Muscovy duck

I need some advice for my Muscovy ducks. I have 4 adults (a year old) 2 are from the same seller and 2 are from another. All was well and fine untel the first of July. One by one they all went 100% off there feet. Totally limp web feet (see attached photo) I thought something hurt them. I started giving them vitamin supplements & electrolytes daily (took a good week) to get them walking again. But if I go just a few days with out giving them a big mouth full they end right back unable to walk again. (A little info about there daily life) free roaming, large pond, lots of corn scratch, laying mash) 2 other breeds of ducks it does not happen to them. The very sad part one has 8 baby ducks 3 weeks old and one has the very same thing going on. Any help solving my problem is greatly appreciate.

I have read somewhere that worms makes ducks go off their legs, or at least lame. I'll see if I can find it again.

Here you are, might be worth a try.

poultrykeeper.com/duck-keeping/worming-ducks/

www.waterfowl.org.uk/docs/IS_ailments.pdf

www.majesticwaterfowl.org/diagnostic%20chart.htm