What Causes Diarrhea in Dogs? WHAT CAUSES DIARRHEA IN DOGS? There are many causes of diarrhea in dogs, says Dr. Jennifer Coates. The most common are intestinal parasites (e.g., roundworms, hookworms, Giardia), bacterial infections or overgrowth, dietary indiscretion (e.g., garbage ingestion or an abrupt diet change), and inflammatory diseases (e.g., diet intolerances or allergies).. Diarrhea can also be a common side effect of some pet medications, including antibiotics, non-steroidal anti-inflammatory drugs and heart medications,. HOW IS DIARRHEA IN DOGS DIAGNOSED? Identifying the cause of the diarrhea will vary in difficulty. To diagnose the cause of severe or prolonged diarrhea, veterinarians will perform a thorough history and physical exam, says Dr. Coates, adding that in some cases, they may also need the results of blood work, a urinalysis, fecal examinations, X-rays, abdominal ultrasounds, specialized laboratory tests, and even exploratory surgery or endoscopy with tissue biopsies.. Sometimes the cause could be as simple as your dog eating something from your trash. Whatever the cause, it is important that the diarrhea is treated accordingly. HOW TO TREAT DIARRHEA IN DOGS Treatment for diarrhea in dogs will be dependent on what’s causing it, so it’s important to consult your veterinarian if you notice an issue with your pet’s health.According to Dr. Coates, many diarrhea-causing diseases are easily diagnosed and treated. However, some disorders are not curable and must be managed with medication and/or dietary modification. WHEN SHOULD YOU GO TO THE VET? Diarrhea that has lasted for more than 24 hours could indicate something more serious than a simple stomach bug, and if left unchecked can lead to dehydration. “However, if your pet’s stool is bloody, dark or tarry, if he seems lethargic or has lost his appetite, or is also vomiting, don’t wait 24 hours,” says Dr. Benson. “In those cases, you should get to the vet as soon as possible.”.