Giardia is a microscopic intestinal parasite that infects cats by colonizing the small intestine, causing soft, fatty, foul-smelling diarrhea. It’s one of the most common intestinal parasites found in cats worldwide, and while many infected cats show no symptoms at all, kittens and cats with weaker immune systems can develop chronic digestive problems and weight loss if the infection goes untreated.
How the Parasite Works
Giardia duodenalis is a single-celled organism, a protozoan, not a worm or bacteria. It exists in two forms. The first is the cyst, a tough, dormant shell that survives in the environment and is responsible for spreading the infection. Cysts can survive for several months in cold water and persist on surfaces, making them surprisingly hard to eliminate. The second form is the trophozoite, the active stage that lives inside your cat’s gut.
When a cat swallows cysts (typically from contaminated water, food, or by grooming after contact with infected feces), the cysts crack open in the small intestine and release trophozoites. Each cyst produces two of these pear-shaped organisms, which then attach to the intestinal lining using a suction-cup-like disk. They multiply rapidly and interfere with your cat’s ability to absorb fat and nutrients. As the parasites move toward the lower intestine, they re-form into cysts and pass out in the stool, ready to infect the next host.
Signs of Giardia in Cats
Many cats carry Giardia without showing any symptoms. The infection is sometimes discovered only during a routine fecal exam. When symptoms do appear, the hallmark is diarrhea with a distinctive appearance: soft, poorly formed, pale, greasy-looking stools that smell worse than usual and often contain mucus. This greasy quality comes from fat malabsorption in the small intestine.
Watery diarrhea is uncommon with Giardia, and blood in the stool is rare. The diarrhea can be continuous or come and go over weeks, which sometimes leads owners to assume the problem resolved on its own before it returns. Kittens are more likely to show noticeable symptoms than adult cats. Weight loss can develop over time, especially in young or already-thin cats. Vomiting is possible but unusual.
How Quickly Symptoms Appear
After a cat swallows Giardia cysts, the prepatent period (the time between ingestion and when cysts start appearing in the stool) ranges from 5 to 16 days. Symptoms, when they occur, typically develop within this same window. This delay matters because a cat can be shedding infectious cysts before you notice anything is wrong, potentially exposing other pets in the household.
How Giardia Is Diagnosed
Diagnosing Giardia can be frustrating because the parasite sheds cysts intermittently, meaning a single stool sample can easily come back negative even when the cat is infected. Veterinarians typically use one of two approaches.
A fecal flotation test using zinc sulfate solution is considered the more reliable method. It concentrates cysts from the stool so they can be seen under a microscope. In chronic carriers, this method picks up the parasite about 85% of the time. A rapid antigen test (sometimes called a SNAP test) detects Giardia proteins in the stool and gives results within minutes. However, its sensitivity drops to around 50% in cats that have been carrying the parasite for a while. Because of this, vets often recommend testing multiple stool samples collected over several days, or combining both methods, to avoid a false negative.
Treatment Options
Two medications are considered the standard treatments for feline Giardia. Fenbendazole is an antiparasitic drug commonly used because it’s generally well tolerated and safe for most cats, including kittens. Metronidazole is the main alternative. At lower doses given over about seven days, it’s effective and unlikely to cause problems. Higher doses carry a risk of neurological side effects including weakness, loss of coordination, and in rare cases, seizures. Metronidazole should not be used in pregnant cats.
Some infections clear with a single course of treatment. In stubborn cases, particularly in multi-cat households where reinfection is common, a second round may be needed. In those situations, a combination of both medications is sometimes used. A third option, ronidazole, has shown effectiveness in both cats and dogs and may be considered when standard treatments fail.
Why Reinfection Happens
Giardia is notoriously difficult to eliminate in multi-cat environments because the cycle of shedding and re-swallowing cysts is hard to break. A treated cat can pick up cysts from its own fur during grooming, from a shared litter box, or from contaminated surfaces. This is why environmental cleanup matters just as much as medication.
Bathing your cat on the last day of treatment helps remove cysts clinging to the fur, especially around the hindquarters. For cats that won’t tolerate a full bath, wiping the rear end and hind legs with a damp cloth is a reasonable alternative. In multi-cat homes, all cats should ideally be treated at the same time, even those without symptoms, to prevent them from passing the parasite back and forth.
Cleaning Your Home
Giardia cysts are hardy, but they’re vulnerable to heat and proper disinfection. Your approach depends on the type of surface.
- Hard surfaces (floors, counters, litter boxes): Clean with a bleach solution of 3/4 cup of bleach per gallon of water. Keep the surface visibly wet for the contact time listed on the product label before rinsing.
- Soft surfaces (carpet, upholstery, cat beds): Steam cleaning is the most effective method. The steam needs to reach 158°F for at least 5 minutes, or 212°F for 1 minute.
- Food bowls, water dishes, and toys: Run them through a dishwasher, or submerge them in boiling water for at least 1 minute. Even lower temperatures work with longer exposure: 113°F for 20 minutes or 122°F for 5 minutes will kill cysts.
Litter boxes should be scooped daily during treatment and fully emptied and disinfected at the end of each treatment course. If possible, replace litter boxes entirely after treatment, since microscopic scratches in plastic can harbor cysts even after cleaning.
Can You Catch Giardia From Your Cat?
The risk is very low. The strains of Giardia that typically infect cats are not the same strains that cause illness in humans. The CDC states that you are unlikely to get a Giardia infection from dogs or cats. That said, basic hygiene still applies: wash your hands after cleaning the litter box, and keep immunocompromised household members away from litter duty as a general precaution with any parasite.