You would never be able to irradicate the possibility of reinfection unless your bun was moved into an irradiated room during the course & never put back where any other bunnies had been. The damn things can survive anything but the most harsh disinfectants & you would have to scrub every spare inch until you wore your brush bare, in all nooks & crannies & never let them on to the same patch of grass.. It is simply impossible. My EC bun sooty had a 30 days course. Everything was bleached every two days through the course & the floors, skirting board & walls washed daily (he had urinary incontinence) & his new girlfriend shared the course. He was 2.6kg & had two grads daily. His symptoms came back in three months. Another 1 month course stopped it but the symptoms came back again about 6 months after that. The final full course knocked it on the head & with a few 9 days in between we seem to have managed to keep him ok.
It's my belief that it is probably a little like MRSA & other bacteria. Many of us are carriers but it only causes us problems when we are weak and our imune systems can't cope with it. As you can never guarantee that you are free of it I would worm all my buns on a regular basis, be it with a 9 day or what seems to be recomended 28 day once or twice a year.