It's been 5 weeks since we discovered 4 chickens in our flock had Coryza. We decided it would be best to cull them and then burn their bodies. We sanitized everything and even burned the shavings from the coop too. Yesterday when I was dusting the chickens for lice I noticed that 3 of them had runny noses. It doesn't smell anything like the others who had Coryza. Just smells like regular chicken breath. So I'm wondering is it still possible that after 5 weeks some of my chickens got Coryza from the others that we culled? There were absolutely no signs over the past weeks until now. I have been carefully checking all of the chickens. So now if it's now Coryza can I treat them with Tylan 50? That's what I have for respiratory problems. What would be the dosage for standard size chickens? Any advice and opinions would be greatly appreciated. Thanks!