Clostridium is a group of bacteria that naturally live in the intestines of many dogs but can cause illness, most often diarrhea, when toxin-producing strains multiply out of control. About 12% of healthy dogs carry toxin-producing Clostridium perfringens strains in their gut without any symptoms at all, which is part of what makes this infection tricky to diagnose and understand. The two species that cause the most trouble in dogs are Clostridium perfringens and Clostridioides difficile (previously classified as Clostridium difficile).
Why a Normal Gut Bacteria Causes Problems
Clostridium perfringens is one of the most common bacteria found in a dog’s digestive tract. Under normal conditions, it exists in small numbers alongside hundreds of other bacterial species, kept in check by the balance of the gut microbiome. Problems start when something disrupts that balance: dietary changes, stress, antibiotic use, or underlying illness. When conditions shift in the bacteria’s favor, toxin-producing strains can proliferate rapidly.
The damage comes from the toxins these bacteria release, not simply from their presence. Clostridium perfringens produces an enterotoxin that attacks the tight junctions between cells lining the intestinal wall. These junctions normally act like seals, keeping the contents of the gut where they belong. The toxin binds to specific proteins in those seals and loosens them, creating molecular leaks between cells. It also forms pores directly in cell membranes, killing intestinal lining cells. The result is a gut wall that can no longer properly absorb water or contain its contents, which leads to diarrhea, often with blood and mucus.
Clostridioides difficile works through a different set of toxins (toxin A and toxin B) but causes a similar end result: inflammation and breakdown of the intestinal lining. C. difficile is more commonly associated with diarrhea that follows antibiotic treatment, since antibiotics can wipe out competing gut bacteria and give C. difficile room to thrive.
Symptoms to Recognize
The hallmark symptom is diarrhea, which can range from soft, mushy stools to watery, bloody episodes. Clostridium perfringens is a well-documented cause of acute hemorrhagic diarrhea in dogs, where the stool contains visible blood (a sign called hematochezia). In one documented case, a dog experienced three to four episodes of bloody diarrhea per week over a ten-month period before the underlying clostridial infection was identified.
Other common signs include:
- Increased urgency and frequency of bowel movements, sometimes daily or multiple times per day
- Mucus in the stool, giving it a slimy or jelly-like appearance
- Straining during or after defecation
- Gas and abdominal discomfort, which your dog may show by restlessness, a hunched posture, or reluctance to eat
Acute episodes often resolve within a few days, but some dogs develop a chronic-recurring pattern where diarrhea keeps coming back over weeks or months. This recurring form is particularly associated with C. difficile infections, where symptoms may improve with treatment only to return shortly after the antibiotic course ends.
How Dogs Pick It Up
Because Clostridium already lives in most dogs’ guts, “infection” is often really an overgrowth of strains the dog already carries rather than exposure to a new pathogen. That said, dogs can pick up additional Clostridium spores from contaminated soil, feces from other animals, raw or undercooked meat, and contaminated water.
Clostridium spores are remarkably tough. They survive in oxygen-rich environments, temperature extremes, and nutrient-poor conditions. Standard household disinfectants like alcohol-based cleaners and chlorine dioxide are largely ineffective against them. Cleaning up after an infected dog requires either bleach solutions with extended contact time or hospital-grade disinfectants. Spores can persist on surfaces, bedding, and flooring for long periods, which matters if you have multiple pets or if your dog has recurring infections.
Can Dogs Pass It to People?
The zoonotic risk appears to be low but not zero. A study examining C. difficile in pets and their owners found that the same strains known to cause human hospital infections (including some potentially virulent types) do show up in dogs. However, researchers did not find simultaneous carriage in dogs and their owners within the same household. Interestingly, the evidence suggests humans may be more likely to pass C. difficile to their dogs than the other way around. Basic hygiene, particularly hand washing after cleaning up stool, is a reasonable precaution, especially for household members with weakened immune systems.
How Vets Diagnose It
This is where things get complicated. Simply finding Clostridium in a stool sample doesn’t mean much, since the bacteria live in healthy dogs too. The key is detecting toxin-producing strains specifically. Vets use a few approaches to make that distinction.
PCR testing on a fecal sample looks for the genes that encode toxin production. For C. perfringens, the genes of interest are netE and netF, which code for pore-forming toxins linked to hemorrhagic diarrhea. In one study, 48% of dogs with acute hemorrhagic diarrhea carried these genes compared to just 12% of healthy dogs. For C. difficile, vets look for genes encoding toxin A and toxin B. PCR testing directly on stool gives a significantly higher detection rate than older culture-based methods.
Your vet will also rule out other causes of diarrhea first, including intestinal parasites, Giardia, and parvovirus. A clostridial diagnosis typically comes after these more common culprits have been excluded, especially if the diarrhea is bloody or keeps recurring.
Treatment and Recovery
Not every case needs antibiotics. Mild, acute episodes often resolve with supportive care: a temporary bland diet, ensuring your dog stays hydrated, and giving the gut time to rebalance. Vets generally reserve antibiotics for dogs with moderate to severe symptoms or chronic-recurring diarrhea.
When antibiotics are prescribed, metronidazole is the most commonly used option, typically given twice daily for seven days. It targets anaerobic bacteria like Clostridium while leaving much of the gut’s other bacteria intact. One limitation: some dogs with C. difficile infections relapse after finishing the antibiotic course, with bloody diarrhea returning within days of completing treatment. This recurrence pattern may require a different approach, including dietary management or longer treatment courses.
Probiotics, particularly the yeast-based strain Saccharomyces boulardii, have shown promise in managing clostridial diarrhea. S. boulardii is considered one of the most effective probiotics for preventing and co-treating C. difficile-associated diarrhea. Psyllium fiber, a soluble fiber supplement, has been shown to increase S. boulardii levels in the gut by about 22%, so combining the two may offer additional benefit. Some dogs with chronic C. difficile infections respond well to dietary changes alone, with symptoms improving significantly on a controlled, consistent diet.
Dogs at Higher Risk
Any dog can develop clostridial overgrowth, but certain situations raise the odds. Dogs currently on or recently finishing a course of antibiotics for any reason are more susceptible, since the medications can disrupt the normal bacterial balance that keeps Clostridium in check. Puppies and senior dogs, whose immune systems are either still developing or declining, tend to be more vulnerable. Dogs in kennels, shelters, or boarding facilities face higher exposure to Clostridium spores due to close quarters and shared spaces. Sudden dietary changes, particularly switching foods abruptly rather than transitioning gradually, can also trigger an overgrowth episode in dogs already carrying toxigenic strains.