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Why Does My Dog & Cat Have Diarrhea?

Ron Hines DVM PhD 10/26

Diarrhea accidents in your pet often warrant a rush to the veterinarian. This article tells you how I go about discovering the cause of diarrhea in you pet and some of the ways I treat it. Not every cause of diarrhea or medication is covered but the most important ones are.

I limit this article to problems in dogs cats and ferrets – but it could apply as well to any of the exotic and wild animals I treat.

I am always delighted when a client actually brings me a sample of the mess that their pet made. In more than half the cases, the diagnosis can be made from a fecal specimen and history alone. Try to bring in a fresh specimen – no need for more than a few tablespoons full.

Because my patients can’t talk, the history that you give me is extremely important. The first question I will ask you is whether this is a sudden acute problem or if it is chronic and has been going on quite a while. The next thing I will ask is if you fed something unusual to the pet or if torn remnants of some household object are strewn about the house.

Sudden diarrhea is most serious in very young, very old and very small pets. They can quickly become severely dehydrated. Dehydration in small pets and frail elderly animals can be fatal because their surface area is large in proportion to their body weight. As dehydration progresses these pets have trouble maintaining their body temperature and may lapse into coma. I hospitalize all pets in this group immediately and concentrate on maintaining hydration and body temperature. Intestinal inflammation in young pets with diarrhea causes the intestines to thrash about vigorously. Intestines sometimes tie themselves in knots (volvulus) or fold in upon themselves (intussusception). Both these conditions can be life threatening.

High Intestinal Problem vs. Low Intestinal Problem:

Cases of diarrhea fall into two groups in approximately equal numbers. In the first group, the problem is in the small intestine and in the second group in the large intestine, cecum or rectum. High diarrheas are the most dangerous. When the problem is high in the small intestine the pet voids large amounts of stool, but the frequency of bowel movements is not increased. If there is blood in this stool it is chocolate-colored, not bright red and there is no straining when these pets eliminate. The stool is never mucoid. These pets may also have a tummy ach. The second group have a problem in their large intestine. These pets have frequent, uncomfortable eliminations. When they go, the amount of stool is small and often covered with mucus. Frequently the stool is coated with flecks of bright red or rust-colored blood. These pets often sit straining to defecate without success. Vomiting may accompany high small intestinal problems but never low colonic ones.

A Primary or Secondary Problem:

I then have to decide whether the problem is intestinal or the result of problems in some other organ. I do a physical examination and perhaps laboratory blood analysis to see if the dog has signs of other illnesses. Generally, there are very few abnormal blood work results when the problems is in the intestine itself. In primary intestinal disease, abnormal physical exam findings are limited to an inflamed, thicken, painful intestine. With primary intestinal problems I can often detect enlarged lymph nodes that collect lymph from the intestines.

Diarrhea also occurs secondary to non-enteric or non-intestinal diseases of the organs of the body. When I am fortunate, blood work panels and a careful physical examination identify these diseases. Kidney failure causing uremia is a common cause of diarrhea in dogs, cats and ferrets. So is liver disease or hepatitis. Addison’s disease or an under active adrenal gland as well as an over-active thyroid or hyperthyroidism also cause diarrhea in dogs and cats. An acute inflammation of the pancreas or pancreatitis also will cause diarrhea in dogs and cats. You should find individual articles on each of these subjects at this website. If you don’t, remind me to write them.

Classification of the Process:

This is the fun part about being a veterinarian – poking through poop. Next I try to decide a bit more about the problem by examining the stool. I often pass the material through a strainer to hunt for objects such as leaves, sticks, glass, or aluminum foil that are the source of the problem. Then I prepare slides of the material to examine under my microscope. Some of these cases show a normal digestive process where food is being digested and absorbed. In others, fat globules are present and protein particles are sharp – evidence of poor digestion. These cases are either maldigestion or malabsorption. They can be due to a hyper-motile intestine or the lack of normal liver and pancreatic enzymes. Some stool is abnormally light due to a lack of these enzymes. Many times, I find things like bug parts that allow me to diagnose the problem without further tests. Other times parasites are visible under the microscope.

Diarrhea Caused By Husbandry:

Pet Food Quality

Buying that marked down generic brand of dog or cat chow instead of your regular name brand is very common cause of diarrhea. Quality has a particular cost; and when you go below this cost the ingredients of pet foods suffer. “High Protein” means very little. Feathers, hooves and beaks are all protein. But they are not digestible protein. Try to select a medium-priced pet food -. neither the cheapest nor the most expensive. And stay with that brand.


Some dogs and many cats over-eat when they are fed once a day. This can result in diarrhea, colic and vomission. I suggest feeding dogs and cat with this problem three times a day or leaving the food where they can munch when they please.

Foods That Don’t Agree With Pet

Some pets are intolerant to certain ingredients in pet foods. This can be beef, liver or chicken, preservatives, dyes and the like. Very few pets are actually allergic to pet food ingredients.


Some pets – toy dogs and ferrets in particular – vent stress through their intestines. This can take the form of vomission, refusal to eat, diarrhea or blood in the stool. After this occurs several times, owners usually draw the connection between events that upset the pet and bouts of diarrhea.

Dietary Indiscretions

The most common cause of diarrhea in dogs and ferrets (not so in cats) is dietary indiscretions – the eating of garbage, grass, plant leaves, etc. It is amazing the things pets will eat. Many of these pets are actually separation anxiety cases where a panicky pet eats everything in sight (see article on separation anxiety). Placing these pets on a large dose of petrolatum-based cat laxative helps slide this material out with the stool. These pets need to be monitored carefully for seventy-two hours to be sure the intestine does not block. I sometimes feed them craft glitter and wait for it to come out the other end. At the first sign of abdominal distress, depression, general weakness, fever or blood in vomitus or stool, they should be rushed to a veterinarian for further testing.

Diarrhea Caused By Medications And Chemicals:

Antibiotics And Sulfonamides

Many antibiotics and sulfas disrupt the normal helpful bacterial flora in the intestine as well as killing pathogenic (dangerous) bacteria. When the normal bacterial flora of the intestine is destroyed diarrhea may result. This problem can occur with injected antibiotics as well as oral medicines. Giving the pet yogurt or bacterial gel pastes, decreasing the antibiotic dose, or changing to a different antibiotic all help.

Other Drugs From The Pharmacy

Many medications that we dispense to your pet can cause diarrhea. Thyroid medication, NSAI drugs such as Rimadyl and aspirin, worming medications, and topical insecticides are only a few of the medications that can cause diarrhea.

Parasites Of The Intestine:

Nematode Parasites

I have never found nematode parasites in ferrets. Hookworms, roundworms, whipworms and strongyloides are some of the nematode parasites that cause diarrhea in dogs and cats.. These organisms are the most common causes of diarrhea in puppies and kittens. They are a threat to children who are accidentally exposed to animal stool. You can read about these parasites and how I deal with them in another article in this series. The eggs of these parasites are usually seen in microscopic examination of the stool.

Protozoan Parasites

Protozoal parasites are single-celled microscopic parasites of the intestine. Diarrhea is their primary sign. This group includes coccidia and giardia. We treat them with bland diets, sulfonamides and metronidazole. Giardia can cause similar problems in pet owners – particularly children.

Viral Infection:

Parvovirus Of Dogs

Parvovirus infection of dogs causes severe foul-smelling diarrhea. Mature dogs recover after a period of diarrhea but the disease is often fatal in young unvaccinated pups. Parvovirus attacks the lining of the small intestine causing bloody diarrhea and vomiting. Dogs are severely depressed, dehydrated and in much pain. These pets die from fluid loss so most of my efforts aim at replacing these fluids. Parvo-infected dogs cannot keep oral fluids and medicines down so I give our fluids and medications by injection.

Coronavirus Of Cats

Feline enteric Coronavirus is highly contagious among cats but rarely fatal. The virus is shed in the feces of carrier cats causing erosion of the lining of the intestine. Recently weaned kitten have fever, vomiting and diarrhea that lasts 2-5 days. They loose their appetite. I treat them with fluids, antibiotics and medications to slow the diarrhea.

Bacterial Infections:

Spoiled Food – Food Poisoning

Spoiled food is often contaminated with toxins and bacteria that cause pets to have diarrhea and vomit. Food poisoning usually resolves itself after the contaminated material is purged from the body. It is unwise to give these pets medications that prevent diarrhea and vomiting because this traps harmful bacteria and toxins within the body. Two common bacteria involved in these cases are Staphylococcus and Escherishia coli (E. coli). Dehydration is the most serious side effect of diarrhea and vomiting. Infant animals can quickly go into shock from dehydration. Products such as Gatorade or Pedialyte are excellent for preventing this. If the pet cannot hold any liquids down the fluids must be given intravenously or subcutaneously.

Salmonella And Campylobacter In Dogs And Cats

Feeding poorly cooked meat products and chicken pass these bacteria to pets and humans. As with food poisonings, keeping the pet well hydrated is extremely important. In catteries, adult cats carrying salmonella often pass it to their kittens with sometimes fatal results. I have found it very difficult to get Salmonella out of a group of cats once it has become established. Batryl (enrofloxacin) is an excellent drug to use in adult pets with diarrhea but it cannot be used in growing animals.

Acute Gastroenteritis of Ferrets

Several bacteria cause infections of the gastrointestinal tract of ferrets that appear as bloody diarrhea, vomiting and dehydration. Stress is often the precipitating factor in these conditions. Ferrets that vomit and pass fluid stools rapidly dehydrate. Dehydration can be fatal in these pets. The first thing I do is give the ferrets large doses of intravenous and subcutaneous fluid. (5% dextrose). I place them on injectable antibiotics, keep them warm and withhold all food and oral liquids. Bacteria most often associated with this problem are salmonella, helicobacter and campylobacter. Because diarrhea and vomission remove these bacteria and their toxins from the body I rarely attempt to lessen them. Instead, I replace the fluids and nutrients, as they are lost.

Problems Of Unknown Cause (Idiopathic):

Hemorrhagic Gastroenteritis Of Dogs

Toy breeds of dogs are susceptible to bloody diarrhea the cause of which remains unknown. No doubt many of these cases fall into other groups I have mentioned but were not diagnosed. Many of these cases are due to stress. Some emotional dogs exhibit stress through their intestines. In other dogs, the problem is probably undiagnosed Adison’s disease in which the body is deficient in cortisone. All these cases respond well to food deprivation for 24-48 hours and then gradually increasing amounts of bland diet fed. I give most of these cases the oral corticosteroids, prednisolone or prednisone and place many of them on the sulfonamide, Tylocin. These problems tend to reoccur in the pet from time to time.

Eosinophilic Enteritis Of Cats

This is a portion of eosinophilic complex of cats. The signs are diarrhea, weight loss and vomission. We suspect the problem is a food allergy. Cats with this problem often have elevated numbers of white blood cells called eosinophiles. These cells are associated with allergic conditions in pets and people. Sometimes the skin is ulcerated as well or ulcers are present on the lips tongue and mouth. The disease responds to injections and oral forms of corticosteroids such as methylprednisolone acetate. It also often responds to the human birth control medication, Depopovera (medroxyprogesterone acetate). Moving the cat to a different home or environment and feeding bland diets also help bring the disease into remission. I have also seen the skin and oral form of this disease disappear when fleas were eliminated from cats.

Inflammatory Bowel Disease of Ferrets

In this condition the lining of the stomach and small intestine becomes inflamed and may also ulcerate. Some feel that this condition is a form of autoimmune disease - others that it is due to allergies to various ingredients in ferret diets (possibly chicken). Perhaps both can play a part. Ferrets with this condition pass frequent loose watery stools and may also vomit. They become very thin. Diagnosis of this condition is often made by elimination of other causes. Exact diagnosis can only be made from biopsies of the pet’s intestine, which may require surgery. Because the symptoms are quite similar to acute and chronic bacterial gastroenteritis of ferrets, stool samples need to be cultured for these bacteria. When these causes of similar disease have been eliminated, the ferrets usually improve on a regimen of amoxicillin, metronidazole (Flagyl) and prednisolone. This disease is rarely cured but it can be managed. I usually also put these pets on Pepto-Bismol and Tagamet. It is hard to put weight on them because food moves too rapidly through their intestines and much of the absorptive power of the intestinal lining has been lost. This is why additional vitamins help these pets.

During flare-ups, liquid diets such as AD can be feed. Then, feeding one of the bland diets marketed for similar conditions in dogs and cats (i/d, EN, ZD, etc.) often minimizes this problem. Bland diets marked for cats have sufficient protein but those manufactured for dogs contain less than the optimal amount for ferrets. If dog products are used, I suggest that they be supplemented with vitamins, cooked egg whites and cottage cheese. Low intermittent oral doses of corticosteroids such as prednisone are also helpful.

Lymphangiectasia Of Dogs

Sometimes called plasmacytic enteritis, this condition is diagnosed by a pathologist’s examination of snippets of the wall of the small intestine which are removed at surgery. Lymphangiectasia is an improperly formed intestinal lymphatic system, which results in protein being lost from the body (protein-loosing enteropathy). We think that in some cases the pets were born with this disease and in others they acquired it later in life. Most affected pets respond to a combination of bland diet and anti-inflammatory doses of corticosteroid drugs such as dexamethasone or prednisolone but the disease is never cured. Diets you prepared should contain minimal fat with ample amounts of high-quality protein.