Risk/opportunity:(from the Climate Change Risk Assessment for Scotland 2012):
AG44 Livestock pests and diseases
Narratives: Suitability and productivity (agriculture)
The liver fluke, Fasciola hepatica, is a highly pathogenic flatworm parasite of sheep and cattle. Animals become infected by ingesting fluke cysts, shed by infected mud snails, on pasture while grazing. Fluke can be particularly devastating in sheep and was responsible for considerable disease and death in Scottish sheep in winter/spring of 2012-2013. As well as death of livestock, fluke also causes significant production losses e.g. reduced liveweight gain in sheep and cattle, poor reproductive performance and reduced milk yield in dairy cattle (Schweizer et al, 2005). Liver fluke also contributes significantly to the carbon footprint of livestock farming through reduced biological efficiency and increased waste (Williams et al, 2013).
Based on the climate modelling study of Fox et al., (2011), the overall risk of liver fluke disease (fasciolosis) is predicted to be higher in the future than in the past 40 years, across all regions of the UK, including Scotland (Fox et al, 2011). Their model is based on a calculated index of the likely intermediate host snail numbers and probability of infection, (termed the ‘Ollerenshaw Index’ (Ollerenshaw & Rowlands, 1959)), interfaced with UKCP09 Met Office Climate projections for the UK, at a resolution of 25km (Williams et al, 2013).
Farmers are possibly more aware of liver fluke now as a result of recent increased incidence, coverage in the farming press and awareness campaigns by the industry. However, farmers tend to be reactive rather than proactive and, typically, try to treat their way out of trouble using chemical flukicides. This approach is not sustainable and we are already seeing problems with the emergence of resistance to frontline flukicides, especially triclabendazole, the drug of choice for controlling liver fluke in sheep (Gordon et al, 2012).
The 2014 risk level is unquantified at time of writing, but is predicted to be lower than that of 2012- 2013, based on the hotter, drier summer in 2013. However, this may be off-set by the extremely mild, wet winter that followed, allowing for better over-winter survival of snails and fluke stages on pasture. Interestingly, the UK Government’s Animal Health and Veterinary Laboratories Agency (AHVLA) and Scottish Agricultural College Veterinary Investigation Service (SAC VIS) have issued fluke alerts for England, Wales and Scotland in May 2014, based on this perceived risk.
Liver fluke has been an issue for farmers over many decades, especially in the wetter west of the country; however its more recent spread into the east/northeast has caught many farmers unawares. There were significant fluke ‘crises’ in the UK as far back as the 1950s-60s, but at that time, farmers were encouraged (and permitted) to drain their land to reduce snail habitat and/or to treat their pasture with chemical molluscicides e.g. copper sulphate, to control the snails. Chemical flukicides only appeared around the 1960s, and until that time, farmers had to make best use of farm management strategies to control fluke on their farms.
The retrospective climate modelling study of Fox et al (2011) shows that , over the past four decades, liver fluke risk has spread across the UK from a relatively restricted distribution in west of the country in the 1970s, to the current level, where large parts of the UK see regular disease outbreaks. This trend is echoed by regional and national passive surveillance reports.
Figure 1 above in Latest figures hows the historic summer fluke risk across the UK at 5x5km resolution (taken from Fox et al, 2011), based on Ollerenshaw Indices interfaced with long term average data from 1961– 1990, based on the HadRM3 regional climate model. Green = low/no risk; orange = disease prevalent; red= serious epidemic.
Because liver fluke spends much of its complicated life-cycle outside the host, either within the snails, or as cysts or eggs on pasture, its prevalence, seasonality and geographic spread are very much affected by climatic conditions, especially temperature and rainfall. Liver fluke risk has previously been forecast using a formula devised in the 1950s (Ollerenshaw & Rowlands, 1959) that is based on the summer infection of snails. The ‘Ollerenshaw Index’, a measure of evapotranspiration rate, is still used to provide regional fluke forecasts online at the National Animal Disease Information Service1.
Fox et al (2011) used a modified Ollerenshaw formula, interfaced with UKCP09 climate projections, to model likely fluke risk across the UK up to 2070. Model outputs show unprecedented fluke risk in parts of the UK, due to a combination of increasingly wet summers and mild winters, with serious epidemics predicted in Wales by 2050. As for Scotland, the high levels of fluke experienced in 2012 could become a more common occurrence, with serious epidemics predicted to become the norm by 2020, especially in the north and west of the country (Fox et al, 2011; Fox & Hutchings, 2013).
Figure 2 below shows projected summer fluke risk across the UK at 25x25km resolution, (taken from Fox et al, 2011), based on the Ollerenshaw Indices interfaced with UKCP09 climate data Green = low/no risk; orange = disease prevalent; red= serious epidemic.
Image taken from Fox et al (2011)
Figure 2. Projected change in Summer Liver Fluke Risk
Changing weather patterns are undoubtedly one of the main drivers of changing liver fluke risk, but they are not the only explanation of recent changes (Kenyon et al, 2009). Animal movements contribute significantly to the spread of the parasite around the country and its establishment in previously fluke-free areas (Van Dijk et al, 2009), especially if bought-in livestock are not given an effective quarantine fluke treatment. The emergence of flukicide resistance is also a contributory factor in our inability to control the parasite in endemic areas (Gordon et al, 2012). Furthermore, wetland/agri-environment schemes designed to improve flood management, enhance carbon sequestration or improve biodiversity, are an acknowledged liver fluke risk as they bring grazing animals into close proximity with ideal mud snail habitat (Pritchard et al, 2005). In fact, the liver fluke’s intermediate host, the mud snail Galba truncatula, is an indicator species for healthy wetland schemes. Furthermore, another mud snail species, Omphiscola glabra, classed as an endangered species in Scotland and subject to conservation, is an acknowledged liver fluke intermediate host.
Wetland birds have also been implicated in the spread of liver fluke as they are capable of transporting liver fluke-infected snails over long distances (Van Leeuwen et al, 2012).
There are a number of limitations in the interpretation of recent and future trends of fluke risk. Changing weather patterns are one of the main drivers of liver fluke incidence, but not the only one. There are also, as yet unidentified, farm-specific factors that dictate a given farm’s fluke risk that can override the influence of changing weather patterns. These may include the underlying geology of the land and/or specific farm management practices that can increase or reduce the fluke risk (McCann et al, 2010). There are also constraints on what farmers are allowed or incentivised to do in terms of farm management, as enshrined in legislation and ongoing CAP reform. There are also some limitations to long term parasite forecasts based on correlative models (see Fox et al, (2012) for full discussion of model limitations). Research to fill some of these critical knowledge gaps is ongoing within the EUFP7 GLOWORM project2.
Fox et al, 2011. Predicting impacts of climate change on Fasciola hepatica risk. PLoS One 6(1): e16126.doi:10.1371/journal.pone.0016126
Fox, et al., (2012). Livestock Helminths in a Changing Climate: Approaches and Restrictions to Meaningful Predictions. Animals 2012, 2, 93-107.
Fox, N and Hutchings, M. (2013) Liver Fluke risk in a changing climate. SRUC Rural Policy Centre Research Briefing, February 2013.
Gordon et al, 2012. Confirmation of triclabendazole resistance in liver fluke in the UK. Veterinary Record, Aug 11, 2012
Kenyon et al, 2009. Sheep helminth disease in south eastern Scotland arising as a possible consequence of climate change. Veterinary Parasitology 163: 293-297
McCann et al, 2010. The development of linear regression models using environmental variables to explain the spatial distribution of Fasciola hepatica infection in dairy herds in England and Wales. International Journal for Parasitology, 40: 1021-1028
Ollerenshaw and Rowlands, 1959. A method of forecasting the incidence of fascioliasis on Anglesey.
Veterinary Record, 71:591-598
Pritchard et al, 2005. Emergence of fasciolosis in cattle in East Anglia. Veterinary Record, Nov 5, 2005. Schweizer et al, 2005. Estimating the financial losses due to bovine fasciolosis in Switzerland.
Veterinary Record, Aug 13, 2005
Van Dijk et al 2009. Climate change and infectious disease: helminthological challenges to farmed ruminants in temperate regions. Animal doi:10.1017/S1751731109990991
Van Leeuwen et al, 2012. A snail on four continents: bird-mediated dispersal of a parasite vector? PhD thesis. Netherlands Institute of Ecology, Chapter 7.
Williams et al, 2013. The benefits of improving cattle health on environmental impacts and enhanced sustainability. Sustainable Intensification Conference abstract, Edinburgh, September 2013
Prof Mike Hutchings, Disease Systems Team, SRUC, e-mail firstname.lastname@example.orgMoredun Research Institute, www.moredun.org.uk
The authors, Dr Philip Skuce (Moredun Research Institute) and Dr Naomi Fox (SRUC), would like to thank Prof Ruth Zadoks, MRI, for helpful input and discussions