It's also worth mentioning that (a), as far as I understand it, worm tests don't show fluke unless you specifically ask for a fluke test, so if you've had a worm test and no-one mentioned fluke it does not mean your sheep don't have fluke and (b) a sheep can have fluke and not have any fluke eggs in the poo, it depends on where the fluke is in its lifecycle - so if you've had a fluke test and no fluke was found it still does not mean your sheep don't have fluke