Do the other hens ever pick on this one?
Do you keep any antibiotics on hand for your animals?
Can you bring her inside?
Is there discharge inside her mouth as well?
What human antibiotics do you have?
Generic names if you can? :)
The closest disease to what symptoms you are seeing would be histomoniasis (blackhead) disease. This is a protozoal parasite. This parasite is carried in chickens but most do not sucuumb to infection.
Another possibility is trichomoniasis. Again this is a protozoal parasite.
A more common infection in chickens.
Can you tell me what she weighs?
I will take me a few minutes to calculate her dose.
Be right back.
Take the capsule and add it to a small amount of water. You will want to divide the water into 4 parts. Each one part (1/4) will be a dose of antibiotics. So start with 4 teaspoons of water. If the antibiotic dissolves well enough then the dose will be 1 teaspoon. If not then go to 8 teaspoons of water and the dose will be 2 teaspoons.
You will want to give this medication every 6 hours. I would continue for 10 days as long as she is doing well. If not then continue the antibiotic until she has improved or until you run out.
Tomorrow I would recommend going to your local farm store and looking at what protozoal treatment they offer. Most of these are mixed into the drinking water and should be offered to your other 2 chickens for whatever the treatment course is for the medication they have available.
Depending upon which of these two conditions she has will impact on how she does long term. Histomoniasis can have a long course 8-10 days before they become sick enough to die. Trichomoniasis infected birds can resolve. The adult animals do better than the immature birds.
Can I answer anything else?
Yes these parasites can come from wild birds.
This is why I would recommend getting a antiprotozoal treatment for chickens at your local feed/livestock store. Mix it into their drinking water for the prescribed time to help prevent any problems in them as well.
I am not familiar with herban. A quick google just brought up spa products.