I honestly don't know the details on that. I know that you shouldn't treat birds that aren't sick. And I know that, if I hadn't treated, I would have lost all of my birds. So I took a chance and did it.
Wish I could give you a more technical answer on that. But... the doses I gave the...
When my chickens had it, I locked them in the coop for 2 days with Sulmet and Duramycin in their water. (They're free-ranged and I have a pond, so I had to be sure that the only water available to them was medicated.) I did have to cull the 8-wk-old chicks that got sick before I figured out...
Does she have any odd odor coming from her head? Facial swelling makes me first think about coryza.
I have The Chicken Health Handbook, which lists diseases by symptoms. Swollen around eyes leads to aspergillosis (rare,) infectious coryza (common,) paratyphoid (common,) cholera (not common,)...