I think our hen may be sick
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She could be gasping, which can be a sign of a respiratory infection. Infectious bronchitis, MG, ILT, coryza, and aspergillosis are some of the common respiratory diseases in chickens that you can read about here: http://edis.ifas.ufl.edu/ps044
Make sure that your coop ventilation is good, that dust or ammonia odors are kept to a minimum, and keep the coop bedding clean and dry. Tylan 50 injectable is some times given orally or as an injection to treat bacterial respiratory infections, or to help prevent secondary infections. Dosage is 1/2-1 ml given ORALLY twice a day for 5 days.