Cats get giardia by swallowing microscopic cysts of the parasite, almost always through the fecal-oral route. This means a cat ingests cysts either by direct contact with infected feces, by grooming contaminated fur (their own or another cat’s), by drinking contaminated water, or by encountering cysts in a shared environment like a litter box. Once swallowed, the cysts open up in the small intestine, release active parasites, and the infection cycle begins.
The Fecal-Oral Route
Giardia exists in two forms. The first is the cyst, a tough, dormant shell that survives outside the body and serves as the infectious stage. The second is the trophozoite, the active parasite that lives inside the gut. When a cat swallows cysts, they travel to the small intestine, crack open, and release trophozoites. These parasites latch onto the intestinal lining, absorb nutrients, and multiply by dividing in two. As they move further down the intestine, they re-form into cysts and pass out in the cat’s stool, ready to infect the next host.
This cycle is what makes giardia so persistent. An infected cat sheds enormous numbers of cysts in its feces, and it only takes a small number of ingested cysts to start a new infection. Cats that groom their paws or hindquarters after stepping in contaminated litter are especially vulnerable to reinfection, which is why the parasite can be difficult to fully clear in multi-cat households.
Where Cats Pick Up Cysts
The most common sources are contaminated water, shared litter boxes, and direct contact with infected animals. Outdoor cats face additional risk from puddles, streams, and soil where infected animals have defecated. Kittens in shelters, catteries, and breeding facilities are particularly prone to infection because of high animal density and shared living spaces.
Giardia cysts are remarkably hardy. According to EPA data, cysts survive two to three months in water below 50°F (10°C) and remain viable for nearly a month even at room temperature around 70°F (21°C). Survivability drops as temperatures rise, but in cool, damp conditions (shaded yards, wet litter, standing water), cysts persist long enough to infect animals weeks after the original contamination.
Indoor cats aren’t immune. A new cat brought into the household, a contaminated water bowl, or even cysts tracked in on shoes can introduce the parasite. In multi-cat homes, a single infected cat shedding cysts into a shared litter box can expose every other cat in the house.
Which Cats Are Most at Risk
Kittens and young cats are more likely to become infected and to show symptoms. Their immune systems are still developing, and they’re more often in high-density settings like shelters where the parasite circulates freely. Adult cats with healthy immune systems frequently carry giardia without ever showing signs of illness. Many infections in mature cats produce no symptoms at all, which means a cat can be shedding cysts and spreading the parasite without anyone realizing it.
Cats with weakened immune systems, those on immunosuppressive medications, or those dealing with concurrent illness are more likely to develop clinical disease from an infection.
Signs Your Cat May Be Infected
The hallmark symptom is soft, pale, greasy-looking diarrhea. Because the trophozoites damage the intestinal lining and interfere with nutrient absorption, affected cats may also lose weight, have a poor coat, or seem lethargic. The diarrhea can be intermittent, coming and going over days or weeks, which sometimes delays diagnosis.
Here’s the complicating factor: many cats test positive for giardia without ever developing diarrhea. Experimental infections have shown that not all cats develop clinical signs even when deliberately exposed. This means a positive test result in a cat with diarrhea doesn’t automatically confirm giardia is the cause. Your vet will consider the full picture before starting treatment. Current guidelines actually recommend against treating cats that test positive but show no symptoms, because unnecessary treatment can contribute to drug resistance.
How Giardia Is Diagnosed
Vets typically use one of three approaches: a rapid in-clinic antigen test, a fecal flotation technique using zinc sulfate solution, or a combination of both. All three methods perform similarly in cats, with sensitivity around 91 to 93% and specificity above 95%. Combining the antigen test with fecal flotation bumps sensitivity up to about 97.5%, which is why vets sometimes run both when they suspect giardia but get an initial negative.
Because cyst shedding can be intermittent, a single negative fecal test doesn’t rule out infection. If your cat has persistent diarrhea and one test comes back clean, your vet may recommend retesting a few days later.
Can You Catch Giardia From Your Cat?
This is a common worry, but the risk is very low. The type of giardia that typically infects cats is not the same type that causes illness in humans. Cats usually carry a host-specific strain, while human giardiasis comes from strains adapted to people. The CDC notes that while animal-to-human transmission is theoretically possible, you are unlikely to get giardia from your cat. Basic hygiene, washing your hands after cleaning the litter box and before eating, is sufficient protection.
Preventing Infection and Reinfection
Reinfection is the biggest challenge with giardia. Even after successful treatment, a cat can immediately pick up new cysts from its own contaminated environment. Prevention comes down to breaking the fecal-oral cycle through aggressive cleaning.
For hard, non-porous surfaces like litter boxes and tile floors, a bleach solution of 3/4 cup bleach per gallon of water is effective at killing cysts. Soft surfaces like carpet or fabric need steam cleaning at 158°F for five minutes or 212°F for one minute. Food and water bowls can be disinfected in a dishwasher with a dry cycle or final rinse that reaches at least 113°F for 20 minutes, 122°F for five minutes, or 162°F for one minute. If you don’t have a dishwasher, submerging bowls in boiling water for at least one minute works.
During and after treatment, scoop litter boxes at least once daily and fully replace the litter frequently. In multi-cat homes, consider temporarily separating the infected cat and providing individual litter boxes. Bathing your cat near the end of treatment can help remove cysts clinging to the fur around the hindquarters, reducing the chance of reinfection through grooming. Keep outdoor access limited if possible, since puddles and moist soil are prime sources of new exposure.
For cats that drink from outdoor water sources, providing fresh, clean indoor water can reduce one of the most common routes of exposure. Giardia cysts are resistant to standard chlorine levels in tap water, but they don’t survive boiling, so if your water supply is from an untreated well, boiling or filtering it through a filter rated for cyst removal adds a layer of protection.