Giardia in Cats: The Parasite Behind Diarrhea That Isn't a Worm
A round of dewormer that doesn't fix a cat's soft, mucousy stool is a common reason vets end up testing for giardia. It's a genuinely common cause of feline diarrhea, and it's not a worm at all, so the medications that clear roundworms or tapeworms don't touch it.
What giardia actually is
Giardia is a single-celled protozoan parasite that infects the small intestine, spread through cysts shed in feces that contaminate food, water, or a shared environment like a litter box or shelter floor. According to the Merck Veterinary Manual, it's one of the more common intestinal parasites found in cats, particularly young cats and those in crowded shelter or multi-cat settings.
What symptoms actually look like
In adult cats, giardia infection is often subclinical, meaning the cat carries and sheds the parasite without any obvious illness, or shows only a mild, transient softening of stool. Kittens tend to have a rougher time of it, developing more noticeable acute diarrhea, often pale, greasy-looking, and unusually foul-smelling, sometimes with visible mucus.
The zoonotic risk question, and why it's more nuanced than it sounds
Giardia gets lumped in with other "can I catch this from my cat" parasites, but the real picture is more specific. The Companion Animal Parasite Council notes that giardia exists as genetically distinct assemblages, and cats are most commonly infected with assemblage F, a strain that's essentially host-specific to cats and not known to cause human infection. Cats can, less commonly, carry assemblages A or B, the strains more associated with human infection, which is why a genuinely zoonotic-risk case is possible but is the exception rather than the rule for a typical cat's giardia infection.
How it's actually diagnosed
Giardia cysts aren't always shed consistently, which makes a single fecal sample an imperfect test on its own. Today's Veterinary Practice recommends a combination approach: a fecal flotation using centrifugation alongside a specific ELISA antigen test, since relying on just a direct smear can miss real infections. In some cases, a PCR test can also identify which assemblage is present, which is the only way to actually confirm whether a given infection carries meaningful zoonotic risk.
Treatment
Fenbendazole or metronidazole are the standard treatments, and a bath to remove cysts stuck in fur around the hindquarters is often recommended alongside medication, since a cat can reinfect itself by grooming. According to the ABCD's clinical guideline, treatment is clearly warranted for any cat showing symptoms, and for asymptomatic cats carrying a zoonotic-risk assemblage, but asymptomatic cats with the cat-adapted assemblage F don't necessarily need treatment at all.
Preventing spread in multi-cat households
Giardia cysts survive reasonably well in a damp environment, so thorough litter box cleaning, prompt removal of feces, and washing bedding during an active infection all reduce the chance of reinfection or spread to other cats in the home. In shelters, where crowding makes giardia especially persistent, prompt isolation of diarrheic cats is standard practice for exactly this reason.
Frequently asked questions
Can dogs give cats giardia, or vice versa?
It's uncommon. Dog and cat giardia assemblages are largely species-specific, so cross-species transmission between the family dog and cat is possible but not the typical route of infection.
Will a dewormer treat giardia?
Not the typical over-the-counter dewormers aimed at roundworms and tapeworms. Giardia needs a specific medication like fenbendazole or metronidazole, which is part of why a stool test to identify the actual cause matters before treating.
Can a cat get giardia again after treatment?
Yes, reinfection is possible, especially in a contaminated environment or multi-cat household, which is why environmental cleaning alongside medication matters for a lasting fix.
Sources
- Merck Veterinary Manual, "Giardiasis in Animals"
- Companion Animal Parasite Council, "Giardia"
- Today's Veterinary Practice, "Feline Giardia: Transmission, Diagnosis, and Treatment"
- ABCD (European Advisory Board on Cat Diseases), "Guideline for Giardiasis"
This article is for general information and isn't a substitute for advice from your own veterinarian, who can properly diagnose and treat diarrhea in your specific cat.
