Denagard aka Tiamulin treats only Mycoplasma Gallisepticum (MG) in poultry. MG is extremely contagious to other birds and there are more visible symptoms than an infected eye.
Birds infected with MG are carriers for life and the disease is passed through the egg to chicks if they survive.
MG infected flocks must be quarantined; no new birds in, no birds to be sold or given away. It's best to cull the flock, disinfect everything including inside the coop, feeders/waterers, roosts, nest boxes etc...
The disease stays in the environment for 3 days due to the lack of a cell wall around the bacteria. It would be best to wait one month before repopulating with new birds.
Sometimes birds can have two respiratory diseases at the same time such as Coryza and MG. Coryza has similar symptoms as MG but with a foul odor around the head area. It's best to cull, sick birds wont lay eggs much less eat or drink.