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?

• Which snail species act as intermediate host for Fasciola hepatica and Calicophoron spp. infection and what is their transmission potential?
• Which wildlife species carry significant fluke burdens and should they be taken into account in epidemiology and control?

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

In each continent there are main intermediate hosts (eg Galba truncatula in Europe), but a large range of other potential intermediate hosts has also been reported. However, their role in fluke epidemiology is not well understood. Infections in these other species of snail may be accidental and may not always lead to the production of cercariae.
There is a lack of information on wildlife reservoirs for F.hepatica, their competence and contribution to metacercariae on pasture used by domestic livestock.

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

• Accurate identification of snails and their infection status and cercarial shedding capacities.
• Quantification of infection levels in wildlife definitive hosts and their potential to contaminate farmland and drive increased infection pressure and spread of resistance genes.

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

• Development and validation of specific molecular methods for snail identification and to determine infection status.
• Development of methods to evaluate cercarial shedding under various conditions.
• Development of/access to the life-cycle in snails. Provision of snail +/- cyst material is an ongoing bottleneck in fluke research.

Existing knowledge including successes and failures

Infection with F. hepatica can be detected by observing shedding of cercariae, crushing or microscopic dissection of the snail. These methods have poor sensitivity in the early stages of infection, and it is difficult to distinguish the intra‐molluscan stages of different species of trematodes. A number of molecular techniques have been used to detect F. hepatica infection in snails, which have generally been found to be more sensitive than microscopic methods; however, the presence of inhibitory factors within the snails can reduce the sensitivity of PCRs, while the presence of parasite DNA does not prove the presence of viable infection. Conversely, 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. Furthermore, few of the published F. hepatica PCRs have been validated for use with snails. A variety of wildlife species have been shown to host F. hepatica and to produce eggs in their faeces, including mammals abundant on farmland such as deer, rabbits and nutria. There have been few attempts, however, to quantify egg shedding rates and to plot spatio-temporal overlap between wildlife, livestock and snail populations in order to make robust estimates of cross-transmission potential.
Should wildlife hosts be identified as a problem, possible solutions would require further research.