Giardia in Dogs: Signs, Testing, Treatment and Reinfection
Last reviewed: August 2026
Giardia Quick Answer
A protozoan parasite causing pale, greasy, foul diarrhoea — or nothing at all. Testing misses it easily.
Giardia is one of the more frustrating parasites to deal with, not because it is dangerous but because it is slippery. It hides from tests, comes back after apparently successful treatment, and produces a diarrhoea vague enough to be blamed on food for weeks before anyone tests for it.
What It Looks Like
The characteristic presentation is chronic diarrhoea or steatorrhoea — fatty stool — which may be continuous or come and go, and which is more common in puppies. The stool itself is described as soft, poorly formed, pale and malodorous, often containing mucus and looking greasy. Weight loss can follow if it persists.
Equally important: plenty of infected dogs have no signs whatsoever and are discovered only when a faecal sample is checked for something else. A healthy-looking dog with giardia is not unusual, which matters if you have other animals in the house.
The single most useful thing to know
Giardia cysts are shed intermittently. A dog can be infected and test negative simply because it was not shedding on the day the sample was taken. If giardiasis is suspected, several faecal examinations should be done — typically three samples collected over three to five consecutive days. A single negative result does not rule it out, and many dogs are cleared by one test when they should not have been.
How It Is Diagnosed
Diagnosis rests on finding either giardia cysts or giardia antigen in the stool. Because of the intermittent shedding above, most vets will either run repeat samples or combine a cyst-based test with an antigen test, which detect different things and fail in different circumstances. If your dog has had persistent diarrhoea and one negative faecal test, asking about repeat sampling is a reasonable conversation to have.
Treatment, and Why It Is Not Straightforward
In the United States, no drugs are approved specifically for treating giardiasis in dogs and cats. Metronidazole oral suspension is FDA-approved for dogs and is widely used. Fenbendazole is approved for giardiasis in most of Europe and is used in the US too. Vets sometimes combine them.
This is worth knowing because it explains why treatment protocols vary between practices in a way they do not for most conditions, and why your vet may take a different approach from something you read online.
Why It Keeps Coming Back
Most apparent treatment failures are reinfection, and there are two routes for it.
The dog itself. Cysts cling to the coat, particularly around the back end, and a grooming dog swallows them straight back. Infected dogs should be bathed to remove cysts from the coat, and a shampoo containing chlorhexidine digluconate has been suggested for this. Owners are rarely told this, and it is a common reason a course of medication appears not to have worked.
The environment. Cysts are inactivated by most quaternary ammonium disinfectants, steam and boiling water, so hard surfaces can be dealt with. Grass is another matter: yards and runs should be considered contaminated for at least a month after infected dogs were last there. Picking up faeces promptly during and after treatment matters more than anything you can spray.
Can You Catch It?
Less easily than most people fear. Giardia duodenalis comes in eight assemblages. Assemblages A and B have a wide host range and can infect humans; assemblages C and D are the ones that typically infect dogs. So a dog carrying a dog-specific assemblage poses little risk to the household. That said, assemblage typing is not routine in practice, so normal hygiene — handwashing, prompt faeces disposal — remains sensible, particularly around young children and anyone immunocompromised.
When It Is Not Giardia
Chronic diarrhoea has a long differential, and giardia is only one entry. If faecal tests are repeatedly negative, the conversation usually moves to dietary causes, inflammatory bowel disease, exocrine pancreatic insufficiency and other parasites — at which point bloodwork becomes useful for checking protein levels, organ function and inflammatory markers. Any blood in the stool shifts the picture and is worth flagging to your vet.
Chronic diarrhoea and bloodwork done?
Upload your dog's panel and see protein levels, organ values and inflammatory markers explained in plain English — what gets checked when faecal tests keep coming back clear.
Check My Dog's ResultsFrequently Asked Questions
What are the signs of giardia in dogs?
Soft, pale, greasy, foul-smelling stool with mucus, often intermittent and more common in puppies. Weight loss can follow. Many dogs show nothing at all.
Can a dog have giardia with a negative test?
Yes. Cysts are shed intermittently, so three samples over three to five days are usually needed. One negative does not rule it out.
How is it treated?
No drug is approved specifically for giardiasis in the US, though metronidazole oral suspension is FDA-approved for dogs. Fenbendazole is approved for it across most of Europe.
Can I catch it from my dog?
Risk is lower than assumed — assemblages C and D typically infect dogs, while A and B are the human-capable ones. Ordinary hygiene remains sensible.
Why does it keep coming back?
Usually reinfection. Cysts on the coat get swallowed during grooming, so bathing is part of treatment, and grass stays contaminated for at least a month.
Related Reading
Blood in Dog Stool
When diarrhoea contains blood, and what the colour means
Dog & Cat Diarrhoea
When diarrhoea needs a vet rather than watching
Gastroenteritis in Dogs
Inflammation of the stomach and intestines
Lethargy in Dogs
When a parasite load starts affecting the whole dog
Dog & Cat Throwing Up
The other half of most GI presentations