If it is a parasite, that's pretty easy to clear up. Better that than IBS or colitis.
A few years ago, when Oscar had had his month-long bladder infection, I had switched to the vet food (I now do grain-free). It WAS more expensive, but he ate less - and it was saving me future expensive vet trips.
I also now follow the measuring instructions on the bag and have a measuring cup just for the food. That helps too. Re: your thread in Cat General, it might be more economical than free feeding.
"Common sense and a sense of humor are the same thing, moving at different speeds. A sense of humor is just common sense, dancing." -- William James