What breed of duck and how old is she?
Many years ago, I had two young Welsh Harlequin’s that both had something that sounds and looks exactly like what you’re describing. I could not find a bumble to work on because the infection was inside the joint. I even tried cutting it open carefully and there was just blood so I stopped.I ended up taking them both to the vet who suggested surgery to clean out the toe joint and then antibiotics , but I could not afford the surgery, so I took the antibiotics and that was enough to clear it up.
Well, I didn’t take them myself lolI took them from him and gave them to the Ducks lol
A couple years later, one of them relapsed so all I did was give her a round of antibiotics didn’t even visit the vet this time and it took care of it. She lived to be 10 years old and passed away recently from something unrelated .
Perhaps you can try an antibiotic. I don’t remember what kind the Vet prescribed. It could be that a simple amoxicillin could clear it up or perhaps Baytril.
Do you have any duck antibiotics on hand?