Turns On and Off — And Can Infect You and Other Pets Even While Asymptomatic

giardia feces contaminated water

Story at-a-glance

  • Giardia is a common infection in pets, especially dogs, and is most commonly acquired through contact with feces-contaminated water
  • Many pets with giardia have no symptoms, but this parasitic infection is very likely the root cause of many cases of chronic gastrointestinal disease in dogs and cats
  • A fecal ELISA or PCR test is the best method for diagnosing giardia, along with analysis by a national veterinary lab versus in-house at the vet’s office
  • Giardia can be difficult to treat successfully and follow-up tests are required to insure the infection has been cleared
  • Infection prevention tips include avoiding crowded boarding facilities, and not allowing pets to sample the poop of other animals or to drink from outdoor water sources

By Dr. Karen Shaw Becker

Giardia is a one-celled parasitic organism found not only in the small intestine of dogs and cats, but also in most wild animals and human populations living in third-world countries. It's generally assumed that while exposure to giardia is common, acquiring disease from the parasite is less common.

Giardia is ubiquitous in the environment and is found in rivers, ponds, puddles and many other cool, moist locations. It's a zoonotic disease, meaning if the family dog has it or a human family member has it, the entire rest of the family — humans and animals — can be infected.

Animals bred in puppy mills and other facilities that house lots of dogs commonly carry the giardia parasite. Your dog can acquire giardia by ingesting an infected cyst contained in another animal's feces. Contamination can occur directly or indirectly through contact with infected cysts, and the most common route of transmission is through feces-contaminated water.

Once inside your pet’s small intestine, the cyst opens and releases the active form of the parasite. These forms are able to move around and attach to the walls of the intestine, where they reproduce. Eventually, the active forms of giardia encyst (build cysts around themselves) and are passed from the animal's body in feces. The poop then contaminates water sources, grass, soil and other surfaces.

Another way the infection can spread is through licking. If a dog is giardia-positive, licks his backside and then licks another dog, cat or human, there's potential for transmission to occur.

Giardia Symptoms

Most giardia infections are asymptomatic, meaning there are no obvious signs your pet is infected. When symptoms are present, the most common is diarrhea, which can be acute, chronic or it can come and go.

Many dog parents consider calling the vet about their pet's soft, mushy stool, but then suddenly it firms up and all seems well again. Due to the intermittent nature of loose stools associated with giardia, many people assume the dog got into something he shouldn't have, or had a meal that didn't agree with him. Consequently, many cases of giardia go undiagnosed.

After a week, month or sometimes years of an undiagnosed giardia infection, a giardia-positive animal can experience an acute and debilitating episode of bloody, dehydrating diarrhea. Most dogs with diarrhea will not lose their appetite, but in chronic cases, they often lose noticeable body weight because the infection interferes with digestion and inhibits absorption of dietary nutrients.

Giardia can also damage the lining of the intestine, and in my experience, this parasite is the root cause of many cases of chronic gastrointestinal (GI) inflammation in dogs and cats. In fact, many pets with inflammatory bowel disease (IBD) have a history of being giardia-positive as puppies or kittens. I also see a number of pets with chronic diarrhea, malabsorption and other digestive issues who test positive for giardia.

How Giardia Is Diagnosed

The giardia parasite is microscopic and can't be seen with the naked eye, so don’t bother trying to find it in your pet’s poop. It’s also important to note that parasite testing performed at your veterinarian's office instead of an independent laboratory may not be accurate. Estimates are that up to 30 percent of in-house tests return a false negative, which means there are a lot of giardia-positive animals testing negative for the infection.

National veterinary labs like Antech and Idexx use standardized equipment that returns consistently reliable results, so if you're having your pet tested for giardia, I recommend asking your vet to send the samples out for analysis.

Another problem in diagnosing giardia is that the parasite isn't shed in every stool. This means there can be cyst-free stool samples from infected animals. If one of these samples happens to be the one collected for analysis, it won't show evidence of giardia, even though your pet is infected.

I recommend an ELISA or PCR test for giardia for any pet with a history of GI issues. A fecal ELISA or PCR test is preferable to a fecal flotation test because it checks for the presence of giardia antigens. A fecal float only detects giardia cysts, which may or may not be in the particular stool sample being tested.

Unfortunately, many veterinarians don't routinely run the ELISA or PCR test and instead, use only stool sample results that may or may not pick up evidence of infection. So make sure to ask your vet for a fecal antigen test in addition to a fecal float. Labs also offer diarrhea panels that check for other common causes of diarrhea and this is an excellent diagnostic choice for any pet with intermittent GI issues.

Effectively Treating the Infection

The standard treatment for a giardia infection is antiprotozoal drugs. However, the parasite is growing resistant to many of these drugs, which means more and more pets are becoming what we call persistent carriers of the infection. Integra­tive vets often blend herbal protocols with standard treatments to enhance their effectiveness. 

That’s why I like to run monthly fecal float tests for three to four months after completion of treatment, sometimes followed by an ELISA test to make sure the infection is fully resolved. The ELISA test can be giardia-positive for up to six months after treatment because it takes awhile for the antigens to clear out of the bloodstream. This is why I don’t run them immediately after treatment completion.

A few fecal floats will give you and your vet the most accurate information about whether the infection has been successfully treated. The reason for more than one test is, again, because giardia cysts aren’t passed in every stool, so a test immediately following treatment may be negative, but a test a week later could be positive. If you stop after one fecal float, it’s very difficult to be absolutely sure the infection is gone.

Infection Prevention Tips

To prevent a giardia infection in your pet, take a few common sense precautions:

  • Don’t kennel your pet in close quarters with other animals.
  • Do clean up your pet’s poop outdoors, and don’t walk your dog in areas where other animals relieve themselves.
  • Don’t allow your pet to drink from outdoor water sources.
  • Do drop off a fecal sample with your vet twice a year for testing. This will help identify the presence of a parasitic infection before digestive function is compromised.

If your dog’s fecal intestinal parasite analysis is negative, don’t routinely deworm your dog. No dewormer kills every parasite in existence, and since no drug is entirely safe, there are always risks associated with giving medications. The same applies to natural dewormers. Don’t use them “just in case,” assuming one medication or herb will knock out roundworm, hookworm, coccidia, whipworm and giardia. It won’t.



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