Hi,
You should probably bypass the Terramycin and Duramycin-those versions are less strong, less effective than the Tylan 50-your goal right now is to subdue the symptoms so no secondary illness can take hold also
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Tylan 50 will help your birds heal/recover from the cough, sneezing etc. BUT it will not cure it. Bottom line is your entire flock has been exposed now even though only some are showing outward signs of the illness. Chickens don't get 'colds', they get viruses and bacterial infections etc. MS and MG are the biggest offenders of respiratory issues. But you will only be guessing at what your birds have unless you take some live birds and dead birds to be necropsied by your local agricultural vet or exchange.
The best you can do besides testing for illness is to disinfect your waterers, feeders DAILY, and clean your roosts etc. and keep bedding as clean as possible. Give Tylan 50 injections (which work quickest & are more effective than oral doses) at a rate of 1/2cc subcutaneously into the breast for full size chickens & 1/4cc for bantam size (switching breast sides each injection) or to inject behind the neck between the shoulder blades subcutaneously 2x's per day for a duration of 5 days. The reason you inject 2x's a day is that Tylan 50 is effective in the system for a period of only about 8 hours. In more severe cases where your bird is struggling to breathe or is very very lethargic, you can give Tylan 50 every 3x's daily.
Injections can be intimidating. Google how to give a chicken an injection and you should find the exact step by step instructions. Tractor supply carries tylan 50, syringes, and you should use about a 22 gauge needle.
Isolating your sick from the 'healthy' ones while under treatment (even though they have all been exposed now) is recommended in order to reduce the airborne and direct swapping of fluids. Isolating will help control the spread of the symptoms, but your birds will always be carriers-there is no cure-no vaccinations to cure what disease they may have. "Vaccinations", only help control and reduce the symptoms of the illnesses=they do not eradicate the disease. You can reduce illness outbreaks by minimizing stress, practicing cleanliness, good nutrition, and antibiotics to reduce the chance of any secondary issues occurring.