While this forum provides great info in caring for our koi, it can be depressing and sad at times. In an attempt to balance it out a bit I have a positive story.
One of our favorite koi came down with a nasty bacteria infection last summer that started at it's mouth and eventually spread throughout it's body.
Luckily we have a vet near us who sees koi. I put him in a big cooler and took him to the vet. At the clinic the vet was able to check him out. Treatment prescribed were injections of antibiotics into his sides every two days for two weeks. It went well but the infection didn't completely clear up. Another two weeks was needed and even with that, the vet gave the fish a 50-50 chance of survival.
While he was getting healthier, winter was coming and I couldn't maintain the hospital tank through the winter. He had to go back into the pond.
He survived the winter but still wasn't 100% this spring.
But over this summer he's gotten healthier and healthier. And while his mouth is probably permanently scarred he's pretty much back to normal.
Bottom line, koi can be very resilient and while not every ending can be happy, this one was!