I only found out now that there is a technical difference between Irritable Bowel Syndrome (IBS) and Inflammatory Bowel Disease (IBD). I had earlier erroneously thought the two were the same thing. My apologies for this.
https://www.petmd.com/cat/conditions/digestive/c_ct_irritable_bowel_syndrome
From the description of the difference in the link above, I think Bunny had IBD. His wasn’t a sudden onset. It’s been there since October last year.
What is Irritable Bowel Syndrome in Cats?
Irritable bowel syndrome (IBS) in cats is typically an acute (severe and sudden) onset episode of gastrointestinal distress.
Episodes of IBS in cats often occur in response to a stressful event, an intolerance or allergy to some component of the cat’s diet, or a change in the normal function of the colon.
What Is the Difference Between IBS and IBD in Cats?
While IBS in cats is severe and sudden inflammation of the gastrointestinal tract, IBD (inflammatory bowel disease) in cats is a syndrome characterized by chronic (long-term) inflammation of the mucosal lining of the intestinal tract.
IBD is usually the result of an underlying disease process such as a bacterial imbalance, food allergies or intolerances, or other genetic factors.
Definitive diagnosis of IBD requires biopsies of the stomach and intestinal tract via laparotomy (exploratory surgery) or endoscopy (investigation with a long, flexible tube).
Cats with IBD typically have thickened intestinal walls due to the chronic inflammatory response. Cats with IBS typically have normal biopsy results.