Excellent page here, would highly recommend that people read it.
http://www.scops.org.uk/endoparasites-liver-fluke.htmlIf you've not got the time to read it all scan through to the two tables. Information on what drugs to treat when.
The other thing to bear in mind is that your animals will keep putting eggs out into pasture even after you've killed the adult fluke (the eggs store in the gall bladder for up to three weeks) so you really really need to do before and after Faecal Egg Counts so you can see a reduction if the medicine has worked.
A quick low down:
Lots of young fluke ingested at once munch through the liver can cause sudden death or death in 2 weeks. This is acute and subacute fluke,
Infection at a low level over a long period of time gives chronic fluke, affects weight gain, fertility, wool yield and anaemia.
Triclabendazole drugs target the very young fluke so is good for use against acute and subacute fluke.
Closantel based drugs target older fluke than triclabendazole but still young. Treating with this means you will miss very young fluke and they can still cause actute or sub acute fluke or chronic fluke depending on the number.
Other drugs just target adults. Fine to use at times of year when you should only have adult fluke.
All drugs only act at the time of treatment so as soon as you turn out they can be re-infected.
Also depending on the weather and how the seasons have been, and your ground you may get fluke at times you're not expecting.
Hope that helps. My brain is a bit fuzzy at the moment so the website may be clearer.
Dans