How do you treat a fistula in a horse?

With great difficulty - fistulous withers in horses is caused by bacteria or parasites in the withers that caused lots of inflammation and pus. When the pus tunnels its way out of the muscle and drains from an opening in the skin, it is said to have a fistula or a fistulous tract. Unfortunately by the time the infection is draining it is usually a severe infection and often is with bacteria/fungi/parasites that are difficult to kill. Aggressive therapy would include intravenous antimicrobials and surgery to cut away the majority of the infection.