I think it is more likely to be an injury than an infection but anything is possible. Are you thinking bumble foot? These are wild ducks who roam as they please so it's hard to know what it could be, but the fact that it is only females makes me think maybe there is a rogue male out there...
I enjoy a group of Muscovies that visit my backyard daily. A week ago one female came with a bad right leg that she couldn't put weight on. A few days later a second female showed up with a bad right leg. I thought it was odd. But then a day later, a third female showed up with the exact same...