Fasciola hepatica, more commonly referred to as liver fluke, is now considered endemic in certain areas of the UK, particularly in the wetter regions to the west. Despite our increased knowledge of liver fluke, the presence of this parasite is still on the increase.
Climate change may be contributing to this increase, with many wet summers and milder winters leading to a longer window where the intermediate host, the mud snail, is active on pastures. Intensification of farming practices, increased animal movements, increased drug resistance and wildlife hosts also play their part.
The incidence of fluke disease diagnosed post-mortem in cattle and sheep has risen year on year (VIDA), and is estimated to cost UK agriculture around £300 million per annum due to production losses; liver condemnations alone cost over £3 million each year (AHDB, 2016).
According to the National Animal Disease Information Service (NADIS), the overall UK average temperature in August 2018 was 15.3°C (0.3°C above the long-term average), while rainfall was 5 percent less than the long-term average. What does this mean for liver fluke?
The provisional fluke forecast for autumn 2018 (based on monthly rainfall and temperature data from May to August 2018) is predicting a low risk across the UK, although some parts of Scotland have been flagged as medium to high risk. These risk levels may change dependant on weather conditions in the following months (NADIS, 2018).
A joint press release issued by Sustainable Control of Parasites in Sheep and Control of Worms Sustainably highlighted that when considering liver fluke risk on individual farms, local factors must be considered. It must not be assumed that the dry hot weather this summer will guarantee low infection levels on all farms across the UK.
Triclabendazole is the only active ingredient available that should be effective against all stages of liver fluke. However, historical over-reliance on this flukicide has contributed to the development of resistance in F. hepatica. This resistance has been confirmed repeatedly in the UK (Hanna et al., 2015), particularly in sheep-rearing areas (Williams et al., 2014). Triclabendazole resistance in fluke causes the biggest disease issues in sheep as they are far more susceptible than cattle to the acute syndrome, where most often the initial clinical signs are multiple sudden deaths (up to 10 percent of the flock).
Triclabendazole resistance in liver fluke is a one-way street. Once resistance has developed, reversion to susceptibility is unlikely; strategies must be discussed with farmers to decrease the likelihood of triclabendazole resistance developing.
Advising farmers on management changes, eg considering fencing off wetter areas of pasture and not grazing sheep on high-risk pastures during the periods of highest risk, can help reduce fluke burdens. When treatment is necessary, it is crucial to make a choice of correct active ingredient based on the season and likely lifecycle stage of the fluke infecting sheep. Triclabendazole should be reserved for times of the year when acute liver fluke disease is most likely.
It is equally important to only prescribe flukicides when strictly necessary, taking into account individual farm history, forecasting and specific diagnostics. Advising farmers on proper dosing procedures can also help ensure effective fluke control. Weighing individual animals would be the gold standard for good dosing practice. Where this is not possible, animals should be grouped according to estimated size and dosed according to the heaviest animal in the group. If there is a wide variation in size, farmers should take care to split the group appropriately, then dose accordingly. Underdosing may increase the risk of fluke surviving, while overdosing may increase the likelihood of adverse events.
In conjunction with farmers, it is crucial to implement a robust quarantine strategy for all parasites, including liver fluke, particularly with the risk of introducing fluke resistant to triclabendazole to farms.
Treatment of liver fluke is not straightforward; different active ingredients are effective against different fluke stages and choice of product should take into consideration the time of year and which stages of fluke you are trying to target.
For example, sheep are most at risk of acute fluke disease caused by migration of a huge number of immature fluke stages in the autumn. Therefore, an active ingredient such as triclabendazole, which targets younger fluke, should be considered. Follow-up testing after treatment with triclabendazole would be prudent to ensure this has worked. Later in the year, active ingredients that target older fluke, such as closantel or nitroxinil, should be used. Where adult fluke are likely to be present, oxyclosanide or albendazole could be considered. No flukicide offers a persistency of action and in high-risk areas, treatments will need to be rotated and possibly repeated throughout the year.
A full reference list is available on request