100% wot Mike said. I've been fighting ongoing respiratory illness for almost three months now and the reason it keeps getting me down is because our unusually mild, damp weather means they keep re-infecting eachother - the mud makes it very difficult for me to keep strict biosecurity among the pens. Ordinarily, I'd just increase the hot mashes, Livamol, and vitamins, and let them get it through their system - just like a human with a cold. Every year, at the turn of autumn into winter, they get colds.
One thing I've discovered is that when chooks get stressed, they sneeze. Dogs and cats get diarrhoea - chooks sneeze. Every chook I've brought in from outside has got an instant case of the sniffles when they get to my place. I've learnt not to take it personally
So, your chooks are unlikely to be a "carrier" of anything dire, and therefore should not require culling.
1. Plan to have them in quarantine for at least two weeks
. This is recommended when bringing in new birds to your existing flock anyway. If it means their movement is a bit restricted, harden your heart - they'll survive, and it's the best way to prevent a cold getting to your whole flock.
2. Give them TLC up the wazoo
. Bird vitamins in their water, hot mashes every day, Livamol in the food if you can get some (it's an excellent general livestock supplement and really helps birds in stressful times), cracked corn and additional protein in the form of cat food, bandsaw dust, or just plain mince.
3. Make sure their quarters are clean, dry, warm, protected, and free of draughts at night.
They'll gurgle and sneeze for about a week, even with this treatment, and then overnight the sniffles will just seem to clear up. Keep them away from your current flock for at least two weeks after you're convinced they're well, and try to plan a staged introduction, to keep stress limits down.
Good luck! I know full well how tiring it is to keep hearing chooks sneeze and splutter and gurgle ...
And also wot Candler said while I was writing this essay