Have you tryed the praziquantel too?
Do you seen some exemplares of this parasites?
Try to identify the parasites first,becouse the medication is different,dependent on species.
If you suspect drug resistance there is some drugs ,in case of cestodes,the paramomycin-sulfate,in case of nematodes the verapamil can break the drug resistence.
This drugs are well nown from the cancer therapies.