Deciding what liver fluke product to use

Recent liver fluke warnings released by the Department of Agriculture and the Agri-Food and Biosciences Institute have stimulated more discussions around treatment for liver fluke and in particular regarding product choice.

The threat from liver fluke varies depending on the time of year and the management of sheep.

Table 1 details a snapshot of the three threats from acute liver fluke, sub-acute liver fluke and chronic liver fluke. In the past, there was a more pronounced seasonal shift in these threats with acute liver fluke becoming less of an issue as you progress into November and chronic fluke becoming a greater threat from then on.

Changing climatic conditions and milder temperatures mean acute and sub-acute liver fluke now pose a risk later into the year with the risk waning from the turn of the New Year.

As such outwintered flocks could still be facing a risk from acute and sub-acute fluke and as such it is advisable to use a product that treats at least mature and immature liver fluke parasites.

Management factors

Management also needs to be taken into account. Where sheep are grazing dry ground with a low history of fluke issues then frequency of treatment can be extended.

However where sheep are grazing waterlogged soils that have traditionally given rise to significant problems then it is important to consult with your vet to put an appropriate management plan in place.

Maximum use should be made of all information sources. The obvious areas are carrying out post-mortems on fallen animals and exploring the health status of the liver in slaughtered sheep where possible.

Faecal egg counts (FECs) can be used strategically but they are not a good option where there is a high risk of an acute liver fluke as a high number of parasites can be present and cause significant danger before adults start laying eggs that will be identified in FECs.

Product selection

Focusing on the active ingredient is an important component to remember as there are numerous products on the market within the same class of active ingredients.

The product range is extensive if you include combination products but the advice from veterinary and advisory bodies is to refrain from using combination products to treat liver fluke unless there is a known issue that requires treatment with the combined active ingredient.

Figure 1 was presented at the Teagasc Hill Sheep Conference a few years ago and is a good diagrammatic picture of the stage of live flukes treated by the main anthelmintic classes.

There are a few slight changes with the two nitroxynil products introduced to the Irish market on an emergency licence to replace Trodax.

It is clear that trichlabendazole is the most complete treatment but there is resistance issues reported on some farms.

It is worth noting that some of these resistance issues were recorded many years ago when there were fewer investigations on liver fluke.

As such it is worth farms taking steps in tandem with their vet to explore if Trichlabendazole resistance was actually the cause of the problem.

Rafoxanide

The Sustainable Control of Parasites, or SCOPS, grouping in the UK released a technical note in 2023 regarding the potential use of rafoxanide as an alternative to other flukicides.

The document stated that in recent years “there has been confusion around rafoxanide, especially with respect to its use as an alternative to closantel on farms where triclabendazole resistance is suspected”.

It explains that rafoxanide is what is known as a salicylanilide anthelmintic and that other products in this class include closantel and oxyclozanide.

It states “therefore, rafoxanide is not a different class of flukicide. Rafoxanide and closantel are similar in chemical structure and mode of action.

There is evidence of cross-resistance between rafoxanide and closantel from both field and laboratory studies (Boray and de Bono, 1989)”.

It adds that this is important because “there is no evidence to suggest that using closantel and rafoxanide interchangeably, or on a rotational basis, will reduce the selection pressure for resistance to closantel.

Indeed, there is a serious risk that such use of rafoxanide will hasten the development of resistance to closantel. Hence, rafoxanide is not considered an appropriate alternative to closantel”.

Closantel and haemonchus

The SCOPS technical note also highlights risks associated with the overuse of closantel where farmers are using it to treat sheep identified with haemonchus contortus.

It says that increased use of closantel for haemonchus contortus will add selection pressure for resistance on liver fluke populations and that this should be noted. Consult your vet if you suspect haemonchus could be a problem.