Colitis is the inflammation of the lining of the colon (large intestine). It may be chronic, symptoms have been present for 14 to 21 days, or acute (sudden onset).
The colon is responsible for extracting water from the feces and storing fecal matter prior to evacuation (bowel movement).
Inflammatory bowel diseases are classified according to the type of inflammatory cell infiltrating the gastrointestinal wall. Lymphocytic-plasmacytic enterocolitis is the most common form of IBD. Lymphocytes (a type of white blood cell) and plasma cells (antibody producing cells) are the predominant types of inflammatory cells present in the mucosa of the small and large intestine. Eosinophilic Enterocolitis is the second most common form of IBD, eosinophils may be found in the stomach, small intestine or colon, Granulomatous (Regional) Enteritis in which macrophages are found in the lower small intestine and colon.
If the inflammation is restricted to the large intestine it is referred to as colitis, if the small intestine is involved it is referred to as enteritis and if both the large and small intestine are involved it is referred to enterocolitis, if the stomach is involved it is referred to as gastritis. 
What causes colitis in cats?
There are several causes of colitis including:
Certain bacteria(Salmonella and Campylobacter), viruses, fungi.
Stress – Moving house, new family member, change in routine etc.
What are the symptoms of colitis in cats?
The most obvious sign of colitis is diarrhea, possibly outside the litter tray because of the ‘urgency’ to go. The stool is often soft or watery and may contain blood or mucus. Other symptoms may include;
Straining (tenesmus) in the litter tray causing discomfort.
Diarrhea (the stool may start out normal but finish off loose or watery)
There may be blood or mucus in the stool.
Reduced appetite (anorexia).
Chronic colitis can lead to weight loss.
How is colitis diagnosed?
Your veterinarian will perform a physical examination of your cat and obtain a medical history, including symptoms you have noticed. Your veterinarian may be able to determine a cause depending on how the colitis has presented. Was it acute (sudden onset), chronic (lasting several weeks) or episodic (comes and goes)?
Fecal test for bacteria, nematodes (worms) and Giardia.
Complete blood count. Most cases will reveal a normal CBC but some cats may have become anemic. Higher numbers of white blood cells may be observed.
Chemistry profile (biochemistry profile) to evaluate for diabetes mellitus, liver disease, renal disease.
X-Rays/Ultrasound don’t help diagnose colitis but are useful to rule out other medical conditions such as cancer.
Colonoscopy and biopsy to determine the type if inflammatory cell present, check for cancer.
Feline trypsin-like immunoreactivity (fTLI) to evaluate for pancreatitis.
FIV and FeLV tests may suggest if a secondary disease is present.
Total T4 (TT4) to evaluate for hyperthyroidism as diarrhea can be a symptom.
How is colitis treated?
If a cause can be identified then elimination of the cause.
A highly digestible, low-fat diet containing a novel protein should be instituted.
If the colon is affected, a high fibre diet should also be provided.
De-worming medications for parasitic worms.
If necessary, supportive care such as fluids to treat dehydration.
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