Is Giardiasis Getting Too Much Attention for Diarrhea?

Only a small percentage of dogs and cats develop diarrhea

October 17, 2016 (published)
This is the small intestine of a gerbil infested with Giardia sp. protozoa. Photo courtesy of CDC/Dr. Stan Erlandsen.

You remember the Pepto Bismol jingle, right? “Nausea, heartburn, indigestion, upset stomach, diarrhea!” Some of you may be running for the toilet paper just thinking about it. But in all seriousness, those signs sum up the bulk of what your dog or cat may experience if they have an active infection with Giardia.

How does my pet get Giardia, you might ask? Well, let’s say you take your dog somewhere a lot of other dogs congregate, perhaps a dog park or a show venue. Before you and Fido got there, some other dog (we’ll call him Patient Zero for anonymity) with Giardia pooped on the ground and Mrs. or Mr. G failed to pick it up. Maybe they were having a hectic morning or maybe they just don’t like their hands getting close to poop! Hey, who can blame them, right? This stuff stinks! Then the ground where the infected feces is sitting got wet, probably from rain or a hose. Your dog comes along and drinks from a puddle at that same spot and voila...the fun begins!

The puddle drinker now has giardiasis and a lovely case of bowel-emptying, watery diarrhea.

Wise as they are, cats are not immune to the effect of our friend Giardia. If exposed to infected stool from another cat, they can also become infected. Yay, more "dire rears," and now you're washing rugs again, plus that brand new pair of Calvin Klein jeans you left on the floor.

How often do pets pick up this nastiness?

Pretty often. Let's just say I frequently see Giardia-positive fecal samples; I happen to be an internal medicine consultant for a large veterinary diagnostic lab. Treatment usually involves antibiotics and/or dewormers.

In an effort to remain good stewards for appropriate antibiotic usage, I think it’s reasonable to talk about when it’s okay not to treat that Giardia positive stool sample because honestly, only a small percentage of dogs or cats develop diarrhea when exposed to this organism.

Giardia duodenalis can be an infectious organism in dogs and cats. But according to the Companion Animal Parasite Council (CAPC), infections in dogs and cats who actually have symptoms only averages about 15.6 and 10.3 percent respectively. If the organism attaches to the inner lining of the small intestine, damage may ensue and this on occasion can result in diarrhea, bloating, or cramping (remember the Pepto jingle?). Giardia is transmitted when an animal ingests cysts shed by infected animals or humans. These cysts are acquired from fecal-contaminated water, food, or inanimate objects. Dog strains of Giardia are not known to infect cats, and cat strains are not known to infect dogs.

In most cases the infection goes away on its own. But if diarrhea is severe or becomes chronic, then medical attention is required.

Zoonosis, meaning transmission of Giardia from pet to human, is a concern commonly cited as a reason to persist with chronic or repetitive treatment. Despite your pet possibly not having any obvious signs of illness, many veterinarians will generally treat a pet with a single course of therapy to see if it can be eliminated it from the stool. I think that once-off treatment is okay.

We can’t completely say for sure whether people universally get affected or sick with Giardia from their dog. What I don’t like to see is young dogs, whose symptoms have resolved, repeatedly treated for Giardia based on seeing the organism in the stool. They had diarrhea the first time they presented, we diagnosed Giardia and treated it, and the darn thing is still there but NOW the diarrhea is completely resolved. In general I usually instruct the veterinarian in those cases to treat once more and then stop, even if the pet is still positive for Giardia on a routine fecal screening.

I think this is okay because I see many young pets who have no gastrointestinal signs who eventually eliminate the organism from their gut without our intervention. And let’s face it: the less we use antibiotics in ourselves and our pets, the better off we are in the long run.

Even if your dog continued to have diarrhea and remained Giardia positive, after several rounds of therapy, I think it’s time to allow your veterinarian to step out from the tunnel vision of blaming the Giardia, put on their thinking cap, and investigate whether or not there may be an alternative cause for your dog to continually soil that new rug you just bought from Rooms to Go.

Human infections are usually acquired from other humans (want to wash your hands after you've wiped? Thank you!). Fortunately, transmission to people from dogs and cats appears to be rare. Giardia also are classified into collections based on their infectivity towards other animals or humans. Dogs have mainly collections of strains C and D, cats have A1 and F, and humans get A2 and B. The collections of Giardia that your dog or cat may be carrying may not be infective to you, so breath a collective sigh of relief!

There is little direct evidence of transmission of Giardia from pets to people naturally. Accordingly, healthy pets positive for Giardia are not considered significant human health risks for HIV-infected people by the Centers for Disease Control and Prevention. Is there absolutely no risk to humans? No, we can't say that because the risk is not absolute zero. It is reasonable to err on the side of caution by treating a Giardia infection initially when your pet has diarrhea to reduce potential spread to other pets, and less likely to their human owner(s). It can only be transmitted by ingesting cysts from the feces of an infected animal or person. Therefore, preventing stool from contaminating water, food, and yourself is the best way to reduce its spread. Basically, try not to eat your pet's poop, appetizing as it may sound!

Recommendations to reduce spread of the organism include:  

  • Proper hand hygiene with soap and water (especially after playing with your pet, changing an infant’s diaper, handling your pet’s stool, using the bathroom, or before food handling)
  • Pick up dog or cat stool as soon as possible after they eliminate (shame on you, Mr. and Mrs. G!)
  • Prevent pets from drinking out of puddles, ponds, lakes or standing water sources in the great outdoors.
  • Bathe the pet after completion of therapy and wipe the hairs near their back end free of any stool that might remain after a bowel movement to reduce possible shedding of Giardia 

As a fellow dog and cat owner, I can understand the concern we may have if our pet is diagnosed with something like Giardia. If your pet suddenly develops diarrhea, your veterinarian can run several different tests if Giardia is suspected. Thankfully, most cases do not make people or animals sick at all. If it does, it will usually appear one to three weeks after the initial infection, and usually the pet has watery diarrhea for a few days. Some dogs and cats may have subclinical infections, meaning they may be passing it in their stool but have no noticeable signs of disease themselves. The CAPC suggests testing only symptomatic dogs and cats.

So what happens if your veterinarian diagnoses Giardia in your pet with sudden diarrhea? Currently there are no drugs approved to treat Giardia in the United States (oops). Most veterinarians, myself included, will use either a deworming medication called fenbendazole or an antibiotic called metronidazole, or even both to help eliminate the organism and resolve the diarrhea, in addition to bathing. A veterinarian will typically use one of these drugs alone and recheck the pet and a stool sample within 14 to 28 days after finishing the medication. If the pet still has diarrhea and is positive a second time, I’ll usually combine both drugs together, complete a second course of therapy, and recheck again in 14 to 28 days. If the pet is still positive but the diarrhea has resolved and the pet is otherwise healthy, then medication is discontinued and I do not recheck again.

If diarrhea and Giardia persists after several rounds of rechecks and subsequent treatments, then we start looking for other causes such as:  

  • diarrhea that responds to dietary adjustments
  • infection with a different bacteria, virus, or other organism
  • a different internal disease causing the diarrhea
  • pet may have ingested foreign material.  

Sure, Giardia can cause diarrhea. Sometimes it is the sole cause. Other times a pet who has it will have absolutely no signs whatsoever. And in some instances a pet may have it but it is not the cause of their diarrhea. The majority opinion these days is that most dogs and cats without symptoms don't get tested or treated. If treatment is deemed necessary, then one or two rounds of therapy are generally sufficient if there are clinical signs (you should all have the Pepto jingle memorized by now for those signs). There remains no approved or labeled medication to eliminate Giardia.

As we become more educated as pet owners, we can hopefully relieve some of the anxiety knowing our pet may be carrying this parasite, and we can understand that it doesn’t always pose a substantial health risk to humans so long as we practice good hygiene. Therefore, we will hopefully pressure our veterinarian in fewer instances to prescribe repetitive rounds of antibiotics that may not be necessary, might be directly harmful to the good and normal bacteria in the GI tract, or even worse delay diagnosis of the true cause of your pet’s diarrhea.

Repeated courses of treatment are not indicated in dogs or cats without clinical signs. If clinical signs persist, then looking for an alternative cause makes sense if the pet doesn't seem to be responding to routine therapy.

Further information can be found on the following websites:

Remember to wash your hands!


October 31, 2021

REPEATING what Brett Wasik, DVM stated above: So what happens if your veterinarian diagnoses Giardia in your pet with sudden diarrhea? Currently there are no drugs approved to treat Giardia in the United States (oops). There remains no approved or labeled medication to eliminate Giardia. In most cases the infection goes away on its own.

Eileen Dannemann
June 5, 2019

Just picked up a 4 month old female lab retreiver-yellow from a nice Kennel.  They had been feeding her only dry food.  She had from Feb.through May...nearly daily panacur for Giardia.  She presented with diarhea.  I slowly changed her diet to real food including chop beef, squash,veggies...and starting some organ meat, probiotics, Charcoal, Diatomacious earth, clay and apple pectin formula that I make.  Vet did test...she has giardia.  However, the new diet is firming up and I rejected medical treatment for a while until her gut is restored. Now that Giardia is confirmed, I am giving her lumbrakaise (worm casing that disolves biofilm), oregano oil/clovebud oil, grapefruit seed extract, yogurt, probiotics, DE, and charcoal and some bonemeal for calcium.  I also give her some ripe banana as a treat. Her stool is firming up.  I will take her in 9 days for a stool test. Otherwise she has no clinical symptoms...happy puppy.

Laura Werner
May 2, 2019

I had 2 little jack mixes with giardia & I am a firm believer in keeping this from spreading. Dogs interact with each other, and this parasite spreads very easily. I treated my pups with panecur as step 1 just to rid them of the overgrown colony, but then started using MSM (keep from giardia reinfection) & several GI soother/probiotic treatments to help them heal. They are now Giardia free.

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