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24 June 2010

How to treat -- or not treat -- giardia in your dog

3d rendered close up of some isolated giardia parasitesHere in California, many dogs, perhaps even a majority, have some giardia organisms in their digestive tracts. Those are the little critters depicted at right, parasites that can cause diarrhea and intestinal distress.

Those high numbers have probably always been present, but we didn't know it until recently. That's because we used to only look for it when the dog had diarrhea. And now we have all these great new tests that can examine the dog's stool and tell you whether or not any giardia critters have set up housekeeping in his innards.

Which means a whole lot of healthy, normal dogs are getting treated for a condition that wasn't causing them any problems.

Since there is no 100 percent effective treatment for giardia, and a dog may test positive for giardia months or even years after treatment, you can see the problem. What, exactly, are you treating here? And should you?

Many veterinarians seem to take a scorched earth approach to digestive health, which is a really good way to end up with resistant organisms and a dog with chronic diarrhea from the death and destruction of his natural intestinal flora. If a dog is doing perfectly fine, and it's probably impossible to really eradicated these little intestinal hitchhikers, why treat?

On the other hand, asymptomatic giardia can become symptomatic if a dog is stressed, has other digestive illnesses, or even is put on antibiotics for anything, with an accompanying die-off of giardia competitors.

Also, while dog-to-human giardia transmission is surprisingly uncommon, it's not impossible.

My personal approach is not to treat a positive test result, and only treat if the dog has symptoms. My treatment goal is to eliminate the diarrhea, and then do everything possible to restore and maintain digestive health, including probiotics (the product FortiFlora has shown some efficacy against giardia in lab mice; I'm not aware of any that has been tested in canines), herbs known to promote good digestion (I'm a fan of peppermint, chamomile, ginger, and the food herb slippery elm, but there are many others). And I also have found that many dogs with chronic diarrhea problems do much better on a grain-free diet.

When it comes to that first treatment step, however, I prefer to use a veterinary drug that has the best chance to work with the least chance to harm my dog. That would be the drug Panacur (fenbendazole), which is considered a particularly safe drug, with few listed side effects. It also clears up around 90 percent of all giardia cases.

For reasons I can't understand, however, a lot of vets still reach first for a less effective, much more dangerous drug, Flagyl (metronidazole). It's only effective in around 67 percent of cases, and carries some pretty scary side effect warnings, particularly at the fairly high giardia dose.

Some very resistant cases will need both drugs, but for a first line therapy, there's no reason I know of to use Flagyl, and plenty of reasons not to, including the risk of neurological side effects.

There's also a giardia vaccine... you can see what I think of that right here, but in short, the 2006 American Animal Hospital Association Guidelines don't recommend it and neither do I.

Photo: Stock 3-d rendered close up of isolated giardia parasites.


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Ingrid King

It would be interesting to hear feedback from vets as to why Metronidazole is still used so frequently, despite the potentially serious side effects.

When I adopted my kitten at 7 months of age, she had blowout diarrhea and tested positive for giardia. The vet clinic I adopted her from treated her with Panacur and Metronidazole. The Panacur made sense to me, the Metronidazole made me cringe - seemed like a heavy duty drug for such a young kitten, considering the potential side effects. By the time I adopted her, she only had one dose left, so I finished it out. She still had raging diarrhea.

They sent me home with a small bag of dry i/d (a bland prescription diet high in by-products and grains) because that's what they started to feed her when she started with diarrhea. I had no intentions of feeding dry food, let alone dry i/d. After two days on a grain-free canned diet, probiotics and digestive enzymes, her diarrhea resolved. From blow out diarrhea to normal stools in two days by taking the grain out of the diet and providing digestive support? Maybe it was also a case of the drugs kicking in, but I thought it was pretty impressive. Two months later, her fecal test came back negative.


This is interesting. My many vets over the years I have lived off and on in Florida have often checked for Giardia. When positive (often) they warns me about potential harm to my kids and our neighborhood kids who may be barefoot on our lawn.

I used to get so freaked out over "zoonotic" diseases.

I like your approach for this specific issue- if no symptoms no treatment.

Dee Green

Excellent post, thank you. I also avoid "treating a disease" that is asymptomatic. I prefer to avoid prescriptions until the very things you've mentioned (probiotics, herbs, grain-free diet) have proved to not resolve the issue. Far too few people question the use of Flagyl and other antibiotics in dogs, IMO. I've often wondered if so many vets prescribe them because they dispense, as opposed to things like probiotics, herbs, etc., which they don't?


Want to hear something really scary? I was at an obedience trial a few weeks ago and overheard a conversation about treating "stress diarrhea" with metronidazole. The handler said that her vet let her buy a huge bottle of it and she uses it for a few days at a time whenever one of her dogs pops up with diarrhea at a trial or what have you. Great stuff! Really cheap!

I found this distressing on so many levels.


Years ago I found a JVMA article that reported that 60% of giardia in California is resistant to metronidazole. The article recommended fenbendazole and metronidazole in combination as the treatment of choice. I have never even heard of a vet suggest anything other than metronidazole. I no longer go to the vet for giardia. Fenbendazole is available OTC in several forms. You can find the dosing info for giardia on the Net. The nice thing about fenbendazole is how incredibly safe it is.

When I got giardia last summer, the doc prescribed metronidazole. It worked. So far as I know fenbendazole is not approved for human use. For sure I would not take it.

H. Houlahan

If I was as sick as Grahund was when he got the Montana Munge, and the metronidazole wasn't working, damned right I'd dose myself with fenbendazole! It would be panacur or a bullet if that went on for long.

I've had giardia, and fortunately the flagyl worked (I'm not in CA -- don't know what the resistance is among our PA cooties.) It is not the worst intestinal distress I've ever suffered, but it's in the top three (right after life-threatening food poisoning and colonoscopy prep).

When it gets bad enough, it's not the fear that you are gonna die, but the much worse fear that you will NOT that dominates.

The dogs do not appear to be nearly so distressed when they are affected. Advantage of the carnivore intestine.


And sometimes the Big D isn't about giardia at all, even though it shows up in stool samples. My standard poodle kept having on and off bloody diarrhea - and tested positive for giardia. He was treated with Flagyl, it would clear up for a while, then he would have another episode. This continued for about a year until he had his first (and only) Addisonian crash. His Addison's has been well controlled for about 7 years now, but he's become one of the "lucky" dogs who can't take Flagyl (he developed liver problems from it, now reversed).

Max Salinas DVM

The reason many vets choose Metronidazole at 5mg/kg for giardiasis is that it is very safe at this dosage (please, give all info before you say something is dangerous) and it has GI anti-inflammatory action that fenbendazole does not. However, I agree with what your main points are:Only clinical pets should be treated, and that fenbendazole is a far better product for this condition. Vets should, in my opinion, follow CAPC guidelines for parasite control.