Roadmap for the development of control strategies for liver fluke

What are we trying to achieve and why? What is the problem we are trying to solve?

How do intermediate host snail biology and infection drive Fasciola hepatica epidemiology in definitive hosts?

What are the scientific and technological challenges (knowledge gaps needing to be addressed)?

• Understanding of snail population dynamics and factors that affect transmission are implicit rather than explicit.
• Biological influences on snail populations, e.g. through predation and parasitism, largely unknown.
• Lack of understanding of the snail immune response to F.hepatica.
• Variation between snail species and strains in receptivity to F.hepatica, and the importance of within-host processes including innate immunity and competition from other trematodes in determining the fate of incoming miracidia, largely unknown.

What approaches could/should be taken to address the research question?

• Using remote sensing methods, particularly soil moisture data from the new generation of Sentinel satellite systems together with other technologies, such as detection of environmental DNA, to identify suitable snail habitat on farms will improve our ability to predict when and where metacercariae may appear on pasture. The timing of high‐risk periods will vary from region to region.
• A practical and accurate tool to measure cercarial levels on pasture and their viability would be a big step forward.
• Basic biological studies in a farming context to determine how far snails roam, the distance cercariae travel before encysting and what factors trigger cercarial shedding

What else needs to be done before we can solve this need?

Further validation of PCRs to detect F. hepatica infection. It is important to ensure that putative trematode‐specific PCR primers do not amplify snail DNA. A number of other trematode species, including those of birds and amphibians, have been isolated from G. truncatula including Calicophoron daubneyi, Haplometra cylindracea, Notocotylus spp., Plagiorchis spp. Some of these trematodes have little or no published DNA sequence available, which makes it difficult to ensure PCRs are F. hepatica specific. Understanding the role of these other trematode
infections on the dynamics of F. hepatica in snails as well as parasite detection should be improved.

Lack of methods to identify when metacercariae appear on pasture and to quantify risk.

Existing knowledge including successes and failures

The presence of the snail intermediate host is essential to the transmission of F. hepatica, and knowledge of the interaction between snail and parasite is important when considering what drives parasite transmission. It is also important to understand how events in the snail influence genetic diversity of parasites in the mammalian host. To fully understand the epidemiology of Fasciola spp., better knowledge of snail habitats, species of snails acting as intermediate hosts, and prevalence of F. hepatica infection within the snail are required.

For many years, the role of the intermediate host in the Fasciola spp. life cycle has been relatively neglected, but modern molecular and genomic tools are becoming available to study events in the snail, and we can start to address how these impact on transmission and the spread of virulence and anthelmintic resistance genes within fluke populations.