Skip to main content

Q&A: Why Does My Dog Have Bloody Diarrhea Again?

  • Author:
  • Updated date:

Dr. Mark is a veterinarian. He has been working with dogs for more than 40 years.

Colitis, an inflammation of the large bowel, can cause bloody stools in dogs.

Colitis, an inflammation of the large bowel, can cause bloody stools in dogs.

Why Is My Dog's Stool Bloody Again a Month After Treatment?

"I rescued a 4-month-old golden retriever about 2 months ago. When I first got her, she had worrisome diarrhea, but we thought she might just need some time to settle. We had a vet appointment for her puppy shots and deworming. She was given a dewormer and seemed to be doing well until she started having blood in her stool.

The vet saw her for an emergency appointment, and they put her on Metronidazole, Simparica, and Pro-Pectalin. They thought it was an infection and that would completely get rid of it. It worked great—her stool was perfect and we were on a good routine—and then she started having blood in her poop again. It wasn’t as much considered diarrhea but very mooshy. Not really watery.

I wanted to reach out to other places to get their advice. She’s on a very limited-ingredient diet and does amazing otherwise. Very energetic and happy." —Ryan

Colitis in Dogs

From your description, it sounds like your dog has colitis, an inflammation of the large bowel.

Causes of Colitis in Dogs

Colitis can have a lot of causes:

  • bacterial infection
  • parasite infection (giardia and some others)
  • stress
  • food allergy
  • inflammatory bowel disease (IBD)

Unfortunately, there are a lot of causes and unless you find out which is causing the problem, treatment will not necessarily work.

Here is a more in-depth look at each of the causes outlined above.

Bacterial Infection

The best way to treat this problem is by having the feces examined, finding the bacteria that is causing the problem (E.coli or Salmonella), and then doing a sensitivity test to find out which antibiotic will work. (1)

That can be pretty hard to do in the real world, so most vets will just prescribe an antibiotic that will usually work.

Scroll to Continue

Read More From Pethelpful


Giardia infection is much more likely in your puppy. Did the emergency clinic do a giardia test? It is not always done, but anytime a dog is a suspect or is positive for giardia they should be rechecked about a month later.

The metronidazole may have killed most but not all of the giardia, or she may have been reinfected from your yard or her water bowl. If either of those things happened, it would explain her signs starting back up. (In humans, studies have found that over 90% of people with giardia are cured by the first treatment, but not 100%.) (2)


A lot of things can stress out your puppy, but you need to go back and ask yourself if this bout and the previous episode had anything to do with a stressful event. It does not sound like it, and I would be much more likely to suspect giardia, but you can answer this better than me since you are around her all of the time.


Some dogs will become allergic to something in their food, and loose stools are one of the signs we see. Almost all of them also scratch their face and ears, though, so if you have not seen this, a food allergy is not as likely.

Inflammatory Bowel Disease (IBD)

Some dogs with colitis need to be on a special diet that is high in fiber, and since your veterinarian already recommended starting this diet, that may have been what they were suspecting. Sometimes probiotics help, and some dogs need to be on a diet of hydrolyzed protein.

Some holistic veterinarians recommend that all toxins be removed from the dog's environment, but that may not help; many of these cases remain idiopathic (we do not know what is causing the problem). IBD will at times flare up even after a dog is on a special diet and seems to be cured.

Check for Giardia Before Proceeding

Be sure to take your dog in again to your regular veterinarian and have him checked for giardia before proceeding. If it is a bacterial infection rather than giardia (a parasite), antibiotics may be very effective. Stress colitis is also easy to treat, with many options such as fasting, probiotics, diet, slippery elm bark, etc.

You may be okay even without the special diet (this is typically a last resort if no other alternatives have worked), which in the next 10 to 15 years is going to be better for you and your dog.


(1) Manchester AC, Dogan B, Guo Y, Simpson KW. Escherichia coli-associated granulomatous colitis in dogs treated according to antimicrobial susceptibility profiling. J Vet Intern Med. 2021 Jan;35(1):150-161.

(2) Gardner TB, Hill DR. Treatment of giardiasis. Clin Microbiol Rev. 2001 Jan;14(1):114-28.

This article is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from your veterinarian. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2022 Dr Mark

Related Articles