I just had a outbreak of a pretty awful respiratory infection in my six month old black and blue Copper Marans pullets. I started them out on Duramycin 10 but it wasn't touching the bug. I ended up treating them with injections of Tylan 50 for three days. They began to show almost immediate improvement after the first shot by the next morning. My dilemma is I can't find anywhere how long these things run their course and if they come down with this junk again do I retreat with Tylan 50 again? I don't want to over do it.