Do not use this site again
I would suggest - if there is room for you to ladder - that you ladder, well away from the troublesome site, while ongoing AB's attempt to settle the infection down, remembering that this can take some time.
Remember a tract is, in essence, a scar tissue tract and that antibiotics may not enter scar tissue easily. Importantly, do not needle near the damaged part of the fistula. Have your nephrologist or vascular surgeon advise you if an surgical 'excision' may be needed.