I had one on a bull last year and the first I have ever had. I think it came from a shot that I may have given but may have been farther back also. The first time I drained it and opened it up, flushed it out and got him back up about two more times to flush out and make sure it was draining. It came back in about 6 months and took him to vet this time. We drained and found a tract inside a tract that went deeper than I could shove my finger in. The vet cut and sawed on scar tissue until I was getting scared. We flushed it out and he would let me flush it later to a point and it finally healed up and has been about 6 months again. It seems like it is finished but I still wonder if it will do it again.