on the Internal Transcribed Spacer 2 (ITS2) region. Among 17 snail species collected from various habitats, 13 were infected by 10 cercarial groups among which parapleurolophocercous, pleurolophocercous, and echinostome cercariae were of zoonotic importance. The monophyletic tree separated cercarial sequences into different groups following the description of the cercariae families in which Haplorchidae, Opisthorchiidae, Echinochasmidae, and Echinostomatidae are important families of FZT. The overall prevalence was different among snail species and habitats and showed a seasonal trend. Parapleurolophocercous and echinostome cercariae emerged as the most common cercariae in snails in Yen Bai, while monostome, echinostome, and megalura cercariae were most common in Thanh Hoa. Using a molecular approach, we identified Parafossarulus striatulus as the first intermediate snail host of Clonorchis sinensis in Thac Ba Lake. Melanoides tuberculata and Bithynia fuchsiana were we identified preferred intermediate snail hosts of a diverse range of trematode species including intestinal flukes (i.e., Haplorchis pumilio and Echinochasmus japonicus) in Yen Bai and Thanh Hoa, respectively.

Foodborne intestinal flukes: A brief review of epidemiology and geographical distribution

Foodborne intestinal flukes are highly diverse consisting of at least 74 species with a diverse global distribution. Taxonomically they include 28 species of heterophyids, 23 species of echinostomes, and 23 species of miscellaneous groups (amphistomes, brachylaimids, cyathocotylids, diplostomes, fasciolids, gymnophallids, isoparorchiids, lecithodendriid-like group, microphallids, nanophyetids, plagiorchiids, and strigeids). The important heterophyid species (15 species) include Metagonimus yokogawai, M. takahashii, M. miyatai, Heterophyes heterophyes, H. nocens, Haplorchis taichui, H. pumilio, H. yokogawai, Heterophyopsis continua, Centrocestus formosanus, Pygidiopsis genata, P. summa, Stellantchasmus falcatus, Stictodora fuscata, and S. lari. The echinostome species of public health significance (15 species) include Echinostoma revolutum, E. cinetorchis, E. lindoense, E. ilocanum, Isthmiophora hortensis, Echinochasmus japonicus, E. perfoliatus, E. liliputanus, E. fujianensis, E. caninus, Acanthoparyphium tyosenense, Artyfechinostomum malayanum, A. sufrartyfex, A. oraoni, and Hypoderaeum conoideum. Among the other zoonotic intestinal flukes, Gastrodiscoides hominis, Brachylaima cribbi, Neodiplostomum seoulense, Fasciolopsis buski, Gymnophalloides seoi, Caprimolgorchis molenkampi, Phaneropsolus bonnei, Microphallus brevicaeca, Nanophyetus salmincola, and N. schikhobalowi (10 species) have drawn considerable medical attention causing quite a fair number of human infection cases. The principal mode of human infections include ingestion of raw or improperly cooked fish (heterophyids and echinostomes), snails including oysters (echinostomes and G. seoi), amphibians and reptiles (N. seoulense), aquatic vegetables (amphistomes and F. buski), and insect larvae or adults (C. molenkampi and P. bonnei). Epidemiological characteristics such as the prevalence, geographical distribution, and clinical and public health significance are poorly known in many of these species. Praziquantel has been proved to be highly effective against most species of intestinal fluke infections. Surveys and detection of human infection cases are urgently required for better understanding of the global status and public health significance of each species.

Epidemiology of Trematode Infections: An Update

Digenetic trematodes infecting humans are more than 91 species which belong to 46 genera all over the world. According to their habitat in definitive hosts, they are classified as blood flukes (Schistosoma japonicum. S. mekongi, S. mansoni, S. haematobium, and S. intercalatum), liver flukes (Clonorchis sinensis, Opisthorchis viverrini, O. felineus, Metorchis conjunctus, M. bilis, M. orientalis, Fasciola hepatica, F. gigantica, Dicrocoelium dendriticum, and D. hospes), lung flukes (Paragonimus westermani, P. heterotremus, P. skrjabini, P. miyazakii, P. kellicoti, P. mexicanus, P. africanus, and P. uterobilateralis), throat fluke (Clinostomum complanatum), pancreatic fluke (Eurytrema pancreaticum), and intestinal flukes (Metagonimus yokogawai, M. miyatai, M. takahashii, Heterophyes nocens, H. heterophyes, Haplorchis taichui, H. pumilio, H. yokogawai, Centrocestus formosanus, Echinostoma revolutum, E. ilocanum, Isthmiophora hortensis, Echinochasmus japonicus, E. lilliputanus, Artyfechinostomum malayanum, A. sufrartyfex, A. oraoni, Fasciolopsis buski, Gymnophalloides seoi, Neodiplostomum seoulense, Caprimolgorchis molenkampi, Phaneropsolus bonnei, and Plagiorchis muris). The mode of transmission to humans includes contact with cercariae contaminated in water (schistosomes) and ingestion of raw or improperly cooked fish (liver and throat flukes, heterophyids, and echinostomes), snails (echinostomes and gymnophallids), amphibia, reptiles (neodiplostomes), aquatic vegetables (amphistomes), or insect larvae or adults (plagiorchiids, lecithodendriids, and pancreatic fluke). Praziquantel has been proved to be highly effective against most species of trematode infections except fascioliasis. Epidemiological surveys and detection of human infections are required for better understanding of the geographical distribution and endemicity of each trematode species.