Reader Question: A Gastrointestinal Cat Query. Is it Feline Colitis; a Colon Infection?

My 18 year-old cat has very foul smelling, watery diarrhea.  Occasionally there’s red blood in the diarrhea, and you can actually hear noises coming from her stomach.  She has a big appetite but is losing weight. She does have a thyroid problem.  Any advice about the diarrhea?

Thank you,

Dear Terry,
It sounds like your cat has colitis – inflammation of the large intestine.  When bleeding occurs from the stomach or small intestine, the blood gets digested, turning the stool black.  When bleeding occurs from the large intestine, it’s past the point where digestion occurs, so the blood will look red, which is what you’re describing.   There are many causes of colitis, including stress, a sudden change in diet, intestinal parasites, and inflammatory bowel disease. 
Hyperthyroidism is a glandular condition that is very common in elderly cats.  Cats with this condition produce excessive amounts of thyroid hormone, which causes cats to lose weight despite having an excellent appetite.  You indicated that your cat does have thyroid disease, but you didn’t mention if or how she was being treated. 
The first thing I would do is to have your cat’s thyroid status evaluated.  Assuming you are administering anti-thyroid medication, you may discover that the dose in no longer adequate and needs to be increased.  When you visit your veterinarian, be sure to bring a stool sample so it can be tested for intestinal parasites.  Diarrhea is sometimes seen in cats with uncontrolled or poorly controlled hyperthyroidism.  If your cat does need the thyroid medication dosage adjusted, you may find that this may halt the weight loss and resolve the diarrhea at the same time. If the thyroid is fine, and other common causes of weight loss in an older cat (kidney disease, diabetes) are ruled out, then a primary gastrointestinal disorder may be present.  Your veterinarian may want to initially prescribe a therapeutic diet (either a high-fiber diet or a hypoallergenic diet) and perhaps a probiotic to control the diarrhea.  If the diarrhea persists, more advance diagnostics may be warranted such as x-rays, ultrasound, or possibly endoscopy, although I would understand your reluctance to pursue an extensive diagnostic work-up for an 18 year-old cat.