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Hello. Thanks for the question.
This could still be IBS or IBD (inflammatory bowel disease). The diagnosis is usually made with biopsies from the small intestine- ideally the ileum. These are obtained surgically. If you want to confirm a diagnosis, that is the way to go.
I would probably try a hypoallergenic diet to completely rule out a food allergy. Even with a single novel protein approach, we can still have food allergies. To completely rule out food allergies, we use a hypoallergenic diet like Hill's Z/D or Royal Canin Hypoallergenic. I would try one of those diets and give it 2-4 months to see if that gets the signs under control.
When there is a flare up of bloody stool, I usually put them on a short course of Metronidazole. It's an antibiotic, but it has some anti-inflammatory properties in the gut so that's why I like it for IBD flare ups in dogs.
There is not going to be a cure here, but we just have to play with meds and diets until we get the right mix to keep the signs at bay. When there is the inevitable flare up, we treat more aggressively and then get back down to the maintenance dose.