10 Most Common Causes of Dog Diarrhea by A. Gaffaud, DVM

The most common causes of dog diarrhea are a sudden change in diet, eating something they shouldn't (garbage, table scraps, toys), food intolerance, intestinal parasites, infections, and stress. Most mild cases in healthy adult dogs clear up within a day or two once the cause is removed. Diarrhea that is explosive, bloody, black, keeps coming back, or comes with vomiting or lethargy needs a vet.

Dog diarrhea is a clinical sign, not a disease on its own. It means loose or unformed stools, and it can come from something as minor as a new treat or as serious as a viral infection, organ disease or cancer. Minor cases can often be resolved quickly with simple care. Still, they may become serious if they are ignored, especially in puppies and older dogs. As always, if you have any concerns about your dog's health, consult your veterinarian.

Here are 10 of the most common causes of dog diarrhea, followed by what explosive diarrhea can mean and when to call the vet.

1) Sudden Change in Diet

The gastrointestinal tract of dogs and other animals requires enough time to adjust to new food; hence gradual changes are always recommended to prevent stomach upset. It usually takes several days to shift to the new food as the system needs to adjust and adapt to the new proteins. Puppies are more sensitive to these changes since their bodies are not fully developed yet.

A quick note on food: for a dog with a sensitive stomach, the goal is high protein with fewer calories, and a reduced-fat recipe makes that easy.

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To prevent this, make sure to introduce the new food gradually. You may start for days 1 and 2 with a mixture of 75% old – 25% new food. For days 3 and 4, change it to 50% old-50% new, and for days 5 and 6, 25% old – 75% new food. By day 7, you may give 100% new food.

2) Dietary Indiscretion

Dietary indiscretion means eating something the gut isn't built for, or simply too much of something. It irritates the stomach and intestines. These include a sock, a toy, dirt, poop, wood, certain fruits, spoiled food, or even table scraps from the garbage, hence the term “garbage toxicosis” or “garbage gut.” This irritation changes the stool consistency.

This can be prevented by discouraging the scavenging behavior of your dogs using clicker training or a food-based reward system. You may also distract them with toys.

3) Food Allergies and Intolerances

Dogs have many allergies, and food is one of the most common. Food allergy could be due to the ingredient or food group to which dogs could be allergic most often a protein such as beef, chicken or dairy. Food intolerances, such as poor tolerance of lactose or high-fat foods, are more common than true allergies and also cause loose stool.

The findings in the study of Rosser indicated that the recommendation of a 3-week elimination-diet trial for diagnosis of food allergy was adequate for only 25% of the dogs. It is recommended that test diets be fed for at least ten weeks before a food allergy is ruled out. Skin testing is another commonly used diagnostic technique to determine food allergies. However, the study of Kunkle involving 100 dogs concluded that skin-testing with selected allergenic food extracts was not helpful in identifying food-sensitive dogs.

If you and your vet decide to try a food with a different protein or less fat, switch slowly using the 7-day schedule above. Our chicken-free lamb recipe is one option. Our guide to the best dog food for allergies covers the most common protein triggers.

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4) Intestinal Parasites

Dogs usually get infected with endoparasites and could suffer from diarrhea. Puppies with weak or compromised immune systems are more susceptible. These internal parasites are whipworms, roundworms, hookworms, Coccidia, and Giardia. They usually get this by fecal-oral route or by exposure to contaminated soil, drinking water, or stool.

Whipworms could cause mucoid and bloody diarrhea as the parasite burrows itself in the intestinal mucosa causing hemorrhage.

You may see roundworms such as ascarids in your dog’s poop or vomit if infected. Toxocara canis is usually found in puppies, but some studies have suggested that these worms may be present in up to 33% of adult dogs.

Hookworms are small but terrible suckers that consume large amounts of blood from their host leading to life-threatening anemia. They attach themselves to the intestines like a hook hence the name.

According to Dr. Moorehead of the University of Georgia, the “unholy trinity” of roundworms, hookworms, and whipworms ranks among some of the most common and well-known infectious agents affecting dogs. You can discuss with your veterinarian the best options for dewormers based on your area and your health status.

Coccidia may cause severe watery diarrhea, vomiting, dehydration, and abdominal distress. In severe cases, death may occur. Your veterinarian may prescribe a sulfa-type antibiotic.

Giardia is a protozoan parasite that attaches itself to the intestinal wall causing an acute, sudden-onset of foul-smelling chronic intermittent diarrhea. The stool is soft to watery, often has a greenish tinge, and occasionally contains blood. You may also observe excess mucus in the feces. This may persist for a week leading to weight loss. Your vet can test for it and prescribe treatment.

5) Bacterial Infection

The dog’s gut is a common target for various microorganisms. Bacterial infections are usually accompanied by severe (explosive and often bloody) diarrhea. The most commonly reported bacteria causing diarrhea include Clostridium perfringens, Clostridium difficile, Campylobacter spp, pathogenic Escherichia coli, and Salmonella spp, among others.

6) Viral Infections

Parvovirus – Canine Parvovirus is a highly contagious viral disease causing acute gastrointestinal illness in puppies transmitted through the fecal-oral route. It has a notable odor; hence experts would usually say, “smells like parvo” when encountering such a case. Some breeds have a higher risk of infection. The virus destroys the enterocytes causing bloody diarrhea.

Distemper – Canine Distemper is a contagious viral disease of carnivores, usually in young animals. It could be acute to subacute and often fatal. It is also known as the “disease of unvaccinated puppies” or “hard pad disease” and can be transmitted airborne with droplet exposure. It invades the respiratory tract but can progress to a multisystemic problem. Clinical signs include diarrhea, vomiting, thick yellow discharge from the eyes and nose, cough, and some neurological symptoms such as seizure, ataxia, and paralysis in severe cases.

Coronavirus – Canine coronavirus is characterized by sudden onset of diarrhea with lethargy and inappetence. The stool is loose, with a fetid odor and orange tint. It may also contain blood or mucus.

Parvovirus and distemper are largely preventable by vaccination, which is why finishing the puppy vaccine series matters so much. Any puppy with vomiting or bloody diarrhea needs a vet the same day.

7) Illnesses

Dog diarrhea can be one of the symptoms of severe health issues like cancer, hemorrhagic gastroenteritis, gastrointestinal inflammations, Inflammatory bowel disease (IBD), liver, pancreas, or kidney diseases. Chemotherapy could also induce diarrhea.

8) Toxin Ingestion

Some food has toxic substances to dogs and could be life-threatening, and its complex manifestation usually starts with diarrhea. Some of these are chocolate, grapes, avocado, and salt. Drugs and chemicals such as household products (detergents, antifreeze) and plants could also be toxic.

Treatment is based on the toxic agent ingested by your dog. If you think your dog ate something toxic, call your vet, an emergency clinic or the ASPCA Animal Poison Control Center right away. Don't wait for diarrhea to start.

9) Foreign Bodies

Ingesting foreign objects such as toys, cloth, sticks, and rocks can cause intestinal blockage and diarrhea.

This is a life-threatening situation and requires immediate veterinary intervention.

10) Stress or Emotional Upset

Any condition that alters the mood of dogs, such as fear, trauma, depression, anxiety, changes in routine, and excitement from playtime, visitors, car rides, or new experiences that could disrupt digestive function and biome balance may lead to inflammation of the colon a.k.a. colitis. The stressful condition leads to the proliferation and overgrowth of harmful bacteria in the gut.

Explosive Diarrhea in Dogs: What It Means

Explosive diarrhea is watery stool that comes out suddenly and forcefully, often with gas and urgency. It usually means the intestines are moving contents along too fast to absorb water. Common triggers include:

• Garbage, spoiled food, or a big serving of something rich or fatty
• A sudden food change
• Bacterial infections and parasites such as Giardia
• Viral infections, especially parvovirus in puppies
• Stress colitis, which often shows as frequent, urgent stools with mucus

A single bout in an adult dog that is otherwise acting normal can be watched closely at home for a few hours. Explosive diarrhea with blood, vomiting, weakness or a puppy involved should be seen by a vet right away because dogs can dehydrate quickly.

Foul-Smelling Diarrhea

All diarrhea smells, but a sudden, unusually foul or "rotten" smell is worth mentioning to your vet. It can go along with Giardia, bacterial overgrowth, blood in the stool, or parvovirus. A fresh stool sample in a clean bag helps your vet test for parasites and infection.

Small Bowel vs. Large Bowel Diarrhea

Your vet may ask questions that sound oddly specific. They help tell where the problem is:

Taking a photo and noting how often your dog goes can save time at the appointment.

What to Do at Home for Mild Diarrhea

For an adult dog that is bright, alert, drinking and eating, these steps are a sensible start while you watch for red flags:

• Keep fresh water available at all times.
• Remove the likely cause, such as a new treat or access to the trash.
• Offer small, plain meals your vet suggests, such as boiled skinless chicken and white rice, for a day or two.
• Go back to the regular food gradually as stools firm up.
• Don't give human anti-diarrhea medicine unless your vet tells you to.

For more, read our step-by-step guide on how to firm up your dog's stool and our guide to helping your dog's upset stomach.

Keep a Lookout for These Common Causes of Dog Diarrhea

Dealing with dog diarrhea is no fun. Fortunately, being aware of these common causes of dog diarrhea can help avoid some of the preventable cases: switch foods slowly, measure meals, keep trash and toxic foods out of reach, keep up with deworming and vaccines, and keep routines steady. Knowing the cause of your dog's diarrhea can also help rule out more serious causes, such as illness. And if your dog is already experiencing mild diarrhea, there are still things you can do to firm up your dog's stool.

A consistent diet is one of the best ways to keep stools steady. Feed one complete food, measured with a real cup, and keep extras small. If you're unsure how much to feed, our dog food calculator can help. Dogs that do better with less fat or without chicken can try Bully Max 25/11 Lamb, and dogs that need more calories can stay steady on Bully Max 30/20. For more help choosing, see our guide to the best dog food for sensitive stomach and diarrhea concerns.

Once your vet has ruled out a medical cause, a daily chew with gut nutrients is an easy way to support your dog's everyday digestion.

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