A major locus confers triclabendazole resistance in Fasciola hepatica and shows dominant inheritance.

Fasciola hepatica infection is responsible for substantial economic losses in livestock worldwide and poses a threat to human health in endemic areas. The mainstay of control in livestock and the only drug licenced for use in humans is triclabendazole (TCBZ). TCBZ resistance has been reported on every continent and threatens effective control of fasciolosis in many parts of the world. To date, understanding the genetic mechanisms underlying TCBZ resistance has been limited to studies of candidate genes, based on assumptions of their role in drug action. Taking an alternative approach, we combined a genetic cross with whole-genome sequencing to localise a ~3.2Mbp locus within the 1.2Gbp F. hepatica genome that confers TCBZ resistance. We validated this locus independently using bulk segregant analysis of F. hepatica populations and showed that it is the target of drug selection in the field. We genotyped individual parasites and tracked segregation and reassortment of SNPs to show that TCBZ resistance exhibits Mendelian inheritance and is conferred by a dominant allele. We defined gene content within this locus to pinpoint genes involved in membrane transport, (e.g. ATP-binding cassette family B, ABCB1), transmembrane signalling and signal transduction (e.g. GTP-Ras-adenylyl cyclase and EGF-like protein), DNA/RNA binding and transcriptional regulation (e.g. SANT/Myb-like DNA-binding domain protein) and drug storage and sequestration (e.g. fatty acid binding protein, FABP) as prime candidates for conferring TCBZ resistance. This study constitutes the first experimental cross and genome-wide approach for any heritable trait in F. hepatica and is key to understanding the evolution of drug resistance in Fasciola spp. to inform deployment of efficacious anthelmintic treatments in the field.

Disease control tools to secure animal and public health in a densely populated world.

Animal health is a prerequisite for global health, economic development, food security, food quality, and poverty reduction, while mitigating against climate change and biodiversity loss. We did a qualitative review of 53 infectious diseases in terrestrial animals with data from DISCONTOOLS, a specialist database and prioritisation model focusing on research gaps for improving infectious disease control in animals. Many diseases do not have any appropriate control tools, but the prioritisation model suggests that we should focus international efforts on Nipah virus infection, African swine fever, contagious bovine pleuropneumonia, peste des petits ruminants, sheeppox and goatpox, avian influenza, Rift Valley fever, foot and mouth disease, and bovine tuberculosis, for the greatest impact on the UN's Sustainable Development Goals. Easy to use and accurate diagnostics are available for many animal diseases. However, there is an urgent need for the development of stable and durable diagnostics that can differentiate infected animals from vaccinated animals, to exploit rapid technological advances, and to make diagnostics widely available and affordable. Veterinary vaccines are important for dealing with endemic, new, and emerging diseases. However, fundamental research is needed to improve the convenience of use and duration of immunity, and to establish performant marker vaccines. The largest gap in animal pharmaceuticals is the threat of pathogens developing resistance to available drugs, in particular for bacterial and parasitic (protozoal, helminth, and arthropod) pathogens. We propose and discuss five research priorities for animal health that will help to deliver a sustainable and healthy planet: vaccinology, antimicrobial resistance, climate mitigation and adaptation, digital health, and epidemic preparedness.

A survey of sheep and/or cattle farmers in the UK shows confusion over the diagnosis and control of rumen fluke and liver fluke.

Calicophoron daubneyi (rumen fluke) is an emerging parasitic infection of livestock across Europe. Despite increasing in prevalence, little is known about the level of awareness of rumen fluke or current control practices used by UK farmers. Fasciola hepatica (liver fluke) is a common parasitic infection of cattle and sheep in the UK. Co-infections with these parasites can present in sheep and cattle, but the only drug with reported efficacy against rumen fluke is oxyclozanide. Between December 2019 and March 2020, 451 sheep and/or cattle farmers completed an online questionnaire, capturing their awareness and current means of control for liver fluke and rumen fluke. Most respondents (70%) were aware of rumen fluke, with 14% recording its presence on their farms and 18% having previously treated for rumen fluke. Almost all respondents (99%) were aware of liver fluke and higher numbers of respondents reported its presence on farm (67%) with 88% having previously treated for liver fluke. Respondents who were aware of rumen fluke said they were concerned about the parasite (81%), although rumen fluke was less of a concern than liver fluke (p < 0.05). Of respondents who reported rumen fluke presence on their farm, 42% cited incorrect diagnostic methods, including those traditionally used to detect liver fluke. Respondents were more likely to treat annually for liver fluke, as opposed to rumen fluke (p < 0.05). The most frequently used drug for the treatment of liver fluke infection was triclabendazole (53% sheep treatments, 34% cattle treatments) and only a minority of farmers treated with a product effective against rumen fluke (oxyclozanide; 42% cattle treatments, 35% sheep treatments). A small proportion of farmers stated that they used a non-flukicide drug to treat sheep for liver fluke infection (1.6% sheep treatments). These results demonstrate a broad awareness of liver and rumen fluke in sheep and cattle, but reveal confusion amongst farmers about their diagnosis and treatment, highlighting the need to provide best practice advice to the livestock industry for the control of both parasites.

Anaerobic fermentation results in loss of viability of Fasciola hepatica metacercariae in grass silage.

The parasitic liver fluke, Fasciola hepatica, has a detrimental impact on food security and poses a welfare concern to ruminant livestock. F. hepatica metacercariae, shed from an intermediate mud snail host, encyst on vegetation and present a source of infection to grazing livestock. Feeding grass silage to ruminants is a common practice, however the role it plays in the transmission of F. hepatica remains largely unknown. Our current understanding relies on historical studies that are not representative of current silage production and did not apply molecular methods to detect F. hepatica DNA persistence within silages. This study determined the impact of specific fermentation factors, including grass dry matter (DM) content (20, 30 & 40 %), length of ensiling period and maintaining an anaerobic environment on F. hepatica metacercariae viability. In vitro excystment assays demonstrated that regardless of grass DM content, metacercariae ensiled under anaerobic conditions were not viable from two weeks post-sealing. Metacercariae recovered from ensiled grass of 20 % DM content subjected to aerobic spoilage, remained viable for up to 10 weeks. DNA of F. hepatica remained detectable for up to 10 weeks in both anaerobic and spoiled silages. This study highlights i) the importance of maintaining an anaerobic ensiling environment to eliminate the risk of F. hepatica transmission from silage and ii) an inverse relationship between grass DM content and duration of metacercariae survival within spoiled silages. Improving our understanding of trematode metacercariae survival rates within silages, especially of highly pathogenic species such as F. hepatica, allows farmers to make informed decisions regarding on-farm parasite control.

The Epidemiology and Control of Liver Flukes in Cattle and Sheep.

Fasciola hepatica, Fasciola gigantica, and Fascioloides magna are liver flukes causing disease in cattle and sheep. Damage to the liver due to F hepatica and F gigantica results in clinical disease and/or production losses. F magna seems to have little effect in cattle but causes high mortality in sheep. The fluke life cycle involves an aquatic or amphibious snail intermediate host and thus requires suitable moisture and temperature conditions. F magna requires the presence of deer. Drug treatment is the mainstay of control and needs to be applied considering the life cycle and epidemiology of the parasite.

Co-infection of cattle with Fasciola hepatica or F. gigantica and Mycobacterium bovis: A systematic review.

The liver flukes, Fasciola hepatica and F. gigantica, are common trematode parasites of livestock. F. hepatica is known to modulate the immune response, including altering the response to co-infecting pathogens. Bovine tuberculosis (bTB), caused by Mycobacterium bovis, is a chronic disease which is difficult to control and is of both animal welfare and public health concern. Previous research has suggested that infection with liver fluke may affect the accuracy of the bTB skin test, but direction of the effect differs between studies. In a systematic review of the literature, all experimental and observational studies concerning co-infection with these two pathogens were sought. Data were extracted on the association between fluke infection and four measures of bTB diagnosis or pathology, namely, the bTB skin test, interferon γ test, lesion detection and culture/bacterial recovery. Of a large body of literature dating from 1950 to 2019, only thirteen studies met the inclusion criteria. These included studies of experimentally infected calves, case control studies on adult cows, cross sectional abattoir studies and a herd level study. All the studies had a medium or high risk of bias. The balance of evidence from the 13 studies included in the review suggests that liver fluke exposure was associated with either no effect or a decreased response to all of the four aspects of bTB diagnosis assessed: skin test, IFN γ, lesion detection and mycobacteria cultured or recovered. Most studies showed a small and/or non-significant effect so the clinical and practical importance of the observed effect is likely to be modest, although it could be more significant in particular groups of animals, such as dairy cattle.

A review of our current understanding of parasite survival in silage and stored forages, with a focus on Fasciola hepatica metacercariae.

Fasciola hepatica, the parasitic liver fluke, is a re-emerging zoonotic infection and an important cause of morbidity and mortality in ruminant livestock worldwide. A significant animal welfare concern, fasciolosis also has a detrimental impact on food security, with the effects of sub-clinical infection on growth rate and milk yield estimated to cost the UK cattle industry £40.4 million annually. It is understood that up to 50% of infective F. hepatica metacercariae may overwinter on pasture and remain viable to infect grazing livestock the following spring. However, the infection risk posed by feeding grass silage to livestock remains largely unknown as the majority of previous studies are outdated in both experimental design and analysis of ensiled metacercariae viability. Anecdotal reports of fasciolosis outbreaks in housed livestock have reignited interest in F. hepatica metacercariae survival during modern ensiling processes and more importantly if they retain viability within stored forages. Consequently, a comprehensive review of the available literature is required in order to identify knowledge gaps and highlight future research opportunities regarding the role of silage in the transmission of F. hepatica.

Refugia and anthelmintic resistance: Concepts and challenges.

Anthelmintic resistance is a threat to global food security. In order to alleviate the selection pressure for resistance and maintain drug efficacy, management strategies increasingly aim to preserve a proportion of the parasite population in 'refugia', unexposed to treatment. While persuasive in its logic, and widely advocated as best practice, evidence for the ability of refugia-based approaches to slow the development of drug resistance in parasitic helminths is currently limited. Moreover, the conditions needed for refugia to work, or how transferable those are between parasite-host systems, are not known. This review, born of an international workshop, seeks to deconstruct the concept of refugia and examine its assumptions and applicability in different situations. We conclude that factors potentially important to refugia, such as the fitness cost of drug resistance, the degree of mixing between parasite sub-populations selected through treatment or not, and the impact of parasite life-history, genetics and environment on the population dynamics of resistance, vary widely between systems. The success of attempts to generate refugia to limit anthelmintic drug resistance are therefore likely to be highly dependent on the system in hand. Additional research is needed on the concept of refugia and the underlying principles for its application across systems, as well as empirical studies within systems that prove and optimise its usefulness.

Stimulation of bovine monocyte-derived macrophages with lipopolysaccharide, interferon-É£, Interleukin-4 or Interleukin-13 does not induce detectable changes in nitric oxide or arginase activity.

BACKGROUND: Bacterial lipopolysaccharide and interferon-γ stimulation of rodent macrophages in vitro induces up-regulation of inducible nitric oxide synthase, whereas interleukin-4 stimulation results in increased activity of arginase-1. Thus different stimulants result in differing macrophage phenotypes, appropriate for responses to a range of pathogens. The current study was conducted in order to determine whether bovine macrophages derived from monocytes and spleen respond similarly. RESULTS: Lipopolysaccharide and interferon-γ did not induce detectable increases in nitric oxide production by bovine monocyte-derived or splenic macrophages in vitro. Similarly, interleukin-4 and interleukin-13 did not affect arginase activity. However, changes in transcription of genes coding for these products were detected. CONCLUSION: Differences between macrophage activation patterns exist between cattle and other species and these differences may occur during the post-transcription phase.

Composite Fasciola hepatica faecal egg sedimentation test for cattle.

Options for diagnosing Fasciola hepatica infection in groups of cattle are limited. Increasing the opportunities for herd-level diagnosis is important for disease monitoring, making informed treatment decisions and for flukicide efficacy testing. The sensitivity of a simple sedimentation method based on composite faecal samples for the detection of fluke eggs in cattle was assessed through a combination of experimental and statistical modelling techniques. Initially, a composite sample method previously developed for sheep was used to investigate the sensitivity of composite sample testing compared with individual counts on the same samples in cattle. Following this, an optimised, validated, qualitative (presence-absence) composite sample field test was developed for cattle. Results showed that fluke egg counts obtained from a composite sample are representative of those expected from individual counts. The optimal sampling strategy was determined to be 10 individual 10 g samples (100 g composite sample) from which a 10 g subsample is taken for sedimentation. This method yielded a diagnostic sensitivity of 0.69 (95 per cent CI 0.5 to 0.85). These results demonstrate the validity and usefulness of a composite faecal egg sedimentation method for use in the diagnosis and control of F. hepatica in groups of cattle, with the caveat that a negative test should be followed up with a second test due to limitations relating to test sensitivity.

Association of Fasciola gigantica Co-infection With Bovine Tuberculosis Infection and Diagnosis in a Naturally Infected Cattle Population in Africa.

Bovine tuberculosis (bTB), caused by Mycobacterium bovis, remains a major livestock and public health problem in both high and low-income countries. With the current absence of an effective vaccine, control in cattle populations is reliant on regular testing and removal of positive animals. However, surveillance and control are hampered by imperfect diagnostic tests that have poorly described properties in naturally infected populations. Recent research in cattle co-infected with the temperate liver fluke, Fasciola hepatica, has raised concerns about the performance of the intradermal skin test in high fluke incidence areas. Further, recent studies of parasitic co-infections have demonstrated their impact on Th1 and Th2 responses, concurrent disease pathology and susceptibility to mycobacterial infections. Here we report for the first time the association of co-infection with the tropical liver fluke, Fasciola gigantica, with the presence of bTB-like lesions and the IFN-γ response in naturally infected African cattle. After adjusting for age and sex we observed a complex interaction between fluke status and breed. Fulani cattle had a higher risk of having bTB-like lesions than the mixed breed group. The risk of bTB-like lesions increased in the mixed breed group if they had concurrent evidence of fluke pathology but was less clear in the coinfected Fulani breed. Further, we observed a slight decline in the IFN-γ levels in fluke infected animals. Finally we explored factors associated with IFN-γ false negative results compared to the presence of bTB-like lesions. Fulani cattle had a higher risk of having a false negative result compared to the mixed breed group. Further, the mixed breed cattle had an increased risk of being false negative if also co-infected with fluke. Interesting, as with the risk of bTB-like lesions, this association was less clear in the Fulani cattle with weak evidence of a slight decrease in risk of having a false negative test result when fluke pathology positive. This interesting interaction where different breeds appear to have different responses to co-infections is intriguing but further work is needed to confirm and understand more clearly the possible confounding effects of different other co-infections not measured here, breed, management or exposure risks.

A mechanistic hydro-epidemiological model of liver fluke risk.

The majority of existing models for predicting disease risk in response to climate change are empirical. These models exploit correlations between historical data, rather than explicitly describing relationships between cause and response variables. Therefore, they are unsuitable for capturing impacts beyond historically observed variability and have limited ability to guide interventions. In this study, we integrate environmental and epidemiological processes into a new mechanistic model, taking the widespread parasitic disease of fasciolosis as an example. The model simulates environmental suitability for disease transmission at a daily time step and 25 m resolution, explicitly linking the parasite life cycle to key weather-water-environment conditions. Using epidemiological data, we show that the model can reproduce observed infection levels in time and space for two case studies in the UK. To overcome data limitations, we propose a calibration approach combining Monte Carlo sampling and expert opinion, which allows constraint of the model in a process-based way, including a quantification of uncertainty. The simulated disease dynamics agree with information from the literature, and comparison with a widely used empirical risk index shows that the new model provides better insight into the time-space patterns of infection, which will be valuable for decision support.

Clonal amplification of Fasciola hepatica in Galba truncatula: within and between isolate variation of triclabendazole-susceptible and -resistant clones.

BACKGROUND: Fasciola hepatica is of worldwide significance, impacting on the health, welfare and productivity of livestock and regarded by WHO as a re-emerging zoonosis. Triclabendazole (TCBZ), the drug of choice for controlling acute fasciolosis in livestock, is also the drug used to treat human infections. However TCBZ-resistance is now considered a major threat to the effective control of F. hepatica. It has yet to be demonstrated whether F. hepatica undergoes a genetic clonal expansion in the snail intermediate host, Galba truncatula, and to what extent amplification of genotypes within the snail facilitates accumulation of drug resistant parasites. Little is known about genotypic and phenotypic variation within and between F. hepatica isolates. RESULTS: Six clonal isolates of F. hepatica (3× triclabendazole-resistant, TCBZ-R and 3× triclabendazole-susceptible, TCBZ-S) were generated. Snails infected with one miracidium started to shed cercariae 42-56 days post-infection and shed repeatedly up to a maximum of 11 times. A maximum of 884 cercariae were shed by one clonally-infected snail (FhLivS1) at a single time point, with > 3000 clonal metacercariae shed over its lifetime. Following experimental infection all 12 sheep were FEC positive at the time of TCBZ treatment. Sheep infected with one of three putative TCBZ-S clones and treated with TCBZ had no parasites in the liver at post-mortem, whilst sheep each infected with putative TCBZ-R isolates had 35-165 adult fluke at post-mortem, despite TCBZ treatment. All six untreated control animals had between 15-127 parasites. A single multi-locus genotype was reported for every fluke from each of the six clonal isolates. Adult F. hepatica showed considerable variation in weight, ranging from 20-280 mg, with variation in weight evident within and amongst clonal isolates. CONCLUSIONS: A genetic clonal expansion occurs within G. truncatula, highlighting the potential for amplification of drug resistant genotypes of F. hepatica. Variation in the weight of parasites within and between clonal isolates and when comparing isolates that are either susceptible or resistant to TCBZ represent inherent variation in liver fluke and cannot be attributed to their resistance or susceptibility traits.

Development and validation of a protocol to identify and recruit participants into a large scale study on liver fluke in cattle.

BACKGROUND: Liver fluke infection caused by the parasite Fasciola hepatica is a major cause of production losses to the cattle industry in the UK. To investigate farm-level risk factors for fluke infection, a randomised method to recruit an appropriate number of herds from a defined geographical area into the study was required. The approach and hurdles that were encountered in designing and implementing this study are described. The county of Shropshire, England, was selected for the study because of the variation between farms in exposure to fluke infection observed in an earlier study. RESULTS: From a sampling list of 569 holdings in Shropshire randomly drawn from the RADAR cattle population dataset, 396 (69.6%) holdings were successfully contacted by telephone and asked if they would be interested in taking part in the study. Of 296 farmers who agreed to receive information packs by post, 195 (65.9%) agreed to take part in the study. Over the period October 2014 - April 2015 visits were made to 100 dairy and 95 non-dairy herds. During the farm visits 40 faecal samples +/- bulk-tank milk samples were collected and a questionnaire administered. Composite faecal samples were analysed for the presence of F. hepatica eggs by sedimentation and bulk tank milk samples were tested with an antibody ELISA for F. hepatica. Forty-five (49%) of non-dairy herds were positive for liver fluke infection as determined by the finding of one or more fluke eggs, while 36 (36%) dairy herds had fluke positive faecal samples and 41 (41%) dairy herds were positive for F. hepatica antibody. Eighty-seven (45.8%) farmers said that they monitored their cattle for liver fluke infection and 118 (62.1%) reported that they used flukicide drugs in their cattle. CONCLUSIONS: Using a protocol of contacting farmers directly by telephone and subsequently sending information by post, 79% of the target sample size was successfully recruited into the study. A dataset of farm-specific information on possible risk factors for liver fluke infection and corresponding liver-fluke infection status was generated for the development of statistical models to identify risk factors for liver fluke infection at the farm-level.

Predicting the distribution of Phortica variegata and potential for Thelazia callipaeda transmission in Europe and the United Kingdom.

BACKGROUND: Male fruitflies Phortica variegata (Drosophilidae, Steganinae) are the intermediate host of the zoonotic nematode Thelazia callipaeda (Spirurida, Thelaziidae). More than 10 years ago, when T. callipaeda was confined to remote regions of southern Italy, ecological niche models were used to predict the potential distribution of P. variegata across Europe and the likely risk of the nematode spreading through infected dogs travelling to/from endemic regions. As predicted, over the last 10 years T. callipaeda has spread rapidly across Europe. Recently, we identified the potential for its introduction to the UK through infected dogs travelling to/from endemic regions of mainland Europe. METHODS: Here updated information is used to re-evaluate the model-predicted European, and specifically, UK distribution to determine the likelihood of T. callipaeda becoming established. Additionally, the UK distribution of P. variegata was further investigated through snapshot fly trapping at model-predicted locations. RESULTS: Ecological niche modelling using Genetic Algorithm for Rule-set Prediction (GARP) analysis suggests a European range similar to that described previously, with some indication of potential spread further eastward. Finer scale UK mapping suggested that P. variegata presence was limited mostly to southern England, but highlighted regions where P. variegata has not been documented previously. The arbitrary fly trapping identified activity of P. variegata at two locations where the species has been found previously late in the season. No specimens were collected at model-predicted locations, although habitat suitable for the species was identified. CONCLUSIONS: GARP-model prediction of P. variegata distribution suggests presence of suitable conditions in previously undocumented regions of the UK and Europe and highlight the possibility for further spread of T. callipaeda across Europe, including the UK. Further work to validate the P. variegata UK model with field data will help improve its accuracy in predicting suitable areas, whilst surveillance of sylvatic definitive host species in such locations is advised to monitor for evidence of autochthonous T. callipaeda transmission.

Co-infection with Fasciola hepatica may increase the risk of Escherichia coli O157 shedding in British cattle destined for the food chain.

Escherichia coli O157 is a zoonotic bacterium that can cause haemorrhagic diarrhoea in humans and is of worldwide public health concern. Cattle are considered to be the main reservoir for human infection. Fasciola hepatica is a globally important parasite of ruminant livestock that is known to modulate its host's immune response and affect susceptibility to bacterial pathogens such as Salmonella Dublin. Shedding of E. coli O157 is triggered by unknown events, but the immune system is thought to play a part. We investigated the hypothesis that shedding of E. coli O157 is associated with F. hepatica infection in cattle. Three hundred and thirty four cattle destined for the food chain, from 14 British farms, were tested between January and October 2015. E. coli O157 was detected by immunomagnetic separation and bacterial load enumerated. F. hepatica infection status was assessed by copro-antigen ELISA. A significant association (p=0.01) was found between the log percent positivity (PP) of the F. hepatica copro-antigen ELISA and E. coli O157 shedding when the fixed effects of day of sampling and the age of the youngest animal in the group, plus the random effect of farm were adjusted for. The results should be interpreted cautiously due to the lower than predicted level of fluke infection in the animals sampled. Nevertheless these results indicate that control of F. hepatica infection may have an impact on the shedding of E. coli O157 in cattle destined for the human food chain.

Dairy Heifers Naturally Exposed to Fasciola hepatica Develop a Type 2 Immune Response and Concomitant Suppression of Leukocyte Proliferation.

Fasciola hepatica is a parasitic trematode of global importance in livestock. Control strategies reliant on anthelmintics are unsustainable due to the emergence of drug resistance. Vaccines are under development, but efficacies are variable. Evidence from experimental infection suggests that vaccine efficacy may be affected by parasite-induced immunomodulation. Little is known about the immune response to F. hepatica following natural exposure. Hence, we analyzed the immune responses over time in calves naturally exposed to F. hepatica infection. Cohorts of replacement dairy heifer calves (n = 42) with no prior exposure to F. hepatica, on three commercial dairy farms, were sampled over the course of a grazing season. Exposure was determined through an F. hepatica-specific serum antibody enzyme-linked immunosorbent assay (ELISA) and fluke egg counts. Concurrent changes in peripheral blood leukocyte subpopulations, lymphocyte proliferation, and cytokine responses were measured. Relationships between fluke infection and immune responses were analyzed by using multivariable linear mixed-effect models. All calves from one farm showed evidence of exposure, while cohorts from the remaining two farms remained negative over the grazing season. A type 2 immune response was associated with exposure, with increased interleukin-4 (IL-4) production, IL-5 transcription, and eosinophilia. Suppression of parasite-specific peripheral blood mononuclear cell (PBMC) proliferation was evident, while decreased mitogen-stimulated gamma interferon (IFN-γ) production suggested immunomodulation, which was not restricted to parasite-specific responses. Our findings show that the global immune response is modulated toward a nonproliferative type 2 state following natural challenge with F. hepatica This has implications in terms of the timing of the administration of vaccination programs and for host susceptibility to coinfecting pathogens.

Fasciola hepatica demonstrates high levels of genetic diversity, a lack of population structure and high gene flow: possible implications for drug resistance.

Fasciola hepatica, the liver fluke, is a trematode parasite of considerable economic importance to the livestock industry and is a re-emerging zoonosis that poses a risk to human health in F. hepatica-endemic areas worldwide. Drug resistance is a substantial threat to the current and future control of F. hepatica, yet little is known about how the biology of the parasite influences the development and spread of resistance. Given that F. hepatica can self-fertilise and therefore inbreed, there is the potential for greater population differentiation and an increased likelihood of recessive alleles, such as drug resistance genes, coming together. This could be compounded by clonal expansion within the snail intermediate host and aggregation of parasites of the same genotype on pasture. Alternatively, widespread movement of animals that typically occurs in the UK could promote high levels of gene flow and prevent population differentiation. We identified clonal parasites with identical multilocus genotypes in 61% of hosts. Despite this, 84% of 1579 adult parasites had unique multilocus genotypes, which supports high levels of genotypic diversity within F. hepatica populations. Our analyses indicate a selfing rate no greater than 2%, suggesting that this diversity is in part due to the propensity for F. hepatica to cross-fertilise. Finally, although we identified high genetic diversity within a given host, there was little evidence for differentiation between populations from different hosts, indicating a single panmictic population. This implies that, once those emerge, anthelmintic resistance genes have the potential to spread rapidly through liver fluke populations.