Duck with swollen hocks

Hi folks,
I haven't posted on here for quite a while but I'm having a problem with two of my ducks. One has recently developed swollen hocks, both of them, and is having difficulty walking. One of my other ducks today has started to limp and her one hock is swollen. Could this be niacin deficiency? They are both 2 years old and I thought that niacin deficiency only affected ducklings. If it isn't that, any ideas what it could be?

Many thanks in anticipation