Canker is very contagious, and many would cull a chicken in the flock with canker. In backyard flocks many will chose to separate the bird and treat with metronidazole 250 mg orally every day. You can buy FishZole online or get the metronidazole from a vet locally, but split the tablet into, and give 1/2 in the morning and half in the evening. Clean and bleach out your waterers, and change water daily to prevent the spread. Once a month you can use 1/4 tsp of acidified copper sulfate per gallon of water for 3 days to help to prevent the disease in your flock. Good luck, and if you need more info (since I have never treated canker,) you can go to the top of this page, and search canker threads.