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Most Common Causes of Diarrhea in Cats

Updated On March 16th, 2016

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Reviewed By Christie Long, DVM

Diarrhea in cats is a frustrating problems for owners, veterinarians, and certainly for cats, although they can’t tell us directly how they feel about it. Usually in cases of chronic diarrhea (symptoms lasting more than a week) the cat usually seems to be feeling pretty well, but if left undiagnosed and untreated over time the patient can lose weight and start to show signs of general illness, like lethargy, poor haircoat, and dehydration.

These problems are due to the effects of reduced nutrient absorption over time. Whatever the inciting agent, nutrients and water aren’t absorbed into the body, and the food that’s ingested makes its way through the intestines much faster than it should, resulting in soft stool.

It can be difficult to pinpoint the exact cause of chronic diarrhea in cats. If the diarrhea doesn’t resolve on its own (or perhaps with the addition of a little dietary fiber, like canned pumpkin) in a few days, the cat needs to see a veterinarian for an examination and testing to determine what’s going on. Here’s more information about the possible causes of diarrhea in cats, and what we do about them.

Food sensitivity

Sometime an abrupt change in diet will cause a cat to develop diarrhea. Cats seem especially sensitive (i.e. more so than dogs) to sudden changes in food, so if you’re considering a switch do it slowly, incorporating larger and larger amounts of the new food in over the course of 10 - 14 days.

Some cats may not tolerate a specific food well. This may not be the result of a true food “allergy”, but if you’ve made an appropriately slow transition to a new food and your cat has continual or recurring diarrhea while eating it, there’s likely an ingredient in the food that your particular cat isn’t digesting well.

Lately we’ve seen a rise in the popularity of grain-free diets for felines. I like many of these diets, because I believe that feeding foods high in grains (and subsequently carbohydrates) has probably lead to the epidemic of feline obesity and diabetes we’re seeing. However, true “grain-sensitivity” is probably rare in cats, just like it is in people. Most of the time it’s unlikely that grains in cat foods are causing diarrhea.

Food allergy

Food allergies can develop at almost any age, from 6 months upwards. Lots of times we’ll see a cat do fine on a food for years before developing an allergy to the food, or more specifically to an ingredient in the food.

Diarrhea is probably the most common sign of a food allergy, but lots of other symptoms can go along with it, including intermittent vomiting, anal gland issues (including impaction of the glands, scooting, constant grooming of that area, and redness around the anus), and itchy red skin. Food allergies develop when the immune system reacts (inappropriately) to an ingredient in the pet’s food. Most of the time the allergy is to the protein component of the food, but sometimes it can be the carbohydrates in the food.

We understand animal allergies about as well as we understand allergies in people, and that’s to say, not very well. Why one animal can eat a food with no problems and another has horrible side effects - we just can’t say. But we do know that these pets greatly improve when we switch them to a hypoallergenic diet that’s specially designed not to engage the immune system.

There’s no good test for food allergies. Typically we perform what’s known as an elimination trial, using prescription hypoallergenic foods, and feeding only that food for at least 8 weeks. If the cat’s symptoms resolve, the diagnosis is made. If not, there’s more work to do.

Parasites

Good old parasites! We always mention these when talking about diarrhea in cats. They’re less likely if the cat doesn’t go outside, but I like to always consider the possibility for an indoor cat that lives with a dog, who can certainly bring parasites indoors. Also, one study showed that as much as 30% of household potting soil may contain roundworm eggs, and while it’s not clear if these eggs were capable of causing disease, cats that root around in houseplants might be at risk for GI parasites.

Most intestinal parasites are easily diagnosed on a fecal egg flotation test, which is cheap and routine. One exception is an organism called Tritrichomonas foetus, a somewhat uncommon parasite that’s impossible to find on a standard fecal test. It’s a common cause of diarrhea in kittens that come from crowded conditions, so when other causes of chronic diarrhea have been ruled out, you should always look for this one.

Not checking for parasites before going on to more invasive and expensive testing is a huge mistake, since de-worming treatments are cheap and easy.

Infection

Mild bacterial infections of the GI tract can cause diarrhea. These usually respond to antibiotics. Many times we’ll combine these with probiotic therapy. These infections usually clear up pretty quickly. But with persistent diarrhea that doesn’t respond to we’ll often consider whether the cat could have a serious systemic viral infection.

Feline Immunodeficiency Virus (FIV), Feline Infectious Peritonitis (FIP), and Feline Leukemia Virus (FeLV) are all serious viral infections that are passed from cat to cat under different circumstances. They can all cause chronic diarrhea, and in cases where a cat has had diarrhea without resolution for some time, and routine causes of diarrhea have been ruled out, it’s time to start considering these viruses as a possible cause.

Inflammatory bowel disease

The symptoms of inflammatory bowel disease are awfully similar to those caused by food allergies. Sometimes cats with IBD even get a little better with a change to a hypoallergenic diet, but the catch is that their signs don’t go away completely, or they come back again and again.

IBD occurs with severe inflammation of the lining of the large and/or small intestines. It’s another mystery to us, just like allergies, because we don’t know what causes it. IBD responds to treatment with drugs that suppress the immune response, so clearly it’s an aberrant immune reaction.

Diagnosis of IBD is made by taking a biopsy of the intestinal lining. A pathologist can determine if there are white blood cells infiltrating the lining, which signals the presence of IBD. While this may seem like an extremely invasive test, it’s important to know for sure whether the cat has IBD, for 2 reasons. The first is that treatment requires using immunosuppressive drugs for life, and these can have many side effects, so you’d want to be certain of your diagnosis. And second, certain cancers of the GI tract look a lot like IBD, and also respond to the same treatment, but clearly have a much different prognosis.

Cancer

Intestinal lymphosarcoma (LSA) is the most common cancer found in the GI tract of cats, and one of the most common feline cancers in general. It typically occurs in older cats, and can often be successfully treated for several years. As mentioned above, because LSA looks a lot like inflammatory bowel disease, accurate and definitive diagnosis is key to designing an appropriate and successful treatment regimen.

Hyperthyroidism

Diarrhea isn’t the only symptom you’ll see if your cat has an overactive thyroid, but it’s certainly a very common one. Typically found in middle-aged to older cats, hyperthyroidism occurs when a benign growth appears on the thyroid gland, causing overproduction of the hormones that regulate metabolism. Hyperthyroid cats have ravenous appetites, yet lose weight despite the fact, due to their ramped-up metabolic state. Diarrhea (and vomiting) are often follow-on problems.

Hyperthyroidism, if left untreated, can lead to serious and fatal heart disease. Detection is fairly easy with blood work, and treatment, which is aimed at either destroying the growth or deactivating the excess thyroid hormone, is usually very successful at controlling signs and improving the cat’s clinical picture.

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