Diarrhea—let's agree upfront that this is an unpleasant topic. It's also one we see in practice every day, and while the cause is often relatively harmless, that isn't always so. This post looks at what may be behind your dog's diarrhea and how serious each possibility is.
Dog Diarrhea Causes
Parvo
Parvovirus is a highly contagious viral illness of young, unvaccinated puppies. It attacks rapidly dividing cells, including those lining the intestines and those that fight infection—so parvo causes severe, often bloody diarrhea and simultaneously leaves a puppy vulnerable to secondary bacterial infection. It's profoundly debilitating and usually requires hospitalization and intensive supportive care.
Untreated, most puppies with parvo don't survive. With prompt and intensive veterinary care—intravenous fluids, anti-nausea medication, antibiotics to control secondary infection, nutritional support, and in some cases an antiviral—the majority do pull through. It's expensive treatment and hard on the puppy, which is what makes prevention so worthwhile.
Parvo is preventable with a series of inexpensive and highly effective vaccines. A puppy who nurses in the first day of life receives antibodies from their mother that give partial protection for the first weeks—but that protection fades, and while it lasts it also interferes with the vaccine. That's why a single shot isn't enough: we start at 6 to 8 weeks and repeat every three to four weeks until the puppy is at least 16 weeks old, so we're certain of covering the window when maternal protection has gone but vaccination hasn't yet taken.
Until that series is complete, keep your puppy away from places where unvaccinated dogs gather—dog parks, pet store floors, sidewalks in busy areas. Parvovirus survives in the environment for months and is very difficult to eliminate.
Worms and Other Intestinal Parasites
Common intestinal parasites include roundworms, hookworms, whipworms, tapeworms, giardia, and coccidia. All can be found through stool sample screening, which is why it's genuinely useful to bring a fresh sample to any appointment for diarrhea—ideally collected that morning.
Parvo and Worms: Separating Fact From Fiction
Fiction: "Parvo has a distinctive smell. If a puppy with diarrhea doesn't smell a certain way, it isn't parvo."
Fact: If only it were that simple. The smell people associate with parvo comes from blood in the stool, and a puppy with bloody stool from hookworms smells much the same. More to the point, a puppy diagnosed early—before there's blood in the stool—has a considerably better outlook than one diagnosed late. Don't wait for a particular smell before calling us.
Fiction: "I don't see any worms in her poop, so she doesn't have worms."
Fact: Adult worms generally stay inside the body. The only time you'd see part of one is when a tapeworm sheds egg-filled segments—those rice-like pieces around the tail aren't the worm itself—or when a whole worm passes after deworming. Roundworms, hookworms, and whipworms release microscopic eggs, which can only be found by examining a stool sample under a microscope. Giardia and coccidia aren't worms at all; they're single-celled organisms, and equally invisible.
Fiction: "My breeder dewormed her, so there's no way she has worms."
Fact: Most worms take around three weeks to develop from egg to adult, and dewormers kill adults. So a single dose leaves behind anything that was still an egg that day—which is why puppies need a series of treatments rather than one. On top of that, many over-the-counter dewormers only cover roundworms and hookworms, leaving whipworms, tapeworms, giardia, and coccidia untouched. Any of those can make a puppy genuinely unwell.
Fiction: "Parvo is the only deadly diarrhea disease of puppies. If it's not parvo, my dog will be fine."
Fact: Puppies can become dangerously anemic from a heavy hookworm burden, malnourished from a heavy roundworm burden, and severely dehydrated from coccidia or giardia. We know that's hard reading, but it's why we take diarrhea in a puppy seriously rather than waiting it out.
Fiction: "Blood in the stool means my dog must be really sick."
Fact: Blood in the stool doesn't automatically mean parvo, and it isn't always as alarming as it looks. Dogs and cats commonly pass a few streaks or drops of fresh blood after a bout of diarrhea, from straining and inflammation in an irritated bowel. That said, more than a few spots, dark tarry stool, or blood alongside a dog who seems unwell all need looking at promptly.
Garbage Gut, Food Poisoning, and Stress
Stress of any kind—anxiety, excitement, a change in routine, boarding, a house move—can disturb the balance of bacteria in a dog's gut. So can eating something they shouldn't, whether that's raw or spoiled food, garbage, or a sudden diet change.
Every dog carries an enormous population of bacteria in their intestines, and those bacteria do much of the work of digesting food. When that population shifts, digestion suffers, and diarrhea can persist for days or even weeks. This is also why any change of diet should be made gradually over a week or so, mixing increasing proportions of the new food in with the old, rather than switching overnight.
Other Causes Worth Knowing About
Not all diarrhea is infectious or dietary. We also see it caused by:
- Something swallowed that shouldn't have been—a partial obstruction can cause diarrhea before anything else
- Toxins, including some plants and household chemicals
- Food allergy or intolerance
- Chronic conditions such as inflammatory bowel disease, exocrine pancreatic insufficiency, or pancreatitis
- Liver, kidney, or thyroid disease
- Medications, antibiotics in particular
- Cancer of the digestive tract
That's why persistent or recurring diarrhea deserves proper investigation rather than repeated courses of the same treatment.
How We Treat It
Treatment depends entirely on the cause, which is why we'd rather examine your dog than guess. Depending on what we find, it may involve fluids to correct dehydration, deworming, anti-nausea medication, a prescription digestible diet, probiotics, and sometimes antibiotics—though these days we use antibiotics for diarrhea considerably less often than we once did, since most cases resolve without them and they can prolong recovery by further disrupting gut bacteria.
What You Can Do at Home
For a mild case in an otherwise bright, well adult dog, there are a few things worth trying while you speak to us.
Make sure they keep drinking. Dehydration is the main risk with diarrhea, and it develops fastest in puppies and small or elderly dogs. Fresh water should always be available.
A bland diet. Plain cooked white rice with a low-fat protein—skinless boneless chicken breast, or lean ground beef, boiled and drained—fed in small, frequent portions. Keep it plain, with no oil, butter, salt, or seasoning. Some vets suggest resting the gut for 8 to 12 hours first in an adult dog, but never withhold food from a puppy without speaking to us.
Probiotics. A veterinary probiotic is a reasonable thing to try. The strains in human yogurt are chosen for human digestion and aren't the most useful ones for dogs, and yogurt also brings lactose that some dogs don't tolerate. Products formulated for dogs use more appropriate strains, and we're happy to recommend one.
Don't feed a bland diet for more than about 48 hours without speaking to us. It isn't nutritionally complete, and continuing diarrhea needs investigating rather than managing.
What About Imodium or Pepto-Bismol?
Please don't give your dog any human medication without talking to us first. There are dosing charts online that look authoritative, and some of those medications aren't harmful at the right dose in the right dog—but that's a considerable number of conditions. Loperamide is dangerous in some breeds, particularly herding breeds with the MDR1 gene variant, and it's the wrong thing entirely if the diarrhea is caused by an infection or a toxin, where slowing the gut keeps the problem inside. Pepto-Bismol contains a salicylate, related to aspirin, which can cause stomach ulceration and is toxic to cats.
Either can also mask the signs of something more serious, which makes our job harder and delays the treatment your dog actually needs. Stick to water, a bland diet, and a call to us.
When to Call Us
Phone us as soon as you notice the problem, even if you don't think your dog needs to come in straight away. It costs nothing, it lets us note the problem and track how often it's happening, and we can give you advice specific to your dog.
Please bring your dog in promptly if:
- Diarrhea continues beyond about 48 hours, even if your dog seems otherwise well
- Your dog seems unwell alongside it—lethargic, off their food, or vomiting
- There's more than a small amount of blood, or the stool is black and tarry
- Your dog has a fever (a dog's normal temperature runs up to about 102.5°F)
- Your dog is a puppy, a senior, a small breed, or has an existing health condition—all of these dehydrate faster and have less reserve
- Your dog may have eaten something toxic, or swallowed a foreign object
- They're on medication, or the diarrhea started after a new medication
Treat it as an emergency if:
- The diarrhea is profuse and bloody, particularly in an unvaccinated puppy
- Your dog is vomiting repeatedly as well and can't keep water down
- Their gums look pale, white, or grayish
- They're weak, unsteady, collapsed, or unresponsive
- Their abdomen looks distended or is painful to touch
- They're retching or trying to vomit without producing anything—this can indicate bloat, which is life-threatening
Diarrhea is no fun for anyone, and as you've read, it can mean more than an inconvenience. If you're unsure whether your dog needs to be seen, please call us—we're glad to talk it through, and we'd far rather hear from you early.