Recurring canker in chicken

[OrpingtonManiac]

Last august, one of my chickens (aspen) died from canker. i now have 5 chickens, and have found canker in their mouths two separate times since. The first time (around january this year), the vet gave me a medicine called Tolracol, which we gave to all the chickens. The canker seemingly disappeared from their mouths, but then in June, one of the hens (juniper) started sneezing a lot. I looked in her throat and found canker in her mouth. It looked to be right inside the opening to her trachea (i'm not completely certain it is her trachea but i've compared our pictures to other pics on the internet and it looks like it is). The vet prescribed me tolracol again, along with metronidazole. A few weeks later, i found canker in another chickens (hazel) mouth and trachea. I've given her a dose of metronidazole and tolracol as the vet said to, and its been 5 days since her last dose. Again, i've found more canker in junipers mouth, and the canker in hazel's mouth hasnt disappeared, though there seems to be less.
I've looked on the internet and seen that giving them to much of the metronidazole might cause toxicity and kill them, so i don't know what to do.
The canker disappears for a few months and then comes back again, and i really don't want to lose another chicken.
Is there anything else i can do for them? I've seen there are other medicines i could give her but idk.

TIA

 

[sourland]

Your treatments sound spot on. In my experience(pigeons), canker was never completely irradicated only controlled. Stress of any sort could cause outbreaks. Scrupulous cleaning of waterers and feed pans was absolutely necessary as well as not allowing access to ground water.