Introduction
Hydropower, as a renewable energy source, has the potential for rapid and sustainable development. It has several advantages, including long lifespan, low maintenance cost, and ease of operation, as it can start and stop generating power much faster than coal-fired plants, which require extended time (e.g., 8 hours) to reach operating temperatures. Additionally, hydroelectric dams contribute to water regulation, flood control, and various social-economic benefits. As a result, hydroelectricity has been implemented in 141 countries worldwide, contributing a total of 4,289 Terawatt hours annually, which accounts for 15% of global electricity production and half of the renewable energy supply (Yearly Electricity Data 2011).
The construction of hydropower dams has significantly altered hydrologic regimes, disrupted natural environments, and reshaped biological community interactions, including those involving humans (Anderson et al. Reference Anderson, Moggridge, Warren and Shucksmith2015; Gao et al. Reference Gao, Zeng, Wang and Liu2010; Guo et al. Reference Guo, Hu, Zhang and Feng2012; Han et al. Reference Han, Fukushima, Kameyama, Fukushima and Matsushita2008; Li and Rees Reference Li and Rees2000; Li et al. Reference Li, Dong, Peng, Yang, Liu, Li and Zhao2013; Magilligan and Nislow Reference Magilligan and Nislow2005; Meentemeyer et al. Reference Meentemeyer, Haas and Vaclavik2012; Morley Reference Morley2007; Rosenberg et al. Reference Rosenberg, Bodaly and Usher1995). These dams, often associated by irrigation schemes, create suitable habitat for vectors and intermediate hosts of emerging or re-emerging infectious diseases (Ong et al. Reference Ong, Wang, Sithithaworn, Grundy-Warr and Pitaksakulrat2016; Patz et al. Reference Patz, Graczyk, Geller and Vittor2000). Changes in water level and flow directly affect the distribution of aquatic vegetation, invertebrate and fish populations, and nesting sites of birds and carnivores, thereby altering the abundance and species composition of parasites (Izyumova Reference Izyumova1987).
Notable examples of water reservoir-linked diseases include Rift Valley fever (Hassan et al. Reference Hassan, Ahlm, Sang and Evander2011) and schistosomiasis (N’Goran et al. Reference N’Goran, Diabate, Utzinger and Sellin1997; Stauffer et al. Reference Stauffer, Arnegard, Cetron, Sullivan, Chitsulo, Turner, Chiotha and McKaye1997) in Africa, as well as malaria in various regions (Keiser et al. Reference Keiser, De Castro, Maltese, Bos, Tanner, Singer and Utzinger2005). Parasitic communities in newly formed reservoirs undergo significant changes within the first 5–8 years, with dominant parasites establishing themselves during this period. Limnophilic parasites, those preferring still water, thrive in the newly created habitats, while rheophilic species, which prefer flowing water, decline sharply (Becker et al. Reference Becker, Carr, Cloutman, Evans, Heard, Holmes, Norman and Owen1978; Hoffman and Bauer Reference Hoffman and Bauer1971; Iskov Reference Iskov1976; Izyumova Reference Izyumova1987; Morley Reference Morley2007). Parasites with indirect life cycles, which require intermediate hosts, take longer to establish. For example, parasites cycling through molluscs typically establish within 5–6 years (Izyumova Reference Izyumova1987).
Fish-borne zoonotic trematodes (FZTs) are a group of medically significant parasites whose transmission dynamics can be influenced by ecological changes caused by dam construction (Ong et al. Reference Ong, Wang, Sithithaworn, Grundy-Warr and Pitaksakulrat2016). Although a large literature exists regarding the effects of dam construction on many medically important parasites, the specific impact on FZTs remains less studied (Madsen and Stauffer Reference Madsen, Stauffer, Morales-Montor, Rio-Araiza and Hernandéz-Bello2022; Morley Reference Morley2007; Part et al. Reference Patz, Graczyk, Geller and Vittor2000). Ong et al. (Reference Ong, Wang, Sithithaworn, Grundy-Warr and Pitaksakulrat2016) reported that dam construction in Thailand’s Ubolrantana river led to ecological changes in assemblages of fish species, resulting in increased human infection by the small liver fluke Opisthorchis viverrini. The prevalence and intensity of infection in fish in reservoirs were notably higher than in lotic fishes, highlighting the impact of dam ecosystems on trematode transmission. The distribution of trematodes depends heavily on the presence of their first intermediate host snails (Madsen and Hung Reference Madsen and Hung2014; Mas-Coma and Bargues Reference Mas-Coma and Bargues1997).
In Vietnam, 385 hydropower plants generate 120,000 Gigawatt hours annually, contributing one-third of the country’s electricity (Vietnam Electricity 2022), placing Vietnam 13th globally (Yearly Electricity Data 2011). The mountainous region of Northern Vietnam, which houses 60% of the country’s hydropower capacity, is of particular interest (Vietnam Energy 2015). Major hydropower reservoirs, such as Thac Ba (built in 1964), Hoa Binh (1979), and Son La (2005), have transformed local aquatic ecosystems, covering vast areas and supporting substantial production of fishes. These reservoirs provide ideal conditions for studying the spread of trematodes, especially FZTs, through the adaptation of snail populations to the altered environments post-dam construction (Morley Reference Morley2007).
The socio-economic conditions and dietary practices of communities in Northern Vietnam further exacerbate the risk of FZT infection (Nguyen et al. Reference Nguyen, Do, Greiman, Nguyen, Hoang, Phan, Pham-Duc and Madsen2022). This region is among the poorest in the country with a poverty rate of 14.22% (compared to the national average of 4.03%) and is home to over 30 ethnic groups living in reservoir-adjacent communities (Vietnamese Ministry of Labour, Invalid and Social Affairs 2023). Many of these communities practice traditional dietary habits, such as consuming raw or undercooked fish, increasing the risk of trematode infection (Bui et al. Reference Bui, Pham, Nguyen, Nguyen, Murrell and Phan2016; Phi et al. Reference Phi, Nguyen, TTH, Do, Lenaerts, Losson, Vandenberg, Dorny, van Gerven, Boere-Boonkamp and Bui2022). In addition, basic sanitation infrastructure, including direct discharge of wastewater into reservoirs and free-ranging livestock, further exacerbate the potential for transmission of trematodes (Nguyen et al. Reference Nguyen, Van, Ho, Tatonova and Madsen2021a). The convergence of these risk factors makes humans in this region particularly susceptible to parasitic infections, including lung flukes, Paragonimus spp., small liver flukes, Clonorchis sinensis, large liver flukes, Fasciola spp., or nematodes, Trichinella spp. (Nguyen et al. Reference Nguyen, Do, Greiman, Nguyen, Hoang, Phan, Pham-Duc and Madsen2022). Reports of C. sinensis infections are sparse, with low prevalence of infections noted in Ha Giang and Hoa Binh provinces (Nguyen Reference Nguyen2004). Doanh and Nawa (Reference Doanh and Nawa2016) noted the diagnosis challenges complicate species-level identification of small liver fluke eggs.
High prevalence of trematode infections has been observed in the Thac Ba reservoir, with FZT prevalence rates of 47.5% in humans (definitive host) (Thieu et al. Reference Thieu, Vinh, Nam, Thanh and Dung2022), 29.3 to 76.7% in fish (second intermediate hosts) (Bui et al. Reference Bui, Pham, Nguyen, Nguyen, Murrell and Phan2016; Dai et al. Reference Dai, Hong, Pak, Le, Choi, Na and Sohn2020; Phan et al. Reference Phan, Bui, Nguyen and Murrell2016), and 0.7 to 11.5% in snails (first intermediate hosts) (Nguyen et al. Reference Nguyen, Van, Ho, Tatonova and Madsen2021a, Reference Nguyen, Hoang, Dinh, Dorny, Losson, Bui and Lempereurb, Nguyen Reference Nguyen2022). These high infection prevalences are linked to local behaviors, high risk of fish containing metacercaria within the reservoir, and the adaptation of snail hosts to post-dam conditions. The expansion of cage culture in these reservoirs has further contributed to the spread of trematode infections (Kim et al. Reference Kim, Nguyen, Greiman, Nguyen, Nguyen, Vu, Hoai and Madsen2022; Madsen et al. Reference Madsen, Nguyen, Lanza and Stauffer2022).
The Hoa Binh, Son La, and Thac Ba reservoirs were selected for this study because they were constructed at different times and are located in two of the largest river systems in Northern Vietnam: the Red River and Da River systems. The variation in reservoir age, environmental condition, and proximity to human activities may influence the prevalence and diversity of trematode infections among snail hosts across these reservoirs. By comparing these factors, we aim to evaluate the impact of hydropower dams on the transmission of trematodes.
Methods
Snail sampling and examination
We selected five communes near each reservoir for collection of snails (Figure 1). All communes were resettled during the construction of these dams, and populations rely heavily on local resources for their livelihoods. The population of selected communes in Hoa Binh ranged from 2,004 to 2,293, with an average density of 212 people/km2. The lowest population density was in Tien Phong commune, with 33 people/km2, while the highest was in Toan Son commune, with 787 people/km2. In Son La reservoir area, the population of selected communes ranged from 3,403 to 16,153, with an average density of 144 people/km2. Hua Trai commune had the lowest population density at 34 people/km2, and It Ong town had the highest at 464 people/km2. In Thac Ba reservoir area, the population of selected communes ranged from 4,312 to 6,678, with an average density at 212 people/km2, An Phu commune had the lowest population density at 115 people/km2, while Yen Thang commune had the highest at 318 people/km2. Each commune is home to 12–15 ethnic groups living together.
Figure 1. Study sites (red pins) in three hydropower reservoirs.
In each commune, snails were collected from the reservoir (three sites) and from canals or paddy fields where water flows in and out of the reservoir (three sites). The distance between two collection sites ranged from 300 to 1,000 meters. Snail sampling was conducted bi-monthly by handpicking by the same person for 20 minutes per site, from March to December 2023. All collected snails were transferred to plastic containers and transported alive to the laboratory, where they were identified to species level using the taxonomic keys by Brandt (Reference Brandt1974), Thanh et al. (Reference Thanh, Bai and Mien1980), and Thanh and Hai (Reference Thanh, Hai, Canh, Thanh, Huynh, Thinh and Tho2017).
Snails were examined for digenean infection using the shedding method (Frandsen and Christensen Reference Frandsen and Christensen1984). Depending on the size, each snail was placed into a small plastic container with 5 or 10 ml of tap water and left for 48 hours to shed cercariae; snails were not fed during this period. Shedding was checked three times daily at 08:00 hr, 13:30 hr, and 17:30 hr.
The morphological characteristics of cercariae were examined using an Olympus BX53 light microscope (Olympus Corp., Tokyo, Japan). Cercariae were classified into major morphotypes according to the keys of Schell (Reference Schell1985) and Ginetsinskaya (Reference Ginetsinskaya1988).
Statistical analysis
Data were entered into an Excel database (Microsoft Corporation, Redmond, Washington) and analyzed using STATA/IC 12 (StataCorp 2011). Trematode infections within snails were analyzed using logistic regressions, with species, habitat type (within reservoirs and canals/ paddy fields), and reservoir as predictors after adjusting for the cluster within collection sites. Differences with P-values below 0.05 were considered significant.
Results
Snail diversity and infection
A total of 25,299 snail specimens representing 16 species within 9 families was collected from the three hydropower reservoirs, canals, and rice-paddy fields nearby (Table 1; Figure 2). Three families (i.e., Bithyniidae, Viviparidae, and Thiaridae) each consisted of 3 species, while the other families had 1−2 species. The Black Apple snail Pila polita was found only in the Son La reservoir area, Brotia swinhoei was found only in the Da River system, and the remaining 14 species were present at all study sites. Among these snails, Melanoides tuberculata, Angulyagra polyzonata, and Orientogalba viridis were the dominant species, comprising about half of all specimens.
Table 1. Snail distribution and infection prevalence across study sites (Prev. = prevalence)
Figure 2. Stacked bar plot comparing the composition of snail families collected from the three hydropower reservoirs and associated paddy fields: Hoa Binh, Son La, and Thac Ba.
Eight species from 4 families were found to shed cercariae. The overall prevalence of cercarial infection within snails differed significantly among the three reservoirs (p<0.001) and snail species (p<0.001) but did not differ significantly between snails collected from reservoirs and canals/paddy fields (p=0.284). Among the three reservoirs, the prevalence of snail infection with trematode larvae was highest in Son La, showing to be significantly different from Hoa Binh (p<0.001) and Thac Ba (p<0.001). The overall prevalence of infection in Thac Ba reservoir was 1.06 times higher than in Hoa Binh, although the difference was not statistically significant (p=0.627).
Two thiarid snails were the most common hosts for trematodes, with prevalence ranging from 5.9% to 15.4% for Melanoides tuberculata, and 5.4% to 10.8% for Tarebia granifera. Bithynid snails were also often infected, with an infection prevalence in Parafossarulus manchouricus and Gabbia fuchsiana ranging from 3.8% to 5.4% and 2.9% to 5.8%, respectively. Other infected snail species had infection prevalence ranging from 0.3% to 2.9%.
Cercaria morphotypes
Out of 959 infected snail specimens, 9 distinct cercarial morphotypes were recorded (Table 2; Figure 3). Pleurolophocercariae was the most dominant morphotype, comprising 36.1% of the total infected snails. This morphotype was shed from 5 snail species. Xiphidiocercariae represented 28.5% and echinostomes 24.6%, shed by 3 and 7 snail species, respectively. The morphotypes amphistomata (2.1%), megalurous (1.4%), and tail-less cercariae (0.5%) were shed by only 1 snail species each. Other morphotypes were shed by 2 to 3 species. The highest prevalence of echinostome type was observed in Tarebia granifera in canals/paddy fields at the Son La reservoir area, at 16.4%, while the second highest prevalence was pleurolophocercaria type from M. tuberculata at the same study site (8.6%). Gabbia fuchsiana, P. manchouricus, and M. tuberculata each harbored 5 cercarial morphotypes. Radix auricularia only released echinostome type, while the other snail species released 2 or 3 cercarial morphotypes. For comparison of cercarial morphotype infection, prevalence did not differ among cercarial morphotypes (p=0.117), environments (p=0.160), and reservoirs (p=0.077), but it did differ among snail species (p<0.001). The prevalence of each cercarial morphotype was similar between Hoa Binh and Son La (p=0.363) but differed from Thac Ba (p=0.019).
Table 2. Percentage of each cercaria group found within different snail species across various environments
Figure 3. Stacked bar plot comparing the prevalence of cercarial morphotypes shed from snails collected from the three hydropower reservoirs and associated paddy fields: Hoa Binh, Son La, and Thac Ba.
Discussion
The presence of mollusk first intermediate hosts, with an average infection prevalence of 3.8%, and a large diversity of cercarial morphotypes in the Hoa Binh, Son La, and Thac Ba hydropower reservoirs reveal that these three reservoir areas constitute endemic regions for trematode transmission. Among the 16 snail species present in this study area, only 3 species – Brotia swinhoei, Sinotaia reevei, and Gabbia longicornis – are distributed in Northern mountainous regions, while other species had a wider distribution (Thanh et al. Reference Thanh, Bai and Mien1980). Brotia swinhoei inhabited running water, while the other snail species were commonly found in stagnant water. Viviparid snails were commonly found in sluggish rivers, lakes, ditches, and rice fields at high densities. They are very popular for human consumption in Vietnam and other countries as well (Madsen and Hung Reference Madsen and Hung2014). Although viviparids did not shed cercariae, they are known to be second intermediate hosts for some food-borne digeneans, such as Echinostoma spp. (Le et al. Reference Le, The and Ngai1990; Nguyen et al. Reference Nguyen, Do, Greiman, Nguyen, Hoang, Phan, Pham-Duc and Madsen2022). Regarding ampullariid snails, the native Black Apple snail, Pila polita, was found only in a small canal in the Son La reservoir, with just 3 specimens collected. The invasive Golden Apple snail, Pomacea canaliculata, was present across all environments with numerous specimens. Although the invasive snails did not release cercariae, they can still pose significant risks as potential hosts for Angiostrongylus cantonensis, which causes eosinophilic meningitis (Lv et al. Reference Lv, Guo, Nguyen, Sinuon, Sayasone, Lo, Zhou and Andrews2018). Both viviparids and Pila snails are known to feed on plants and the egg masses of other snails. Therefore, they could potentially serve as biological control agents for disease-transmitting snails, such as those responsible for schistosomiasis, by competing for food and eliminating the eggs of target snails (Joshi and Sebastian Reference Joshi and Sebastian2006).
Bithyniid snails prefer relatively stable environments, such as ponds, lakes, and rice fields. Although 3 bithyniid species were collected, only P. manchouricus and G. fuchsiana have been reported to shed cercariae, while G. longicornis did not. Nguyen et al. (Reference Nguyen, Greiman, Hoang, Ngoc, Nguyen, Pham and Madsen2024) reported that both G. fuchsiana and P. manchouricus shed 6 cercarial morphotypes and served as the first intermediate hosts of 8 and 12 digenean species, respectively. In this study, both snail species shed only 5 cercarial morphotypes. Additionally, Nguyen et al. (Reference Nguyen, Van, Ho, Tatonova and Madsen2021a) proved that only P. manchouricus is a suitable host for the small liver fluke Clonorchis sinensis in Vietnam, while G. fuchsiana and the 2 other thiarid snails, M. tuberculata and T. granifera, are not, although they were previously reported as first intermediate hosts. Results of recent studies highlighted Thac Ba reservoir as a highly endemic area for C. sinensis (Bui et al. Reference Bui, Pham, Nguyen, Nguyen, Murrell and Phan2016; Phan et al. Reference Phan, Bui, Nguyen and Murrell2016), and Nguyen et al. (Reference Nguyen, Van, Ho, Tatonova and Madsen2021a) found the prevalence of infection of C. sinensis cercariae in P. manchouricus to be 2.85%. In the present study, pleurolophocercaria, which could be of C. sinensis or other species of families Opisthorchiidae and Heterophyidae, were released from P. manchouricus in Son La, Hoa Binh, and Thac Ba with infection prevenance of 1.27%, 2.3%, and 2.08%, respectively. The presence of the first intermediate host of C. sinensis in Son La and Hoa Binh reservoirs, combined with the high prevalence of infection of pleurolophocercaria and the similar hygiene conditions, practices, and culture of local people living nearby these reservoirs, suggests that Son La and Hoa Binh could also be endemic areas for C. sinensis, similar to Thac Ba reservoir. The limitation of this study is that cercariae were only identified to major morphotypes. Therefore, the diversity of digeneans within all infected snail species within the study areas should be studied further.
Thiarid snails inhabit slow-moving water with muddy bottoms as well as stagnant water. They are often found attached to rocks or buried in the soft mud of shallow waters. All 3 thiarid species in the present study are known hosts of various digenean species. For example, M. tuberculata is known to shed 5 morphotypes of cercariae and serves as a host for more than 10 trematode species, including Microparyphium sp., Philophthamus gralli, Catatropis vietnamensis, Transversotrema patialense, Centrocestus formosanus, Notocotylus intestinalis, Haplorchis spp., Acanthatrium sp., Lecithodendrium spathulatum, and Paralecithodendrium sp. (Besprozvannykh et al. Reference Besprozvannykh, Ngo, Ha, Hung, Rozhkovan and Ermolenko2013; Nguyen et al. Reference Nguyen, Greiman, Hoang, Ngoc, Nguyen, Pham and Madsen2024; TropMed Technical Group 1986). Thiara scabra did not release any cercariae in this study, though it has been reported to do so in other studies (Nguyen et al. Reference Nguyen, Hoang, Dinh, Dorny, Losson, Bui and Lempereur2021b).
Stenothyra messageri was mainly collected in rice fields and near the banks of reservoirs where vegetation and muddy bottom are present. While previous reports noted that S. messageri shed only tail-less cercariae morphotype (Nguyen et al. Reference Nguyen, Greiman, Hoang, Ngoc, Nguyen, Pham and Madsen2024), this study found two morphotypes of cercariae, with a higher prevalence of pleurolophocercaria compared to tail-less cercaria.
Pulmonate species were not commonly found in reservoirs themselves, but they were observed in high densities within sites with floating plants (e.g., soft hornwort Ceratophyllum submersum and common water hyacinth Eichhornia crassipes) and emergent plants (e.g., water taro Colocasia esculenta and various grasses) near the banks, especially in the areas connected to paddy fields. Two lymnaeid species, Orientogalba viridis and Radix Auricularia, collected during this study, are known hosts for rumen and large liver flukes (e.g., Gastrothylax spp. and Fasciola spp). Additionally, these species, and Gyraulus convexiusculus, play an important role as hosts for echinostomatids (Le Reference Le, Thanh, Huynh, Cao, Nguyen, Le, Thai, Nguyen and Ta2000; Nguyen 2020; Nguyen et al. Reference Nguyen, Greiman, Hoang, Ngoc, Nguyen, Pham and Madsen2024). Although the prevalence of fasciolosis in humans at the study sites was low (less than 10 cases per year), prevalence in cattle was high, exceeding 30% (Hien et al. Reference Hien, Doanh, Duc, Luc and Thach2011). Cattle, which roam freely during the day and are confined at night, often graze for periods extending up to six months. Manure from cattle likely enters the water environments, contributing to the prevalence of fasciolosis due to the presence of suitable intermediate hosts (snails and vegetation) and the consumption of raw or insufficiently cooked food.
Among the three reservoirs, Hoa Binh and Thac Ba reservoirs have similar characteristics, forming large lakes behind built dams with many islands and bays, while the Son La reservoir dam creates an expanding upstream river. This may explain the higher prevalence of trematode infection in snails in Son La reservoir compared to Hoa Binh and Thac Ba reservoir. Izyumova (Reference Izyumova1987) noted that the distribution of parasite fauna is not always homogeneous across regions. In river reservoirs with continuous flow and few shallow areas, the parasitic fauna is usually distributed relatively uniformly. However, the distribution can develop into distinct zones depending on factors such as aquatic vegetation coverage, the extent of shallow areas, and bird population density, which is attracted to the reservoir (Bauer and Stolyarov Reference Bauer, Stolyarov, Dogiel, Petrushevski and Polyanksi1961; Morley Reference Morley2007). Furthermore, Patz et al. (Reference Patz, Graczyk, Geller and Vittor2000) and Morley (Reference Morley2007) highlighted that responses of aquatic ecosystems to reservoir construction are complex and variable, influenced by various biotic and abiotic factors. As a result, the difference in parasite fauna, including fish-borne zoonotic trematodes, do not adhere to a single universal model.
About 350 species of snails are estimated to be of potential medical or veterinary importance associated with diseases such as schistosomiasis and various food-borne trematodes (Madsen and Hung Reference Madsen and Hung2014; Madsen and Stauffer 2021). In Vietnam and Southeast Asia, caenogastropods are dominant over heterobranches in freshwater ecosystems. The regional pattern may be related to the long-standing ‘wet rice civilization’ in these areas, where stable, slow-moving water bodies, such as rice paddies, reservoirs, ponds, provide suitable habitats for caenogastropods. The dominance of caenogastropods likely influences the infection rate of snail-borne diseases in the region. Many of these snails serve as intermediate hosts for trematodes, contributing to the transmission of diseases like Clonorchis sinensis, heterophids, and other food-borne trematodioses, which are closely linked to local agricultural practices, cultural food habits, and human-water contact. The coevolution of trematodes with specific caenogastropods hosts, along with agricultural and cultural practices, likely shapes the parasite-host relationships in this region and influence disease transmission dynamics. Further research into the ecological interactions between these snails and their trematode fauna could provide valuable insights into the risks of emerging or changing snail-transmitted diseases in Southeast Asia.
Digenean infections in snails can be observed by either crushing the snails or by natural emergence (shedding). Crushing snails can yield more accurate results regarding infection prevalence, as different morphotypes of intramolluscan larval digenean stages, such as mother sporocysts, daughter sporocysts, and rediae, may be found in snail tissues prior to release of cercaraie. Crushing snails may release immature cercariae, which are unsuitable for identification, even at higher taxonomic levels (Frandsen and Christensen Reference Frandsen and Christensen1984). In the present study, we used the shedding method to collect cercariae. Therefore, the actual prevalence of trematode infections in the first intermediate snail hosts may be higher than what is reported in the result section. Another limitation of this study is that cercariae identification was based solely on morphological characteristics and only classified into major morphotypes, not to the species or genus level using molecular approaches. This limits the statistical analyses, as differences in infection prevalence among different cercarial morphotypes were not observed, but such differences may exist at species level for specific digenean species.
The anthropogenic movement of fish and invertebrates into new reservoirs presents challenges for aquaculture by introducing new parasites to the indigenous fauna, which may lead to epizootic outbreaks of disease (Hoffman and Bauer Reference Hoffman and Bauer1971; Madsen et al. Reference Madsen, Nguyen, Lanza and Stauffer2022). The harvesting of wild populations of fish can affect transmission intensity of trematodes, while the expanding aquaculture in reservoirs is also linked to antimicrobial resistance, metal bioaccumulation, food safety and food security, vector-borne diseases, environmental contamination, and other health threats shared by people, animals (including fish), and the environment (Madsen et al. Reference Madsen, Nguyen, Lanza and Stauffer2022). Moreover, the high prevalence of cercariae in aquaculture environments demonstrates the risk of trematode transmission to fish, which can compromise food safety (Phan et al. Reference Phan, Bui, Nguyen and Murrell2016). This highlights the need for improved aquaculture management practices to control trematodes transmission. Therefore, the evaluation of human health risk in reservoirs should be viewed within a One Health/EcoHealth context.
Furthermore, the water levels in the three reservoirs vary significantly depending on seasonal rainfall, dam management, and local hydrology. Fluctuations of water levels impact snail populations, particularly by altering the availability of suitable habitats for feeding and reproduction (Morley Reference Morley2007). Rapid changes in water levels can reduce the presence of aquatic vegetation and shoreline habitats, which are critical for many snail species. These abnormal fluctuations can affect overall biodiversity and the transmission dynamics of snail-borne diseases. For instance, flooding increases the chance of trematode eggs entering the reservoirs, leading to a higher risk of trematode transmission (Nguyen et al. 2021). Further monitoring of water levels and their effects on invertebrate communities in these reservoirs would provide valuable insights into the ecological health of these ecosystems.
Acknowledgements
During the preparation of this work, the author(s) used [OpenAI-ChatGPT tool] to polish the language and grammar. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication. The authors would also like to express gratitude to the two peer reviewers and editor(s) for their insightful comments and valuable suggestions, which helped improve the manuscript.
Author contribution
HAL and HMN identified the snail species and wrote the manuscripts; HVH, TAT, CNN, BNN, and HNT organized snail collection, examined the infection of trematode larvae, and conducted data entry; HMN and HVN identified cercaria morphotypes; HVN was responsible for funding acquisition and project administration; SEG, JRS Jr, HM, and HMN analyzed the data, reviewed, and edited the manuscript. All authors read and approved the final version of the manuscript.
Financial support
This work was supported by the Vietnamese Ministry of Natural Resources and Environment, with the delegation of power to the Vietnam Academy of Science and Technology (grant number UQ.SNMT.01/23-25).
Competing interest
The authors declare there are no conflicts of interest.
Ethical standard
Not applicable
