Feline diarrhea: Let the diagnostic clues flow

dvm360dvm360 June 2020
Volume 51
Issue 6

Causes of feline diarrhea range from simple infection to cancer. By characterizing the diarrhea and running simple tests, the practitioner can identify the pathology and stop the mess.

cat in litter box

Lightspruch / stock.adobe.com

The first step in managing diarrhea is characterizing it, according to Scott Owens, DVM, MS, DACVIM, an internist at MedVet Indianapolis. “There are many causes of diarrhea in cats,” Dr. Owens said during a recent Fetch dvm360 conference. “Is it acute or chronic? Is is intra- or extra-intestinal? How severely are the intestines affected?”

Differentiating between small- and large-bowel diarrhea can streamline the diagnostic process. Small-bowel diarrhea, which is more common in cats, tends to be subtle and smoldering. It typically features large volume, low frequency, marked chronicity, associated weight loss and lack of straining, mucus or blood.

Large-bowel diarrhea, more common in dogs and often associated with dietary indiscretion, is more dramatic because it is generally acute and characterized by frequent trips to the litter box, tenesmus and bloody, mucoid stool. In cats, Dr. Owens said, diarrhea is usually mixed bowel in nature.

Diagnostic steps

Before anything else, a fecal test should be run to rule out parasites. Large-bowel diarrhea signals intestinal parasites in cats and, particularly in kittens, is typically caused by roundworms, hookworms and coccidia. False-negatives are possible, as ova are shed intermittently, so a broad-spectrum dewormer should be considered.

Routine blood work to detect underlying causes of diarrhea might point to pancreatitis (low sensitivity for amylase/lipase), liver disease or nephropathy. In older cats, especially those with concurrent weight loss, a thyroxine (T4) test should be run to assess for hyperthyroidism.

If diarrhea is acute and severe, abdominal ultrasonography can be used to evaluate the five layers of the intestines (lumen, mucosa, submucosa, muscularis and serosa), mesenteric lymph nodes, pancreas and common bile duct.

Food-responsive diarrhea

For chronic and non–life-threatening diarrhea, a feeding challenge might uncover food-responsive diarrhea. A hydrolyzed diet, such as Royal Canin Ultamino or Hill’s Prescription Diet z/d, or a novel protein diet, is fed exclusively for a minimum of three weeks.

Dr. Owens summarized a study of 55 cats with idiopathic diarrhea.1 Thirty percent responded to hypoallergenic diets and were diagnosed with food allergy. Large-bowel diarrhea and wheat/gluten sensitivity were found to be smoking guns in cats.

Dr. Owens said he sees a lower prevalence of feline food allergy in his own practice. While no definitive testing is available, he noted that he is more likely to diagnose food allergy in cats that are also pruritic.


Infectious diarrhea, more prevalent in young cats and/or those from catteries, generally causes acute large-bowel diarrhea. A fecal polymerase chain reaction (PCR) panel can identify common agents: Campylobacter, Clostridium, Salmonella, Giardia, Cryptosporidium, coronavirus, feline panleukopenia virus, Toxoplasma and Tritrichomonas.

Antibiotics, such as metronidazole and tylosin, can treat many of these pathogens effectively, as well as the general dysbiosis associated with them. Probiotics such as FortiFlora (Purina Pro Plan) can also be used to level out the disrupted gut flora that attends primary infectious and other enteritides.

The IBD-cancer conundrum

Inflammatory bowel disease (IBD) and intestinal lymphoma can mimic one another. Both feature chronic diarrhea, typically small bowel, accompanied by malabsorption and weight loss. On physical examination, body condition is typically poor. Abdominal palpation may reveal thickened, ropey intestines and some degree of sensitivity.

Small-cell, versus large-cell, lymphoma, is often manageable, particularly the T-cell version. But diagnosis is hampered by the fact that lymphoma behaves like IBD.

Furthermore, histopathology sometimes fails to differentiate between the two, although immunohistochemistry and PARR testing (PCR for antigen receptor rearrangements) can identify monoclonal lymphocyte populations suggestive of lymphoma.

“A very high percentage of cats might fly through as IBD,” Dr. Owens said. “But when you get to PARR, you see that 80% of them have lymphoma.” Dr. Owens shared two look-alike case examples.

Case 1: Sophie

Sophie, a 14-year-old female, spayed, domestic medium-hair cat, had a six-month history of watery to soft small-bowel diarrhea and progressive weight loss. An indoor-outdoor cat, Sophie was active and her appetite was good. On physical exam, her body condition score (BCS) was 3/9, she had some muscle wasting and her coat was unkempt. Her intestines felt ropey, and her abdomen tensed on palpation.

Sophie’s fecal was negative. Urinalysis, blood chemistry and T4/free T4 results were normal. Her complete blood count (CBC) was normal but for a mild, nonregenerative anemia. Specific gastrointestinal (GI) tests—indicated in cases of chronic enteropathy with weight loss—were conducted. Sophie’s trypsin-like immunoreactivity (TLI) test was normal, essentially ruling out exocrine pancreatic insufficiency (EPI); her Spec fPL (Idexx Laboratories) was in the gray zone, indicating mild pancreatitis or triaditis; her folate level was normal; and her cobalamin level was low, suggesting infiltrative small-bowel disease with intestinal malabsorption.

Abdominal ultrasonography revealed hypertrophied small intestines and, more importantly, diffuse thickening of the muscle. “This very prominent thickening of the muscularis layer stands out in cats with prominent GI disease,” Dr. Owens said.

The primary diagnostic differentials were IBD and GI small cell lymphoma, with idiopathic enteritis,chronic pancreatitis and intestinal parasitism also on the list. Endoscopy was performed and biopsy samples were taken: The intestinal mucosa was inflamed and gelatinous. Histopathology showed massive lymphocyte infiltration, and diffuse small cell lymphoma was diagnosed.

After the first few weeks of treatment, which included corticosteroids, metronidazole, chlorambucil and vitamin B12, Sophie’s diarrhea began to resolve and her weight rebounded. She lived another few years on the chlorambucil protocol.

Case 2: Mona

Mona, a 14.5-year-old female, spayed Manx cat, had a 5-month history of diarrhea, some vomiting and inappetence, and weight loss. On physical exam, Mona had a BCS of 3/9, muscle wasting, unthrifty appearance, patchy alopecia, and mild pain on cranial abdomen palpation. Her CBC and T4 were normal, and her alanine aminotransferase was mildly elevated. Urinalysis showed epithelial cells and white blood cells. Her Spec fPl was normal, her folate high (sometimes seen with dysbiosis) and her cobalamin low.

Abdominal ultrasound revealed prominent mesenteric lymph nodes, mildly thickened intestinal muscularis and a large bladder stone. The differential diagnosis included IBD, GI small cell lymphoma and EPI (TLI results pending).

During surgery to remove the bladder stone, the pancreas was observed to be shrunken and the bowel thickened. Intestinal and mesenteric lymph node biopsies were taken. Histopathology revealed reactive lymph nodes and a moderate, diffuse lymphoplasmacytic enteritis, as well as numerous mucus-producing goblet cells. The diagnosis was mild-to-moderate IBD, plus EPI (TLI ultimately came back low).

Mona was treated with pancreatic enzymes, a steroid taper and a course of cobalamin injections. She lived another four years.

Exocrine pancreatic insufficiency

EPI is uncommon in cats. The main clinical sign is weight loss. Affected cats also experience diarrhea due to maldigestion, malabsorption and dysbiosis. “It’s not uncommon to see some pretty severe bacterial overgrowth with EPI,” Dr. Owens said.

In cats, the chief underlying cause of EPI is chronic pancreatitis. Acinar atrophy can also lead to EPI, but less so in cats than in dogs.

Dr. Capuzzi is a small animal veterinarian and journalist based in the Philadelphia area.


1. Guilford WG, Jones BR, Markwell PJ, et al. Food sensitivity in cats with chronic idio[athic gastrointestinal problems. J Vet Intern Med 2001;15:7-13.

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