As I said in my very first post, UTIs are very rare in mares. Urine pooling, fecal contamination, chronic infections and suturing are all connected to uterine infections.
The only time a mare's uterus can be flushed is when you infuse enough fluids laced with whatever you are using to treat the infection that you actually get part of it back out. Older mares and those with poor vulva conformation often times are also low backed and have quite a bit of age on them. This allows the mare's uterus to rest lower than the her vulva and lets it collect fluid in the uterus and the vaginal cavity. When you put a gloved hand and arm in a mare like this, you invariably reach in and down. I have literally 'scooped' several handfuls of urine and fluid out of the front part of a mare's vaginal cavity and uterus when they are conformed like this. Part of this condition is what contributes to the mare's rectum being pulled forward to exacerbate the entire situation. The end result is a chronically infected reproductive system (Not urinary tract at all) and a constant nasty discharge from the vulva from fluids over-flowing the uterus and vagina.
When we have treated mares like this, we have often used Oxytocin following infusing them. This causes the uterus to contract helping expel the fluid and urine.
I hope this clears up ay misunderstanding.