I'm wondering if it might be a bout of giardia (or even coccidia). Giardia is very, very hard to spot in a fecal test, so it gets missed a lot. Conditions have to be "just right" to catch it on the slide.
Whipworms can also be very tricky to spot on a slide. I've had many shelter techs miss them, only to have our rescue's vet find them a week or two later. Re-running the fecal test is possibly worth doing. I have known a lot of dogs who had "negative" fecal tests one week who then had "positive" tests a week later upon being re-tested, when we were skeptical of the "negative" result.
With that many food changes, I wonder if the frequent food changes themselves may be stoking this. That's a lot of foods -- how long were you on each one of them? Have you tried novel proteins and a limited ingredient diet long enough (~10 weeks) to see if it's an allergy?
What probiotic did you use? For how long? What about digestive enzymes?
Last edited by Magwart; 06-10-2014 at 08:37 PM.