Is it possibly swayback? (And unrelated to tapeworm or drenching.)
As a general rule, we are told that the tapeworm which has the sheep as its terminal host (so that you see segments in the poo) doesn't do any harm to the sheep, although I guess it does compete for nutrients to an extent.
It used to be the case that most general wormers did not therefore address this worm, as there was not believed to be any economic impact of the worm.
However it is scary to see a pile of tapeworm segments in your sheep's poo! And you feel you want to do something about it! So now several wormers have been tested and are licensed as effective against tapeworm - but if you do the research you will see that only in extreme infestation is the tapeworm really causing the sheep any problem.
The tapeworm which affects sheep and which does have a marked economic impact is the tapeworm which has the sheep as the intermediate host and the canid as the terminal host. You do not see segments in the sheep's poo with this one, you see only condemned meat at the abattoir. (The dog excretes the segments, the sheep eat the eggs from blades of grass.) There is nothing you can do to the sheep to prevent this. Control is by preventing unwormed dogs from defaecating where your sheep graze.
:bookmark: sheep tapeworm