I would try to keep the gnats and flies away from her, that can exacerbate your problems.Thank you so much for the help! The rooster is with the flock already and she was with them for a day before we had to separate her because they were picking on her. No symptoms of trouble breathing or anything. I don’t know about the odor but, I do notice a lot of gnats surrounding her. If it’s an infection, is it treatable?
Is it treatable? A lot of people do treat symptoms and infection from respiratory disease. Some send in samples/get testing to find out what they are dealing with.
Birds, even if recovered remain carriers and can pass it on to others.
Treatment options are in my previous post - I quoted it below in case you missed it.
Has she and the rooster already had contact with your existing flock?
Can the people take them back?
Looks like she has an eye infection to me - most common cause of infection is respiratory disease. Does she have a foul odor? Mycoplasma and Infectious Coryza come to mind with pus and swelling like that. It's possible it could be viral like Infectious Bronchitis or ILT, but you don't report other symptoms like sneezing, coughing or wheezing.
Without testing, it's really hard to know the exact cause of the infection. Respiratory diseases are contagious and depending on which one you are dealing with the bird and those that are exposed can be carriers of the disease for life. https://edis.ifas.ufl.edu/ps044
If you are wanting to treat her, then the eye needs to be flushed well with saline and that pus needs to be pressed out. Chicken pus will not dry up on it's own, it has to be taken out. After you clean up the eye, then apply an eye ointment like Terramycin. Do this 2-3 times a day. See the video below for some ideas of how to clean out the eye.
She may also need antibiotics - Tylan50 can be used, but it seems hard to find, check with your feed store to see if they have any in stock. If not, you can order Tylosin online. Let us know what you are able to find.
I'm very sorry that you ended up with a sick chicken. I would keep her hydrated. Poultry vitamins if you have them too. Try to encourage her to eat - if she won't eat dry feed, try it wet. Scrambled egg is sometimes enticing as well.
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