Yes, that's true, it is a different tapeworm in dogs ... but my thinking is that if you have one you might have the other
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When was the last FEC count done of this lamb? Did they check for fluke? is it losing weight? or any other symptoms apart from a dirty bottom?
Worm egg hatches are dependant on weather conditions so although she was clear earlier in the year that doesn't mean she's clear now. It's all a matter of getting the timing right with your wormer doses. She may be suffering more from the tapeworm because there is something else affecting her (be it other worms or something else).
What was the super expensive wormer that your vet suggested? Maybe its one of the new ones that no worms are resistant to yet (any why they suggested using it). What's the history of your fields, how long have your sheep been on them, and what was there before? You might have inherited some resistant worm strains.
Btw, I pulled a 50cm long tapeworm segment out of a lambs bum last year, now that was unpleasant!!
Only the white/BZ/group 1 class of wormer will kill the tapeworm, so if you want to get rid of that then white is the way to go (but like I say it does mean you don't have other problems). This document lists all the products available and what they kill:
http://beefandlamb.ahdb.org.uk/wp/wp-content/uploads/2015/03/BRP-Parasite-control-guide-030315.pdf