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Sounds like a respiratory disease like MG or MS. A cheap treatment would be Duramycin-10. My State Vet said to mix 5 teaspoons per gallon of water for all birds over 4 weeks. Give for 7 days. If you want to give them injections, you can get Tylan. I'm not familiar with it and it's dosing but you can find that info in some of the threads here.
I would urge you to find a vet or state lab if you lose a bird to this so they can fully diagnose what you are dealing with. Both diseases make carriers of the surviving flock and can spread it to any new birds you get. Here is a link to poultry mycoplasmas.
Edited by nuttinfancy - 3/24/14 at 10:22am
If there arnt any environmental issues causing the symptoms you're observing, it could very well be a respiratory disease. The proper dosage for duramycin 10 at the 400mg dose is 1/2 tablespoon per gallon of water, the 800mg dose is 1 tablespoon per gallon of water. For either dosages, it should be given 7 to 14 days. There's a 21 day withdrawal period after the last dosing of duramycin 10.
As always with respiratory diseases, I recommend that sick birds be culled.
The problem with respiratory disease is that they have all been exposed either by contact with you or by being housed together so the vet would tell you to cull them all and wait a month at least to ensure the disease is gone. If you are raising these chickens as pets, you may elect to keep them but you must know that you are responsible for making sure to not spread a disease to other chickens and their owners. When working with sick (or carrier) birds you should wash and change clothes before going to the feed store or even grocery store. You risk carrying the disease on your tires (especially if they free-range) to other places, and you should never sell your birds nor give them away. Any bird you bring to the property could develop the disease and would become a carrier also.
I recommend contacting the Ag Dept in your state or looking for a State poultry lab for more assistance. They are usually more than happy to help you figure out what is best for you in managing your flock. You may be able to offer a sick chicken for free (in most states) testing to find out exactly what you are dealing with before you decide to cull or not.
Here is what others have said about eating them. That is just mycoplasma but as stated, respiratory issues are in the respiratory tract and that part is discarded. I wouldn't personally eat one that is showing any signs of sickness. Also you should wait 14-21 days after the last dose of antibiotics before processing. Hope that helps