Jocelynn Jacobs DVM, CVC

Diarrhea and competitions go hand in hand with some performance dogs. And if your dog has diarrhea at an event, it can be both embarrassing and frustrating. They may have normal, healthy stools at home, but once at the event (or after a few days of competition), their stools literally fall apart. What are the common causes for diarrhea in dogs? Many things cause diarrhea in dogs – the most common include viral infections, intestinal parasites, small intestinal bacterial overgrowth, other intestinal conditions, stress, and diet.


There are many intestinal viruses that cause diarrhea – parvovirus, coronavirus and rotavirus are the most common. Viral diarrhea can be particularly dangerous, as in the case of parvovirus infection since it infects, damages, and kills normal intestinal lining. Dogs infected with parvovirus have severe, bloody diarrhea that can be fatal if not quickly given supportive and symptomatic care. Coronavirus and rotavirus infections also damage lining of the small intestine, but not as severely as parvovirus infection. However, if concurrent with other types of intestinal disease, they may be harder to overcome.

Vaccines are available to develop protective immunity against both parvovirus and coronavirus infections, but currently there are none against rotavirus infections.


There are many different species of worms that live in the digestive tract of the dog. Roundworms, hookworms, tapeworms, coccidia, and giardia, are just a few. Intestinal worms can have a detrimental effect on your dog’s ability to properly absorb and digest nutrients, and can cause diarrhea if they are present in sufficient numbers.

Many parasites physically damage the lining of the digestive tract. Because of the propulsive movements of the small intestine, many worms latch on to the intestinal wall of their host so they won’t be swept away with food material. This results in inflammation of the lining and inability to digest and absorb nutrients properly which can result in diarrhea.

Bacterial Overgrowth

Bacteria is normally found in the intestinal tract of dogs. There are good species of bacteria, such as lactobacillus, and harmful bacteria, such as salmonella or certain strains of E. coli. Good bacteria are important because they help with digestion and provide nutrients for healthy intestinal lining. Harmful bacteria may be introduced to the intestines by feeding infested or spoiled foods, or by eating bacterial-contaminated objects from the environment. Many dogs have some harmful strains of bacteria present in the intestines but don’t become ill. However, during times of stress or infection with other intestinal diseases, harmful bacteria counts can replicate to high levels, and can kill off or crowd out the beneficial bacteria. This condition is known as small intestinal bacterial overgrowth (SIBO).

SIBO is more common in dogs than most realize. A study done in 1995 documented 41 of 80 dogs (51%) representing 23 different breeds had SIBO in conjunction with chronic diarrhea.1 Dogs with SIBO can have many different clinical signs associated with malabsorption such as weight loss, diarrhea, flatulence (gas), anorexia, coprophagy (eating stools), and an increased appetite. Diagnosing dogs with SIBO can be difficult because there is no easy test to determine the levels of harmful bacteria present in the intestines. Thus many veterinarians must rule out all other causes of diarrhea before they can diagnosis SIBO as the cause of diarrhea and malabsorption.

Common Causes of Diarrhea

Other Diseases

Certain types of intestinal cancer, metabolic diseases, and allergies can also cause diarrhea. Inflammatory bowel disease (IBD) is another fairly common cause of diarrhea in dogs. In IBD, the intestinal lining is inflammed and filled with many different types of white blood cells. Although small numbers of these cells are beneficial, when in large numbers, the intestinal lining becomes irritated which results in diarrhea.


Stress also can cause diarrhea. Anxiety causes the sympathetic nervous system to increase intestinal motility. When the intestines move food through too quickly, nutrients are not absorbed properly. Dogs that are under stress for long periods of time may have chronic diarrhea and weight loss.


Even your dog’s diet can cause diarrhea. Changing from one diet to another can cause intestinal upset and diarrhea if not done gradually over a period of a week. Different brands of dog foods have different protein, fat, carbohydrate, and fiber sources, and the intestines need time to acclimate to a new diet.

Some diets are too high in fat for particular breeds or individual dogs and that can cause diarrhea. If your dog develops diarrhea while on a high fat diet, feeding a lower fat diet will usually resolve the problem.

There are many different causes of diarrhea in dogs. Identifying and eliminating the cause and healing the intestines through diet and medication are proper steps in resolution of the problem.


1. Rutgers HC et al. Small intestinal bacterial overgrowth in dogs with chronic intestinal disease. J Am Vet Med Assoc. 1995;206:187-193.