Thanks, HersheysKiss. I did have some reservations about the "whatever we give them will kill T. foetus" comment too, since everything that I've seen indicates just what you said, that only ronidazole works on it. The vet only mentioned amoxicillan, Flagyl, and Tylosin as options.
I'm not so worried about T. foetus being there, though, because they always take a sample from a live cat, they do both a smear and a flotation every time, and they look for giardia, which T. foetus is supposed to look a lot like. I could be overconfident, of course -- I need to do more research on it than I have.
But another factor is that the diarrhea isn't constant at all. It usually lasts a few days at most in each cat, and they pass it back and forth. I've been able to make the symptoms go away entirely for months at a time by going crazy on the litter box hygiene -- weekly bleach-sterilization of the litter boxes and the room they're in, using only World's Best Cat Litter to keep the dust down. I'm spending $200+ and 16+ hours a month on litter box cleaning. But every time I think I've beaten it, and relax the cleaning even a little bit, whatever-it-is comes back. In your experience, would that be characteristic of T. foetus ?
Well, I have lots more reading to do, including your latest article -- thank you!
I'll post the test results when I get them. And I hope you can get a diagnosis and safe remedy for your little one.