I have a 12 week old Muscovy duckling that is limping. She barely uses her right leg. The "knee" joint is warm and swollen. There was no known injury. When she first started to limp 1 week ago I checked her feet and they were fine. The joint was not swollen or warm then. It has gotten progressively worse. She is being fed nonmedicated chick & duckling starter feed. None of my other ducklings have this problem. I have put her in a pen because she is having difficulty getting around. She uses her left leg fine but does not want to bear any weight on the right leg and drags it. Any suggestions of what the problem is and what to do for it?