Foodborne zoonotic trematode infections and integrated control in Vietnam: Aspects related to snails and vegetables

The foodborne zoonotic trematodes (FZT), belonging to sub-class Digenea, constitute a group of liver flukes (Clonorchis sinensis, Opisthorchis spp., and Fasciola spp.), intestinal flukes (Echinostoma spp., Fasciolopsis buski, heterophyids) and lung flukes (Paragonimus spp.) that are considered as parasites responsible for animal diseases which can potentially be transmitted to humans (foodborne trematodiasis). The complex life cycle of FZT is characterized by several larval stages which require a freshwater snail as the first intermediate host.
FZT infection in humans has been reported in many provinces throughout Vietnam, particularly in endemic areas with high prevalence of both large liver flukes (Fasciola spp.) and small liver flukes (C. sinensis and Opisthochis spp.). Traditional aquaculture and agricultural practices that use unprocessed manure as fertilizer and food for fishes, unrestricted animal roaming, and specific food culinary traditions (e.g raw or undercooked fish/crab/vegetable preparations) are risk factors for the spread and maintenance of FZT infection. This thesis combines surveys and experiments to gain insight into essential aspects of FZT infection and the integrated controls relating to freshwater snails and vegetables in two different provinces in North and Central Vietnam due to their known high infection rate of FZT in both humans and fishes. Therefore, the epidemiology and transmission dynamics of FZT infection in freshwater snails, the biological control aspects of selected snail host species, and the food safety aspects relating to fascioliasis were studied in detail.
The cross-sectional and longitudinal studies were conducted in Yen Bai and Thanh Hoa to investigate the diversity and the spatio-seasonal variation of the prevalence of FZT infection in snails using morphological and molecular approaches (Chapter 3, Section 1, Articles 1 and 2). The study recorded twelve infected snail species among the seventeen snail species collected in two provinces. A total of 16 distinguished cercarial types within 11 cercarial groups were found in which seven FZT species were identified. They consisted of two medically important human liver flukes, i.e. C. sinensis and F. gigantica, and several intestinal flukes, i.e. Haplorchis spp, Echinochasmus japonicus, Echinostoma revolutum, and Centrocestus formosanus. Seasonal variation in FZT species composition was detected with the highest diversity in the spring. The study also revealed that seasonal variations of FZT infections in correlation with environmental factors differed depending on specific FZT species and their snail hosts.
The study in Section 2 was conducted to evaluate the potential of malacopathogenic and entomopathogenic nematodes (MPN and EPN, respectively) as well as essential oils as biomolluscicides under laboratory and field conditions. The study indicated that in the laboratory conditions, the freshwater snail A. viridis is susceptible to both molluscicidal nematode Phasmarhabditis hermaphrodita and the three strains of EPN (Heterorhabditis indica (DL1 and CB3 strains) and Steinernema longicaudum (DL3 strain)), as well as four different essential oils (i.e. lemongrass, cinnamon, holy basil, and peppermint) (Article 3). The ability of P. hermaphrodita to regenerate on snail cadavers is an important outcome in considering their application in the long-term control of snail hosts in the field. The treatments with the highest lethality under laboratory conditions were selected for field trials (4m x 4m plots) including the molluscicidal nematode P. hermaphrodita and the two essential oils of lemongrass and cinnamon (Article 4). Despite using the standard field dose of P. hermaphrodita (300,000 individuals/m2) as recommended and double LC90 of lemongrass and cinnamon essential oils under laboratory conditions, none of the treatments caused complete death of A. viridis in the field. Environmental factors such as temperature, humidity, light, and soil properties can influence the efficacy of both nematodes and essential oils, suggesting the need for further study to determine the best conditions for the effective treatment procedures. Although the use of parasitic nematodes and essential oils in mollusk control is considered safe for some non-target species and crops, its actual implementation in the field needs more research.
Human fascioliasis in Vietnam is distributed throughout the country due to the habit of consuming raw or undercooked vegetables and drinking water contaminated with Fasciola metacercariae in many regions. The goal of Section 3 (Article 5) was to evaluate the efficiency of several chemicals in detaching and inactivating F. gigantica metacercariae on aquatic vegetables typically consumed raw in Vietnamese cuisine while keeping the freshness and safety for human consumption. In the laboratory, Austropeplea viridis snails were infected with F. gigantica miracidia to produce metacercariae. Vegetables containing metacercariae were washed with running water and soaked in different chemicals. Rinsing vegetables under running water and scrubbing them by hand has little effect on detaching metacercariae. Acetic acid was shown to be the most effective in killing metacercariae while sodium hypochlorite could detach 92-94% of metacercariae on aquatic vegetables. Despite the fact that the decrease of vitality of metacercariae is positively associated with exposure periods and treatment concentrations, a high percentage of alive metacercariae has been documented after treatments. This emphasizes the need of discovering the optimal techniques and more effective agents for controlling metacercariae on vegetables in future research.