Once a risk assessment and/or diagnostic testing has been completed and it is clear that treatment is needed, choosing the right flukicide is key. The choice of product and frequency of use will depend on the level of fluke challenge, the time of year, and the management and husbandry systems on the farm. Weather and the time of year is important because of the liver fluke lifecycle .
Depending on the farm and weather, the acute disease risk period is in late summer / autumn when acute fasciolosis is caused by immature fluke migrating through the liver. The chronic disease risk period is in winter and spring (and early summer depending on the season) when chronic fasciolosis is caused by adult fluke.
Things to consider
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Alternatives to triclabendazole
Closantel is a useful drug to control immature fluke in autumn, but alternatives such as nitroxynil, albendazole, oxyclozanide or clorsulon can be used at other times of year when adult fluke predominate. This is important, as using closantel as the only alternative to triclabendazole will put similar selection pressure on closantel.
Closantel and rafoxanide are both salicylanilide anthelmintics and particularly similar in chemical structure and mode of action. There is evidence of cross-resistance between rafoxanide and closantel from both field and laboratory studies. There is no evidence to suggest using closantel and rafoxanide interchangeably / on a rotational basis will successfully reduce the selection pressure for resistance to closantel. Indeed, there is a serious risk that use of rafoxanide in this way will hasten the development of resistance to closantel. Hence rafoxanide, while it can have a role in fluke control, is not considered an alternative to closantel.
Although oxyclozanide is also a salicylanilide anthelmintics, fluke isolates resistant to rafoxanide and closantel show no side resistance to oxyclozanide. It is thought this is because of the different pharmacokinetics of oxyclozanide and that it is only effective against adult fluke.
Combination fluke and worm products
Due to the unlikelihood of sheep needed to be treated for worms and fluke at the same time, the use of combination fluke and worm products is discouraged. These products can put selection pressure on nematodes and liver fluke, speeding up the development of anthelmintic resistance.
There is some evidence that closantel combinations have a synergistic activity that may enhance their activity against resistant liver fluke (and the Barber's pole worm ) and also help delay the emergence of resistance to either class of compound. However, it is important to take specific advice from your vet or adviser before exploring this on an individual farm.