Pinched Mantle is not a disease , it is a symptom.
There is something causing it, which we dont know about ,
FW dips can help, sometimes, but sometimes they're not, and that's why the partial suceess.
What causing the pinched mantle can be various problems, bacteria, parasite, virus, etc.
We know so little about clams, we know actually nothing about clam diseases, it's not like fish that we know we have to treat this with copper and that with formalin-malachite.
With clams, we're actually just doing experiment as basiclly we know nothing much and there's nowhere to get more info.
If you read the Clam Bible=Knop book, you'll find out it's quite a basic book.