After you figure out what they have and they are cured.
Or you can tell people they are sick and you don't know what they have. Yeah, OK, not a good plan.
Where are you located?
Your quickest way out of this is to send one of the sickest birds to your state or national poultry lab for euthanasia and necropsy, treat accordingly and you're done.
In the future, outsiders shouldn't walk anywhere chickens are. I have disposable plastic boots I make people put on when they get out of their vehicle and they still aren't allowed near the runs or forage area.
The thing is that many things that cause coughing aren't treatable with antibiotics. They might not even be bacterial, it could be viral, fungal, protozoal, environmental or nutritional.
However, since outsiders have been in the hen house, it is most likely viral or bacterial.
Edited by ChickenCanoe - 11/25/15 at 9:55am