Introduction: the Fasciola life cycle and the reservoir question
Fasciola hepatica and Fasciola gigantica are snail-borne trematodes of major veterinary and zoonotic importance. Adult flukes live mainly in the bile ducts of mammalian definitive hosts, where eggs are produced and passed with faeces. Eggs do not infect other mammals in the intestine or dry terrestrial environments. They must reach freshwater or sufficiently wet habitats, embryonate, and release miracidia that penetrate compatible freshwater lymnaeid snails. Within the snail, asexual development through sporocyst and redial stages can generate many cercariae, which leave the snail and encyst as metacercariae on vegetation or other wet substrates. Infection of the next definitive host occurs when viable metacercariae are ingested by the host. Thus, interactions among definitive hosts in this system usually indicate ecological coupling through the shared or sequential use of contaminated wet habitats, grazing areas, water sources, or hydrological networks rather than direct parasite transmission, competition, or predation (Alba et al. Reference Alba, Vazquez and Hurtrez-Boussès2021; Beesley et al. Reference Beesley, Caminade, Charlier, Flynn, Hodgkinson, Martinez-Moreno, Martinez-Valladares, Perez, Rinaldi and Williams2018; Mas-Coma et al. Reference Mas-Coma, Valero and Bargues2022; Sabourin et al. Reference Sabourin, Alda, Vazquez, Hurtrez-Boussès and Vittecoq2018).
Natural infections have been documented in cervids, wild bovids, suids, lagomorphs, camelids, and semi-aquatic rodents in several ecological settings (Didkowska et al. Reference Didkowska, Klich, Filip-Hutsch, Matusik, Krajewska-Wędzina, Wojciechowska, Kaczor, Olech and Anusz2025; French et al. Reference French, Zadoks, Skuce, Mitchell, Gordon-Gibbs, Craine, Shaw, Gibb and Taggart2016, Reference French, Zadoks, Skuce, Mitchell, Gordon-Gibbs and Taggart2019; González Hidalgo et al. Reference González Hidalgo, Diez Baños, Hidalgo Argüello and Martínez-Delgado2025; Issia et al. Reference Issia, Pietrokovsky, Sousa-Figueiredo, Stothard and Wisnivesky-Colli2009; Labruna et al. Reference Labruna, Costa, Port-Carvalho, Oliveira, Souza and Castro2018; Lamb et al. Reference Lamb, Doyle, Barwick, Chambers and Kahn2021; Martins et al. Reference Martins, Failla, Martins, Uzai, Viana, Gomes and Verocai2021; Mezo et al. Reference Mezo, Gonzalez-Warleta, Castro-Hermida, Manga-Gonzalez, Peixoto, Mas-Coma and Valero2013). These observations establish a broad host range, but do not all represent the same epidemiological state. Here, this epidemiological state refers to the position supported by the available evidence along the sequence from exposure to established infection, active parasite antigen production, patent egg shedding, and, ultimately, realised environmental transmission. This distinction is important because different diagnostic endpoints support different conclusions.
Reservoir terminology is also target-dependent. Haydon et al. (Reference Haydon, Cleaveland, Taylor and Laurenson2002) defined a reservoir relative to a specified target population and allowed it to comprise one or more epidemiologically connected populations or environments in which an agent is maintained and from which infection reaches the target. In fascioliasis, the targets might be dairy cattle in a wet grazing system, sheep sharing a seasonal marsh, people exposed to a human-endemic area, or a threatened wildlife population in a reserve. Therefore, the same infected wildlife population could be epidemiologically important for one target but irrelevant for another if the required environmental connection is absent. More generally, reservoir attribution requires complementary epidemiological and ecological evidence rather than a positive test alone (Fenton et al. Reference Fenton, Streicker, Petchey and Pedersen2015; Haydon et al. Reference Haydon, Cleaveland, Taylor and Laurenson2002; Roberts and Heesterbeek Reference Roberts and Heesterbeek2020; Viana et al. Reference Viana, Mancy, Biek, Cleaveland, Cross, Lloyd-Smith and Haydon2014).
Parasites are not directly transmitted from deer to cattle or from wild boars to sheep. A wildlife-derived egg must reach a suitable wet environment, hatch successfully, encounter a compatible lymnaeid, complete intramolluscan development, produce non-bifurcated gymnocephalous cercariae, encyst as metacercariae, and be ingested by a susceptible target host. This sequence explains why confirming infection in a definitive host is biologically important but still insufficient to assign a reservoir role to the host.
Figure 1 illustrates the indirect life cycle of Fasciola and the ecological filters that determine whether wildlife infections contribute to transmission in a defined target population. Therefore, the central question of this study is not simply which wildlife species can harbour Fasciola, but when infection in a wildlife population becomes sufficiently connected to the snail-mediated cycle to contribute to parasite persistence or infection of a defined target population. We addressed this question by linking diagnostic evidence, definitive host competence, environmental contamination, intermediate host amplification, ecological connectivity, and intervention into one reservoir-attribution framework.
Conceptual scope and definitions
This article is framed as a critical perspective or opinion synthesis rather than a systematic review. Its purpose is not to calculate pooled prevalence or provide an exhaustive inventory of wildlife hosts, but to use selected empirical and conceptual literature to ask what evidence is required before wildlife infection can be interpreted as a reservoir contribution. This argument integrates established reservoir theory with Fasciola-specific studies of definitive hosts, lymnaeids, molecular epidemiology, environmental connectivity, and intervention (Buhnerkempe et al. Reference Buhnerkempe, Roberts, Dobson, Heesterbeek, Hudson and Lloyd-Smith2015; Haydon et al. Reference Haydon, Cleaveland, Taylor and Laurenson2002; Viana et al. Reference Viana, Mancy, Biek, Cleaveland, Cross, Lloyd-Smith and Haydon2014; Webster et al. Reference Webster, Borlase and Rudge2017).
The literature cited below was selected to illustrate the successive transmission steps and, where possible, to contrast cases in which apparently similar host positivity led to different epidemiological interpretations. This narrative approach is intended to support a mechanistic argument rather than an exhaustive evidence enumeration exercise.
The framework follows a causal sequence: definitive host infection can generate egg contamination; contamination can become transmission only where compatible snails are ecologically present and support parasite amplification; and infection of a target population requires spatial or temporal connectivity to the resulting metacercarial exposure. Therefore, the reservoir status depends on the coupling of these processes, not on any single one of them (Fenton et al. Reference Fenton, Streicker, Petchey and Pedersen2015; Haydon et al. Reference Haydon, Cleaveland, Taylor and Laurenson2002; Viana et al. Reference Viana, Mancy, Biek, Cleaveland, Cross, Lloyd-Smith and Haydon2014). Similar logic is used in other multi-host systems, in which a host may be infected yet function mainly as a recipient, amplifier, bridge/interface population, or part of a maintenance community, depending on the target and ecological network (Caron et al. Reference Caron, Cappelle, Cumming, de Garine-Wichatitsky and Gaidet2015; Roberts and Heesterbeek Reference Roberts and Heesterbeek2020).
Three terms are used as analytical aids rather than as validated classifications. The wildlife reservoir paradox describes a mismatch between measured infections in wildlife and the realised contribution to a defined transmission system. The interface connector describes a population whose behaviour or movement increases ecological coupling among contamination sites, snail habitats, and target hosts; it does not replace the established concept of a bridge host (Caron et al. Reference Caron, Cappelle, Cumming, de Garine-Wichatitsky and Gaidet2015). Fasciola Interface Management (FIM) is a hypothesis-generating management workflow that converts these concepts into explicit questions for surveillance and intervention and must be prospectively tested.
From host positivity to reservoir contribution
Diagnostic states and the maximum inference they support
The diagnostic endpoint sets an upper limit for epidemiological inference. Seropositivity supports previous or current exposure but does not prove a present patent infection (Arias et al. Reference Arias, Pineiro, Sanchez-Andrade, Suarez, Hillyer, Diez-Banos, Paz-Silva and Morrondo2013; Djemai et al. Reference Djemai, Ayadi, Boubezari, Djafar and Mekroud2024). Detection of parasite DNA confirms DNA in the sampled matrix and can improve species assignment, but does not establish viability, adult establishment, or egg shedding (Abbas et al. Reference Abbas, Kozel, Daramola, Selemetas, Ali, Ashraf, Ibrahim, Deza-Cruz, Fingerhood, Robinson, Morgan, Chaudhry and Betson2026; Arifin et al. Reference Arifin, Höglund and Novobilský2016). Coproantigen detection is stronger evidence of current parasite activity and can become positive before faecal eggs are detectable; consequently, a positive coproantigen result does not always correspond to a patent egg-shedding infection (Calvani et al. Reference Calvani, George, Windsor, Bush and Šlapeta2018; French et al. Reference French, Zadoks, Skuce, Mitchell, Gordon-Gibbs, Craine, Shaw, Gibb and Taggart2016). Adult fluke recovery confirms definitive host infection, whereas confirmed Fasciola eggs establish patency and immediate potential for environmental contamination (Bosco et al. Reference Bosco, Ciuca, Maurelli, Vitiello, Cringoli, Prada and Rinaldi2023; González Hidalgo et al. Reference González Hidalgo, Diez Baños, Hidalgo Argüello and Martínez-Delgado2025; Reigate et al. Reference Reigate, Williams, Denwood, Morphew, Thomas and Brophy2021). None of these endpoints alone demonstrates successful infection of local snails or transmission to the target population.
Roe deer in north-western Spain provide a useful example of why these states must be separated from each other. Approximately 29% of 301 roe deer were seropositive for F. hepatica, but neither faecal eggs nor adult flukes were detected; in the sympatric cattle, 23% shed eggs and 67% were seropositive (Arias et al. Reference Arias, Pineiro, Sanchez-Andrade, Suarez, Hillyer, Diez-Banos, Paz-Silva and Morrondo2013). In that study, wildlife positivity demonstrated exposure within an endemic landscape but did not provide equivalent evidence of a patent wildlife contribution to the local cycle.
In contrast, studies recovering adult flukes or repeatedly detecting eggs in red deer, fallow deer, and wild boars establish definitive host competence and contamination potential more directly (French et al. Reference French, Zadoks, Skuce, Mitchell, Gordon-Gibbs, Craine, Shaw, Gibb and Taggart2016; González Hidalgo et al. Reference González Hidalgo, Diez Baños, Hidalgo Argüello and Martínez-Delgado2025; Lamb et al. Reference Lamb, Doyle, Barwick, Chambers and Kahn2021; Mezo et al. Reference Mezo, Gonzalez-Warleta, Castro-Hermida, Manga-Gonzalez, Peixoto, Mas-Coma and Valero2013). However, the reservoir question remains open until contamination is connected to competent intermediate hosts and a defined target population (Table 1).
Diagnostic evidence and maximum epidemiological inference in wildlife fasciolosis
Table 1. Long description
The table consists of four columns: Evidence, Directly supports, Does not establish alone, and Supporting references.
* Seropositivity: Directly supports previous or current exposure; does not establish current patent infection or maintenance.
* Parasite D N A: Directly supports presence of parasite D N A in the sampled matrix and potentially species identity; does not establish viability, patency, maintenance, or transmission direction.
* Coproantigen: Directly supports current parasite antigen and can precede egg shedding; does not establish patency, egg shedding, maintenance, or source direction.
* Adult flukes: Directly supports current definitive-host infection; does not establish population-level transmission contribution.
* Confirmed eggs: Directly supports patent infection and contamination potential; does not establish successful snail infection or maintenance.
* Repeated egg shedding: Directly supports sustained contamination potential; does not establish population maintenance.
* Compatible lymnaeids: Directly supports biological transmission potential; does not establish active local transmission.
* Naturally infected lymnaeids: Directly supports active intermediate-host component; does not establish dominant definitive-host source.
* Shared parasite genotypes: Directly supports epidemiological connectivity; does not establish directionality.
* Host-snail molecular or ecological linkage: Directly supports strong evidence of a functional transmission cycle; does not establish exclusive maintenance by one host population.
* Persistence after reduction of an alternative source: Directly supports evidence towards maintenance attribution; does not establish generality outside that system.
From detection to epidemiological contribution: ecological filters and wildlife reservoir attribution in Fasciola transmission. Conceptual framework distinguishing parasite detection from effective transmission. The detection hierarchy progresses from seropositivity and parasite DNA to adult flukes and egg shedding. Two ecological filters determine transmission: the definitive-host filter, which governs patent infection and environmental egg deposition, and the snail-mediated filter, which requires compatible lymnaeid snails and successful development to infective metacercariae. Wildlife may act as incidental hosts, amplification contributors, interface connectors, or maintenance components. The wildlife reservoir paradox illustrates that a high prevalence of infection does not necessarily imply substantial transmission, whereas a lower prevalence may coincide with a greater epidemiological contribution when ecological connectivity is favourable.
Figure 1. Long description
The flowchart is divided into three main horizontal sections.
Top Section: Three numbered panels.
1. Detection Hierarchy: A vertical scale from More Indirect to More Direct. It lists Seropositivity (Y-shaped antibody), Parasite D N A (double helix), Adult flukes (leaf-shaped parasite), and Egg shedding (oval egg).
2. Definitive-Host Filter: A linear progression showing a deer ingesting metacercariae, excystment in the liver, maturation in bile ducts, egg output, and fecal deposition near water. A central box notes Competence is not contribution.
3. Snail-Mediated Filter: A vertical cycle showing a lymnaeid snail, miracidia infection, intramolluscan development, and the release of gymnocephalous cercariae that encyst on vegetation to be eaten by a deer.
Middle Section: Epidemiological outcome categories.
- Incidental Host: A deer with an X, indicating low habitat overlap.
- Amplifying Contributor: A group of deer providing abundant eggs in wetlands.
- Interface Connector: A deer moving between landscapes, seeding new areas.
- Maintenance Component: A snail and deer in a red circle, sustaining transmission during lean periods.
Bottom Section: Wildlife Reservoir Paradox.
- Scenario 1: High detected infection but weak contribution due to low abundance or limited wetland access. It shows many red-dotted circles but few cercariae.
- Scenario 2: Lower detected infection but stronger contribution due to high abundance and frequent fecal deposition near water. It shows fewer red-dotted circles but a high density of cercariae and snails.
A legend at the bottom right defines symbols for infection status, snails, cercaria, and metacercaria.
Context-dependent wildlife functions in Fasciola transmission. Wildlife populations may act as incidental hosts, amplifying contributors, interface connectors, or maintenance components, depending on patent infection, egg shedding, population abundance, habitat use, and connectivity with suitable lymnaeid snail habitats. Incidental hosts may provide sentinel information without substantially contributing to transmission, whereas maintenance components support parasite persistence across hosts and seasons. These functions represent context-dependent population-level roles, rather than permanent species attributes.
Figure 2. Long description
At the center of the four panels is a circular text box stating that functions apply to populations within landscapes, not permanently to species.
* Panel 1, Top-Left: Incidental Host. Shows a single deer in an upland dry habitat. A dashed line with an X indicates a lack of connection to a productive wet habitat. Icons show little delivery of feces to water and poor connection to snail habitats, resulting in low transmission contribution. A banner notes they may act as a sentinel.
* Panel 2, Top-Right: Amplifying Contributor. Shows a high density of deer at a water's edge. Blue arrows point from fecal piles and deer to the water containing snails. Text indicates many eggs are shed into shared wet habitats and high host density sustains egg input, increasing contamination pressure.
* Panel 3, Bottom-Left: Interface Connector. Shows a deer moving between three zones: Upland Forest, Wetland, and Pasture/Farmland. Curved green, blue, and brown arrows illustrate movement and ecological coupling that enables transmission across the landscape.
* Panel 4, Bottom-Right: Maintenance Component. A circular flow diagram shows wildlife, livestock, and snails (intermediate hosts) linked by fecal deposition and egg shedding. A side panel titled Seasonal Persistence shows the cycle continuing through Spring, Summer, Autumn, and Winter.
A legend at the bottom defines symbols for Fasciola eggs, fecal deposition, Lymnaeid snail, wildlife host, livestock, movement/connectivity, and transmission contribution. A final box notes that sentinel value does not equal reservoir value.
Hydrological connectivity and environmental reorganisation of Fasciola transmission interfaces. (A) Connected streams, wetlands, and drainage networks may link wildlife-derived egg contamination, suitable lymnaeid snail habitats, and livestock exposure without direct contact between definitive hosts. Successful transmission requires viable eggs, compatible snails, cercarial development, and subsequent exposure to infective metacercariae. (B) Environmental changes may create, intensify, displace, or disrupt transmission interfaces. Water infrastructure may establish new transmission sites; habitat contraction may concentrate hosts around remaining wetlands; climate and land-use changes may shift transmission hotspots spatially or seasonally; and loss of suitable habitats or hydrological connectivity may interrupt transmission. These processes are context-dependent and may occur simultaneously.
Figure 3. Long description
Panel A, titled Hydrological connectivity, shows a continuous landscape divided into four zones from left to right. 1. Upland or forest with headwaters and springs featuring a deer wildlife host and Fasciola eggs. 2. Stream network with hydrological linkage featuring Lymnaeid snails. 3. Wetland or floodplain with snail-mediated amplification. 4. Pasture or farm downstream interface featuring a cow livestock host. Blue solid arrows indicate surface water flow, and dashed arrows indicate subsurface flow, connecting these zones via wet nodes. A Transmission pathway flowchart at the bottom right shows the cycle: Egg contamination to Miracidial hatching to Lymnaeid infection to Cercarial emergence to Metacercarial encystment on grass to Definitive-host exposure. Panel B, titled Environmental change reorganizes interfaces, contains four numbered boxes. 1. Creation: Shows an irrigation canal creating new transmission nodes. 2. Intensification: Shows habitat contraction concentrating hosts at remaining wet sites. 3. Displacement: Shows climate or land-use change shifting hotspots due to seasonal conditions like rain or sun. 4. Disruption: Shows human intervention like construction machinery causing loss of hydrological connectivity. A legend at the bottom defines green solid arrows as Connected, orange dashed arrows as Weakened, and red dashed arrows as Disconnected.
Reservoir status is target- and system-dependent
A reservoir need not be the most abundant or heavily infected host species. A population with long-lived patent infections that only occasionally contaminates productive snail habitats could still be a reservoir component if those events are sufficient to maintain the parasite or repeatedly seed infection in the defined target population. Conversely, a high prevalence may have little downstream effect where faecal deposition is spatially disconnected from competent lymnaeids. Therefore, the appropriate question is not whether a species is intrinsically a reservoir but whether a population contributes materially to the maintenance or source transmission within a defined ecological system (Haydon et al. Reference Haydon, Cleaveland, Taylor and Laurenson2002; Roberts and Heesterbeek Reference Roberts and Heesterbeek2020; Viana et al. Reference Viana, Mancy, Biek, Cleaveland, Cross, Lloyd-Smith and Haydon2014).
For Fasciola, this contribution depends jointly on host competence, duration and intensity of egg shedding, population abundance, where faeces are deposited, the presence and productivity of compatible snails, and exposure of the target population. A rare contamination event may be significant if it enters a highly productive snail focus, whereas frequent egg shedding may be insignificant if it occurs where the intermediate host phase cannot operate. This explicitly allows for situations in which long-lived flukes and occasional contamination are epidemiologically important, while requiring that their contribution be demonstrated in relation to maintenance or transmission, rather than inferred from host longevity alone.
The wildlife reservoir paradox in practice: contrasting host examples
We used the wildlife reservoir paradox for the mismatch between the magnitude of infection measured in a host population and its realised contribution to transmission. This concept does not imply that low-prevalence hosts are unimportant; rather, it directs attention to the processes that convert infection into parasite output, snail amplification, and target host exposure.
Llama and domestic pig studies from Andean endemic areas provide a deliberately contrasting pair. Llamas can experimentally support the complete F. hepatica life cycle; however, lower infection intensities and egg outputs, reduced downstream cercarial production, and characteristic dung-pile behaviour away from freshwater led Mas-Coma, Cafrune et al. (Reference Mas-Coma, Cafrune, Funatsu, Mangold, Angles, Buchon, Fantozzi, Artigas, Valero and Bargues2021) to conclude that their reservoir contribution in the studied Andean systems was negligible and did not warrant control priority. In contrast, experimental and field data showed that domestic pigs could sustain the complete cycle, shed substantial numbers of eggs, and participate in transmission; therefore, pigs were considered a genuine reservoir requiring inclusion in One Health control, although with lower priority than sheep and cattle, as concluded by Mas-Coma, Funatsu et al. (Reference Mas-Coma, Funatsu, Angles, Buchon, Mas-Bargues, Artigas, Valero and Bargues2021). These studies show that the same evidence categories – host competence, egg output, snail compatibility, behaviour, and field context – can lead to opposite conclusions regarding the identity of the reservoir.
The practical consequence is that prevalence alone is not an epidemiological quantity of interest. Reservoir attribution should ask how often an infection becomes patent, how much contamination is produced, where that contamination is deposited, whether productive lymnaeids amplify it, and whether a specified target population is subsequently exposed.
Context-dependent wildlife functions: examples that clarify the categories
For interpretation, wildlife populations can be provisionally classified into four context-dependent functions: incidental hosts, amplifying contributors, interface connectors, and maintenance components (Table 2). These are not permanent species attributes. The same species may occupy different functions in different landscapes, seasons, or target-host systems as abundance, behaviour, snail distribution, and livestock contribution change.
Context-dependent wildlife functions in Fasciola transmission with empirical examples
Table 2. Long description
The table consists of three columns titled Function, Definition, and Interpretation, with four rows of data.
* Row 1: Function is Incidental host. Definition is Becomes exposed or infected but contributes little demonstrably to onward transmission. Interpretation is May remain valuable as a sentinel; roe deer example: Arias et al. 2013.
* Row 2: Function is Amplifying contributor. Definition is Adds meaningful environmental contamination without demonstrated necessity for persistence. Interpretation is Can increase transmission pressure; wild boar example: Mezo et al. 2013.
* Row 3: Function is Interface connector. Definition is Behaviour or movement increases ecological coupling among transmission compartments. Interpretation is Can connect contamination with snail amplification; coypu hypothesis: Issia et al. 2009.
* Row 4: Function is Maintenance component. Definition is Continuing contribution materially supports local parasite persistence. Interpretation is Part of a maintenance community; Hokkaido deer-snail-cattle system: Ichikawa-Seki et al. 2017; Ohari et al. 2017.
A note below the table states that these functions apply to populations within defined ecological settings and are not permanent species attributes, and that interface connector is a descriptive term, not a replacement for bridge host.
Note. These functions apply to populations within defined ecological settings and should not be interpreted as permanent species attributes. The term ‘interface connector’ is used descriptively and does not replace the established epidemiological definition of a bridge host.
Figure 2 illustrates the four context-dependent wildlife functions, emphasising that reservoir contributions depend on host competence, environmental contamination, and ecological connectivity, rather than species identity alone.
Incidental hosts and sentinels
An incidental host becomes exposed or infected but makes little demonstrable contribution to onward transmission. Roe deer in the study by Arias et al. (Reference Arias, Pineiro, Sanchez-Andrade, Suarez, Hillyer, Diez-Banos, Paz-Silva and Morrondo2013) approximated this function because exposure was evident, but patency was not demonstrated. Such populations can still be valuable sentinels of environmental exposure; therefore, sentinel value should not be confused with reservoir value.
For surveillance, the question should be specified in advance: is the objective to detect exposure, confirm infection, identify contaminating definitive hosts, or attribute maintenance? This distinction is especially important in wildlife, where capture and necropsy data are often sparse (WOAH and IUCN 2024).
Amplifying contributors
An amplifying contributor is a patent host that adds meaningful environmental contamination without evidence that it is independently required for persistence. Wild boars in Galicia provide a concrete example; Mezo et al. (Reference Mezo, Gonzalez-Warleta, Castro-Hermida, Manga-Gonzalez, Peixoto, Mas-Coma and Valero2013) demonstrated F. hepatica infection and interpreted wild boars as a secondary reservoir. The qualifier secondary is epidemiologically useful because it supports meaningful participation without claiming that transmission would collapse if wild boars were removed.
Abundance can change this contribution without changing prevalence. If a wildlife population expands while the prevalence remains stable, the absolute number of egg-shedding individuals can increase markedly, increasing environmental input. This is one reason why prevalence should be interpreted together with host density, patency, and contamination patterns.
Interface connectors
An interface connector is a population whose movement or behaviour increases the ecological coupling among terrestrial hosts, wet habitats, and lymnaeid populations. It is a descriptive function rather than a synonym for a bridge host (Caron et al. Reference Caron, Cappelle, Cumming, de Garine-Wichatitsky and Gaidet2015). Semi-aquatic mammals are particularly plausible candidates because their behaviour can cause faecal contamination close to the snail habitat.
In two Patagonian reserves, all nine sampled coypus had patent infections and faecal egg counts comparable to those reported in sheep, whereas patent infections were rare in guanacos (Issia et al. Reference Issia, Pietrokovsky, Sousa-Figueiredo, Stothard and Wisnivesky-Colli2009). The small coypu sample precludes a precise population prevalence estimate; however, the combination of patency, egg shedding, and semi-aquatic habitat use makes the species a strong interface hypothesis. No infection was detected in the 175 lymnaeids sampled at that time; therefore, the wildlife-to-snail link remained unresolved.
Capybaras present a similar testable hypothesis. Severe fascioliasis and molecularly confirmed patent infections have been reported in Brazil (Labruna et al. Reference Labruna, Costa, Port-Carvalho, Oliveira, Souza and Castro2018; Martins et al. Reference Martins, Failla, Martins, Uzai, Viana, Gomes and Verocai2021). These observations establish definitive host competence and contamination potential but do not demonstrate population-level maintenance or contemporaneous linkage with naturally infected local lymnaeids.
Maintenance components: what stronger evidence looks like
A maintenance component is a population whose continuing contribution materially supports parasite persistence in a local transmission community. The term component is intentional because persistence may emerge collectively from several host populations rather than from one independently sufficient reservoir species (Fenton et al. Reference Fenton, Streicker, Petchey and Pedersen2015; Haydon et al. Reference Haydon, Cleaveland, Taylor and Laurenson2002; Roberts and Heesterbeek Reference Roberts and Heesterbeek2020).
Hokkaido provides stronger evidence of a connected wildlife transmission system. Fasciola larvae were detected in naturally infected Galba truncatula, and the parasite haplotype was consistent with that previously recovered from adult flukes in wild sika deer (Ohari et al. Reference Ohari, Hayashi, Mohanta, Kuwahara and Itagaki2017). A separate comparison of 105 Fasciola from sika deer and 96 from cattle found almost identical molecular characteristics, supporting the shared transmission between the two host populations (Ichikawa-Seki et al. Reference Ichikawa-Seki, Shiroma, Kariya, Nakao, Ohari, Hayashi and Fukumoto2017). These studies link wildlife, naturally infected snails, and livestock more strongly than host prevalence alone, although they still do not establish which definitive host is the dominant source of infection.
This distinction is central to reservoir attribution: evidence becomes progressively stronger when patent wildlife infection is linked to naturally infected intermediate hosts and infection in the target population, but direction and necessity still require longitudinal or intervention evidence.
The lymnaeid bottleneck: low field prevalence can still sustain transmission
The intermediate host phase is obligatory; however, natural snail infection prevalence should not be treated as a simple measure of transmission intensity. Lymnaeids may be experimentally susceptible, but rarely participate in a particular field system. Conversely, a very low natural infection prevalence can sustain intense transmission because each successfully infected snail can amplify the parasite asexually and release many cercariae. Thus, the relevant field variables are not only the proportion of infected snails but also snail abundance, infection intensity, cercarial production, habitat persistence, and contact with definitive host contamination.
Ngcamphalala et al. (Reference Ngcamphalala, Nyagura, Malatji and Mukaratirwa2025) reported substantially higher pooled infection rates in experimentally exposed lymnaeids (50%) than in field-collected snails (6%), although these estimates reflect different exposure conditions and considerable heterogeneity. Nevertheless, a low prevalence of natural snail infection does not necessarily indicate negligible transmission. In western Cuba, Vazquez et al. (Reference Vázquez, Sánchez, Alba, Pointier and Hurtrez-Boussès2015) recorded a mean F. hepatica prevalence of 2.94% in Galba cubensis, suggesting that transmission can occur even at low infection levels. Similarly, Loan et al. (Reference Loan, Nguyen, Bich, Vi and Matsumoto2025) reported Fasciola infection rates of 2.0–3.7% in Lymnaea viridis in the Mekong Delta, compared with cattle prevalences of 32.9–35.7%. Djemai et al. (Reference Djemai, Ayadi, Boubezari, Djafar and Mekroud2024) also documented a high prevalence of F. hepatica (61.06%) in a selected sample of grazing cattle. Together, these findings suggest that intramolluscan amplification, snail abundance, and repeated host exposure may sustain transmission despite a low observed prevalence of snail infection, although direct quantitative relationships between snails and definitive-host infection remain insufficiently established.
Snail identity, compatibility, and local ecology
Lymnaeid taxonomy matters because parasite-snail compatibility, habitat use, and geographical distribution differ among lineages. Therefore, the simplification of F. hepatica transmission to Galba and F. gigantica transmission to Radix is inadequate. Galba, Pseudosuccinea, Radix, Austropeplea, and Orientogalba lineages are important in different regions, and both F. hepatica and F. gigantica may use different members of these groups depending on local compatibility and biogeography (Vázquez et al. Reference Vázquez, Alda, Lounnas, Sabourin, Alba, Pointier and Hurtrez-Boussès2018, Reference Vázquez, Alba, Alda, Vittecoq, Chapuis, Faugère, Pointier, Hurtrez-Boussès, Vinarski and Vázquez2023). Ecological differences are equally important: amphibious lymnaeids occupying mud margins, seepages, and temporary wet habitats create a different contamination interface than more permanently aquatic forms in ponds, channels, or rice-associated systems.
Therefore, field participation depends on more than just temperature and broad land cover layers. Relevant variables include pH, conductivity, hardness, dissolved ions and nutrients, substrate and soil characteristics, hydroperiod, vegetation, water depth, snail abundance, co-occurring predators or competitors, and the spatial relationship between these microhabitats and definitive host faecal deposition. Recent fieldwork has shown strong associations between lymnaeid occurrence and physicochemical properties such as pH, total hardness, calcium, and dissolved solids (Neira et al. Reference Neira, Sierra, Godoy, Logarzo, Gonzalez and Scarcella2025), while a study of temporary wetlands found that macroinvertebrate assemblages and habitat characteristics were associated with lymnaeid abundance (Kafula et al. Reference Kafula, Mgelwa, Mtemi and Mnyoro2026). Cercarial production should also be interpreted as an outcome of a specific parasite-snail combination rather than a fixed species trait (Vázquez et al. Reference Vázquez, Alda, Lounnas, Sabourin, Alba, Pointier and Hurtrez-Boussès2018, Reference Vázquez, Alba, Alda, Vittecoq, Chapuis, Faugère, Pointier, Hurtrez-Boussès, Vinarski and Vázquez2023). Consequently, ecological niche models based mainly on macroclimate or coarse land use should be treated as suitability layers, not direct evidence of active transmission.
Reconstructing wildlife-livestock transmission interfaces
Environmental DNA (eDNA) is useful in reservoir studies when it helps locate the environmental stage that links a suspected definitive host to the rest of the cycle. Jones et al. (Reference Jones, Brophy, Davis, Davies, Emberson, Rees Stevens and Williams2018) detected G. truncatula eDNA in all visually positive habitats and additional sites where conventional searches failed, whereas F. hepatica eDNA was detected in several water bodies.
Rathinasamy et al. (Reference Rathinasamy, Hosking, Tran, Kelley, Williamson, Swan, Elliott, Rawlin, Beddoe and Spithill2018) developed a multiplex qPCR for F. hepatica and Austropeplea tomentosa in environmental water. These tools can identify candidate transmission nodes for targeted wildlife, livestock, and snail sampling.
However, their reservoir inference is limited. Detection of Fasciola eDNA does not identify the definitive host that deposited the eggs, demonstrate parasite viability, quantify metacercarial production, or establish the direction of transmission. Snail eDNA indicates habitat occupancy rather than infection. Temporal detection also varies according to local conditions, such as temperature, pH, and rainfall (Jones et al. Reference Jones, Davis, Jones, Tyson, Davies, Cutress, Brophy, Rose, Williams and Williams2021). Therefore, eDNA should be used as a complementary spatial screening tool in reservoir investigations, followed by conventional malacology, parasite detection in snails, and host-specific evidence where source attribution is required.
Hydrological and temporal connectivity
Wildlife and livestock do not need to meet for Fasciola transmission to connect them. A wildlife host may deposit eggs at an upstream wet site, snails may amplify the parasite locally or along connected water margins, and livestock may later encounter metacercariae at another site in the same functional-drainage network. This is a hypothesis regarding environmentally mediated connectivity, not direct host-to-host transmission.
Figure 3A illustrates how hydrological connectivity may link wildlife, compatible lymnaeid snails, and livestock through shared transmission habitats without direct contact between definitive hosts. Hurtrez-Boussès et al. (Reference Hurtrez-Boussès, Hurtrez, Turpin, Durand, Durand, De Meeüs, Meunier and Renaud2010) should be interpreted narrowly in this context. Their study showed that genetic differentiation among G. truncatula populations was more closely related to distance along a hydrographic network than to Euclidean separation. It did not demonstrate a wildlife-to-livestock parasite flow. Its relevance is that drainage structures can organise the intermediate host populations through which transmission must pass, providing a rationale for testing catchment-based rather than straight-line connectivity in future Fasciola source-attribution studies.
Therefore, source attribution should focus on transmission nodes, such as springs, seepages, drainage channels, wet pasture depressions, irrigation ditches, pond margins, marshes, and floodplain edges, where host faeces, competent snails, and later target-host exposure can converge (Bennema et al. Reference Bennema, Ducheyne, Vercruysse, Claerebout, Hendrickx and Charlier2011; Charlier et al. Reference Charlier, Bennema, Caron, Counotte, Ducheyne, Hendrickx and Vercruysse2011). These components may also be separated in time; for example, wildlife can contaminate a wet site during one period, snail amplification can occur later, and livestock can encounter metacercariae weeks later. Consequently, longitudinal sampling is more informative than a single cross-sectional survey.
Molecular epidemiology: connectivity is not directionality
Molecular tools can identify parasite relationships among host populations; however, genetic similarity usually establishes connectivity more readily than direction. In Hokkaido, the near-identical characteristics of Fasciola from sika deer and cattle support a shared transmission system; however, the data remain compatible with deer-to-environment-to-cattle transmission, cattle-to-environment-to-deer transmission, or exposure of both hosts from the same environmental source (Ichikawa-Seki et al. Reference Ichikawa-Seki, Shiroma, Kariya, Nakao, Ohari, Hayashi and Fukumoto2017).
The Camargue study by Vázquez et al. (Reference Vázquez, Sabourin, Alda, Leroy, Leray, Carron, Mulero, Caty, Hasfia, Boisseau, Saugné, Pineau, Blanchon, Alba, Faugère, Vittecoq and Hurtrez-Boussès2021) illustrates more precisely what population genetics can add. Cattle had a higher prevalence than nutria and wild boar, and microsatellite analysis showed differentiation among host-associated parasite populations but also inferred migrants, particularly between untreated cattle and nutria. This supports the epidemiological connection and possible parasite exchange between domestic and wildlife hosts, while still not identifying the direction of individual transmission events. The same study also showed that the introduced G. truncatula and Pseudosuccinea columella could be highly compatible intermediate hosts, reinforcing the value of integrating parasite genetics with local snail ecology.
Accordingly, source-attribution studies should link parasite genotypes with definitive host identity, intermediate host infection, place, time, movement, and hydrological context. A sequence or multilocus genotype becomes epidemiologically informative when embedded in a plausible ecological pathway; molecular similarity alone should not be considered proof of a reservoir-to-target route.
Species-level molecular identification remains important where F. hepatica, F. gigantica, or introgressed forms coexist. However, taxonomic resolution and source attribution are separate issues. Recent qPCR and amplicon sequencing approaches have improved species discrimination (Abbas et al. Reference Abbas, Kozel, Daramola, Selemetas, Ali, Ashraf, Ibrahim, Deza-Cruz, Fingerhood, Robinson, Morgan, Chaudhry and Betson2026), whereas inferring who infected whom still requires ecological and temporal evidence.
Environmental and managed-landscape modifiers
Climate, land use, irrigation, and animal movement can change the contribution of reservoirs by reorganising the interfaces between egg contamination, snail habitats, and target-host exposure; they should not be treated as uniform risk multipliers. Temperature and moisture affect egg development, snail survival, and intramolluscan development, whereas drainage, irrigation, stocking, wildlife access, and livestock movement can create, intensify, relocate, or disrupt productive transmission sites (Alba et al. Reference Alba, Vazquez and Hurtrez-Boussès2021; Caminade et al. Reference Caminade, van Dijk, Baylis and Williams2015; Sabourin et al. Reference Sabourin, Alda, Vazquez, Hurtrez-Boussès and Vittecoq2018). Therefore, the relevant question for wildlife attribution is whether environmental change increases or decreases the coupling between a contaminating wildlife population and productive snail habitat, not simply whether climatic suitability increases.
Figure 3B illustrates how environmental and anthropogenic changes may create, intensify, displace, or disrupt Fasciola transmission interfaces. This interpretation is consistent with recent observations of geographically heterogeneous changes in fascioliasis suitability and range, including high-altitude and high-latitude settings (Cuervo et al. Reference Cuervo, Bargues, Artigas, Buchon, Angles and Mas-Coma2024, Reference Cuervo, Bargues, Artigas, Buchon, Angles and Mas-Coma2025). At finer scales, artificial ponds, irrigation channels, seepages, and poorly drained enclosures can create local transmission opportunities that are weakly predicted by the regional climate alone.
Managed wildlife settings provide useful natural experiments because host numbers, movements, water systems, treatment, and enclosure boundaries can be documented. In captive impalas in Zambia, F. gigantica eggs were detected in 46% of examined faecal samples; snails from an implicated fountain shed gymnocephalous cercariae, and combined triclabendazole treatment, vegetation removal, drainage, and temporary drying were followed by the absence of detectable eggs three weeks later (Phiri et al. Reference Phiri, Mudenda, Luwe and Phiri2017). Because host and environmental interventions were simultaneous, their individual effects could not be separated; however, the study demonstrates the practical value of acting on both the contaminating host and the suspected snail habitat.
The same logic applies to translocations. Moving infected wildlife or managed animals introduces parasite stages, but establishment at the destination additionally requires patent shedding, a suitable wet habitat, compatible lymnaeids, successful intramolluscan development, and exposure to another definitive host. Pre-movement assessment should therefore consider both infection status and ecological opportunity for establishment rather than treating a positive animal as equivalent to an established transmission cycle (WOAH and IUCN 2024).
Fasciola Interface Management (FIM): an operational workflow
The FIM is intended to make reservoir attribution actionable without turning wildlife positivity into an automatic control target. The workflow uses five sequential gates (Table 3; Figure 4). Progress to the next gate requires evidence that the preceding link is plausible; failure at a gate redirects surveillance towards missing evidence rather than towards more intensive host removal.
Operational Fasciola Interface Management (FIM) decision workflow
Table 3. Long description
The table consists of five columns and five rows representing sequential gates in a decision workflow.
* Gate 1. Contamination. Question: Which population contributes environmentally relevant eggs? Minimum evidence: Patency or adult infection plus repeated egg shedding, abundance, and faecal deposition near candidate wet habitat. If evidence is insufficient: Do not target the host; obtain stronger host and contamination data. Action when supported: Define candidate contaminating populations.
* Gate 2. Snail amplification. Question: Where can eggs enter a productive intermediate-host cycle? Minimum evidence: Correct lymnaeid identity plus local habitat; preferably natural infection, shedding, or repeated parasite evidence. If evidence is insufficient: Map or snail-sample the site; use e D N A only as complementary screening. Action when supported: Define confirmed or high-priority amplification nodes.
* Gate 3. Connectivity. Question: Is the suspected source linked to the target population through the same node? Minimum evidence: Movement, grazing or water use, hydrology, timing and/or genetics consistent with a shared pathway. If evidence is insufficient: Do not infer source direction; test alternative nodes or hosts. Action when supported: Prioritise plausible source-target interfaces.
* Gate 4. Intervention. Question: Where can parasite flow be interrupted with least ecological disruption? Minimum evidence: Feasible action at a supported link, considering welfare, conservation, drug efficacy, and environmental effects. If evidence is insufficient: Avoid broad wildlife removal or indiscriminate habitat treatment. Action when supported: Treat, exclude, or modify the validated link.
* Gate 5. Reassessment. Question: Did transmission decline after intervention? Minimum evidence: Repeated host, snail and environmental indicators comparable with baseline. If evidence is insufficient: Revisit source attribution, drug efficacy, untreated hosts and overlooked nodes. Action when supported: Maintain, modify or stop the intervention.
Operational Fasciola Interface Management (FIM) workflow. Reservoir attribution and intervention proceed through five decision gates. Gate 1 asks whether a wildlife, livestock, or managed population contributes environmentally relevant eggs, using evidence such as patency, repeated shedding, abundance, and faecal deposition. Gate 2 asks whether those eggs can enter a productive lymnaeid amplification node, requiring correct snail identification and preferably evidence of natural infection or parasite development. Gate 3 tests whether the suspected source and the defined target population are spatially or temporally connected to the same node through habitat use, movement, hydrology, or compatible molecular evidence. Gate 4 selects the least disruptive intervention at a supported link; options may include treatment, temporary exclusion, changes in grazing or movement, or modification of artificial water systems. Gate 5 repeats host, snail, and environmental indicators to determine whether transmission declined. If evidence is insufficient at any gate, the workflow returns to targeted surveillance rather than progressing to wildlife intervention. If transmission fails to decline after intervention, source attribution and alternative transmission nodes are reassessed. FIM is therefore an adaptive, falsifiable workflow rather than a permanent reservoir classification.
Figure 4. Long description
A central landscape illustration of a river with livestock and wildlife is surrounded by five numbered pentagonal gates connected by arrows in a clockwise loop.
Gate 1 at the top is green and titled CONTAMINATION. It includes icons for Patent hosts, Fecal deposition, and Host abundance.
Gate 2 at the top right is blue and titled SNAIL-MEDIATED AMPLIFICATION. It includes icons for Wet microhabitats, Compatible lymnaeids, and e D N A forward slash snail survey.
Gate 3 at the bottom right is teal and titled CONNECTIVITY. It includes icons for Movement paths, Grazing patterns, Hydrology connections, and Seasonality and climate.
Gate 4 at the bottom left is purple and titled INTERVENTION. It includes icons for Treatment, Temporary exclusion, Water management, Quarantine forward slash movement control, and Focused surveillance.
Gate 5 at the top left is orange and titled REASSESSMENT. It includes icons for Longitudinal follow-up, Monitoring hosts, snails, and environment, and Evaluate and adapt.
A dashed outer circle labeled EVIDENCE AND DATA INPUTS surrounds the gates with various diagnostic icons including Fecal egg counts E P G, Coproantigen assays, Snail surveys, e D N A sampling, G P S tracking and movement, Camera traps and remote sensing, Hydrology and rainfall data, Pasture and habitat mapping, Molecular typing, Climate and seasonal indices, Host population surveys, and Serology antibody.
Step 1 – Confirm environmentally relevant contamination
Identify which definitive host populations are capable of contributing eggs to the focal environment. Priority evidence is repeated patency or adult infection, together with egg output, host abundance, duration of shedding, and faecal deposition near potentially suitable snail habitats. Serology or host DNA alone is insufficient for targeted interventions.
Step 2 – Confirm a productive snail-amplification node
Establish which lymnaeid species is present, whether it is locally compatible, and whether natural infection or another strong indicator of active parasite amplification is present. Snail abundance, habitat persistence, parasite-snail compatibility, and cercarial production are more informative than the presence of a lymnaeid taxon alone. eDNA may help locate sites but should trigger, rather than replace, malacological confirmation.
Step 3 – Demonstrate ecological connection to the target population
Movement data, camera traps, grazing records, seasonal water use, hydrological mapping, fencing, and parasite genetics can be used to test whether the suspected contaminating population and the target host are linked to the same amplification site. Direct contact between definitive hosts is not required for transmission. If no credible spatial or temporal connection exists, the wildlife population should not be prioritised as the source for that target.
Step 4 – Intervene at the least disruptive validated link
Potential actions include effective treatment of livestock or managed wildlife, temporary exclusion from focal wet habitats, pre-movement treatment, changes in grazing timing, modification of artificial water systems, and focused management of demonstrably relevant snail habitats. The FIM does not support indiscriminate wildlife treatment or removal, destruction of natural wetlands, or generalised molluscicide use. Increasing triclabendazole resistance further supports the need for measures that reduce reinfection pressure rather than relying on repeated chemotherapy alone (Choi et al. Reference Choi, Rosa, Fernandez-Baca, Ore, Martin, Ortiz, Hoban, Cabada and Mitreva2025; Kelley et al. Reference Kelley, Elliott, Beddoe, Anderson, Skuce and Spithill2016).
Step 5 – Reassess transmission and revise the attribution
After an intervention, repeat host, snail, and environmental measurements using the same indicators that justified the action. A decline in egg shedding, snail infection, or target host reinfection supports the working pathway; no change should trigger reassessment of source attribution, untreated host populations, drug efficacy, or overlooked transmission nodes. In this form, the FIM is a testable learning cycle rather than a reservoir label or one-time risk score.
The resulting decision rule is explicit: do not move from wildlife positivity to wildlife intervention unless the chain from contamination, through snail amplification and ecological connectivity, to the target population is supported strongly enough to justify action. Figure 4 presents this workflow as a sequence of decision gates with feedback after reassessment.
Research priorities: tests that can falsify the framework
Future wildlife fascioliasis studies should prioritise source-attribution designs over additional host lists. The first test is whether models combining patency, host abundance, egg output, and connection to productive snail habitats predict downstream transmission better than wildlife prevalence alone. The second is whether semi-aquatic hosts, such as coypus or capybaras, deposit a greater fraction of viable Fasciola eggs within productive snail habitats than comparably infected terrestrial hosts. Both hypotheses can be tested using longitudinal host diagnostics, global positioning system (GPS) or camera data, faecal deposition mapping, and concurrent snail sampling.
The third priority is to directly test hydrological connectivity. Catchment mapping, repeated parasite genotyping, and host movement data can determine whether drainage-network distance explains shared parasite populations or reinfection better than Euclidean distance. The Hurtrez-Boussès et al. (Reference Hurtrez-Boussès, Hurtrez, Turpin, Durand, Durand, De Meeüs, Meunier and Renaud2010) snail population study provides a rationale for this design, but not evidence of wildlife-to-livestock flow.
Finally, reservoir attribution should be challenged by perturbation or prospective management. Suppressing one suspected source of egg input, where ethically and practically feasible, can reveal whether transmission disappears, declines, or persists, and therefore whether maintenance is dominated by livestock, wildlife, or a multihost community. The FIM should be modified or rejected if interface-based predictions fail to outperform simple host-positivity models. Such prospective tests are more informative than the retrospective assignment of permanent reservoir labels. The proposed testable hypotheses, their expected predictions, and the corresponding study designs are summarised in Table 4.
Testable hypotheses generated by the proposed framework
Table 4. Long description
The table consists of three columns: Hypothesis, Prediction, and Informative design.
* Row 1: Reservoir paradox. Prediction: Wildlife prevalence alone will predict downstream transmission less accurately than patent infection plus abundance, egg output, and snail connectivity. Informative design: Longitudinal multihost study.
* Row 2: Semi-aquatic interface. Prediction: Semi-aquatic hosts will deposit a greater proportion of eggs within productive snail habitats than comparable terrestrial hosts. Informative design: G P S / camera, faecal and snail / environmental sampling.
* Row 3: Hydrological connectivity. Prediction: Hydrological distance will explain shared transmission better than Euclidean distance in some landscapes. Informative design: Catchment mapping plus host movement and parasite genetics.
* Row 4: Maintenance attribution. Prediction: Transmission will persist after suppression of livestock egg input where non-livestock populations materially maintain the system. Informative design: Natural experiment or controlled source-reduction study.
* Row 5: Molecular-ecological attribution. Prediction: Genotype plus host, snail, place, and time will resolve pathways better than genotype alone. Informative design: Longitudinal integrated genotyping.
* Row 6: F I M. Prediction: Interface-based models and interventions will explain or reduce reinfection better than host-positive status alone. Informative design: Prospective comparative validation.
Conclusions
Research has established that Fasciola infects a broad range of mammalian hosts, including humans. A more difficult and useful question is which infected populations contribute to the maintenance of, or transmission to, a defined target. Serology supports exposure, DNA supports parasite detection, coproantigen supports current parasite activity, adult flukes confirm definitive host infection, and eggs establish patency and contamination potential. Reservoir attribution also requires the following links: productive lymnaeid amplification and ecological connection to the target population.
Therefore, the wildlife reservoir paradox does not claim that heavily infected wildlife are unimportant or that low prevalence implies low transmission. This highlights the possibility that contributions and measured prevalence can diverge. Llamas and pigs in Andean studies illustrate opposite outcomes, while coypus, capybaras, wild boar, and sika deer show how host behaviour, snail linkage, and molecular connectivity can progressively strengthen or weaken reservoir hypotheses.
The lymnaeid stage is central to this interpretation of the life cycle. Low natural snail prevalence can sustain substantial transmission because of asexual amplification. The identity, ecology, abundance, community context, and compatibility of local lymnaeids determine whether definitive host contamination results in metacercarial exposure. Environmental DNA (eDNA), hydrological mapping, and molecular epidemiology can identify pieces of this pathway; however, none of these methods should be used alone to infer the source direction.
The FIM converts these principles into five operational questions: who contributes environmentally relevant eggs; where does snail amplification occur; which target populations are connected to those sites; where can parasite flow be interrupted with the least ecological disruption; and did transmission decline after intervention? This workflow deliberately makes the reservoir attribution provisional and testable.
The epidemiologically relevant unit in wildlife fascioliasis is therefore not simply the infected species but the connected transmission interface. Progress will depend on measuring and perturbing the interface rather than adding increasingly long lists of positive wildlife hosts.
Acknowledgements
The authors acknowledge the institutional support of Benemérita Universidad Autónoma de Puebla under project VIEP-VIMA-NAT-25-I.
Author contributions
A.V.-M.: Conceptualisation, literature synthesis, visualisation, writing – original draft, writing – review and editing. J.M.R.-R. and S.O.-V.: literature evaluation, validation, and writing – review and editing. All authors have reviewed and approved the final manuscript.
Financial support
This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Competing interests
The authors declare none.
Ethical standards
Not applicable.
Declaration of generative AI and AI-assisted technologies in manuscript preparation process
Generative artificial intelligence tools provided through ChatGPT (OpenAI; accessed July–August 2026) were used during the manuscript development to support language editing, structural refinement, and conceptual visualisation. All scientific interpretations, references, taxonomic assignments, and final wording were critically reviewed and verified by the authors, who take full responsibility for the content of the manuscript.
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