The incidence of liver fluke (Fasciola hepatica) infections in sheep has been rising steadily over recent years, and these have spread into parts of the country traditionally considered to be fluke-free.
Liver fluke has a complex lifecycle involving life stages within the mud snail (Galba truncatula). This snail is not active in the cooler months of the year, so there will be periods with no new infective life stages (metacercariae) deposited onto the pasture. When the infective life stage is consumed by the sheep, this migrates to the liver and the immature liver fluke spend a period migrating through the liver parenchyma.
When significant numbers of these are consumed, migration through the liver in sheep leads to acute fasciolosis over the following weeks. Acute fasciolosis is characterised by sudden death, while others in the group may appear weak, dyspnoeic and have pale mucous membranes. Some sheep may have enlarged, painful livers with ascites and may be reluctant to move. Once cooler weather arrives, the mud snail will no longer be active and there will not be any fresh metacercariae on the pasture.
Treatment of liver fluke
The flukicides available do not all kill liver fluke at all stages of their development within the animal. There is only one active ingredient available that is effective against all stages of liver fluke: triclabendazole. However, historical over-reliance on this flukicide has contributed to resistance development in F. hepatica, which has been confirmed repeatedly in the UK, particularly in sheep-rearing areas.
When choosing a product, the time of year and which stages of fluke are being targeted should be taken into consideration. For example, when sheep are likely to be at risk of acute fluke disease, caused by ingestion of a huge number of immature stages in late summer, and their subsequent migration through liver tissues, a product targeting younger fluke should be considered.
No flukicide offers a persistency of action and so in high-risk areas, treatments will need to be rotated and possibly repeated throughout the year
Later in the season, active ingredients which target older fluke should be used. It should always be borne in mind that no flukicide offers a persistency of action and so in high-risk areas, treatments will need to be rotated and possibly repeated throughout the year.
Pasture management is also important. Fencing off wet areas of fields, or draining these effectively, can prevent sheep being exposed to high numbers of metacercariae. Long-term, planting trees in these wet areas may reduce the number of snails present as the algae they live off need sunlight to grow. It has also been suggested that birds, such as ducks, will consume these snails, and this may aid control of liver fluke disease.
A major concern is what might happen to the incidence of acute fasciolosis and the ability to control the disease if resistance continues to rise.
The use of triclabendazole for acute fasciolosis cannot be avoided; however, where possible, it is advisable to use alternative flukicide drugs where early, immature fluke are unlikely to be causing a problem.