Your cat's stool tests came back clean for the usual worms, but the diarrhea won't stop. Your vet mentions a word you've never heard before Giardia, or maybe coccidia and suddenly you're Googling something that sounds nothing like the "worms" you were expecting.

Giardia, coccidia, and other protozoa are single-celled parasites that live in a cat's intestine and can cause chronic, hard-to-pin-down diarrhea. Unlike roundworms or tapeworms, these organisms are microscopic and require a specific stool test to detect. Most cases are treatable with the right medication, but diagnosis depends entirely on your vet knowing to look for them.

Key Takeaways

• Protozoa are single-celled organisms, distinct from intestinal worms, and several types can infect a cat's digestive system.
• Giardia is common in cats and has been found in up to 39% of fecal samples from pet and shelter cats in some studies, with younger cats affected more often.
• Coccidiosis is very common in cats, but most infected cats never show any signs of illness at all.
• Amebiasis is much rarer in cats than the other two, but can cause more serious, persistent diarrhea.
• Diagnosis for all three depends on finding the organism, its cysts, or its eggs in a fecal sample and because shedding is often intermittent, more than one test may be needed.
• Cats in crowded conditions, such as catteries or multi-cat households, are at higher risk of infection and reinfection.

What Are Protozoa, and How Are They Different From Worms?

Protozoa are single-celled organisms, some of which are free-living and some of which are parasites capable of causing disease in cats. They are fundamentally different from intestinal worms like roundworms or tapeworms, which are multicellular organisms visible to the naked eye. Protozoa are microscopic, which means they can only be confirmed through a fecal exam under a microscope rather than by spotting anything in your cat's stool.

The three protozoal diseases most commonly seen in cats are giardiasis, coccidiosis, and amebiasis. Each behaves differently some cause obvious illness, while others are carried silently by otherwise healthy cats but all three are diagnosed the same basic way: by finding direct evidence of the organism in feces.

Giardia: The Most Common Protozoal Culprit in Cats

Giardiasis is a long-term intestinal protozoal infection that is common in cats and found worldwide in most domestic and wild mammals, many birds, and people. Giardia has been reported in up to 39% of fecal samples from pet and shelter cats in some studies, with a notably higher infection rate in younger animals. The parasite lives in the small intestine, where it attaches and multiplies before producing cysts that pass out in the feces.

Cats become infected by swallowing these cysts, either through direct contact with an infected cat or through a contaminated environment. Giardia cysts survive well outside the body, and it takes only a small number of organisms to cause infection which is part of why it spreads so easily in crowded settings. Shedding of cysts can be continuous for days or weeks, but it's often intermittent, meaning an infected cat may test negative on any single day even while still carrying the parasite.

Many cats with Giardia show no signs at all. When signs do appear, they typically include poor appetite, weight loss, and long-term diarrhea that may be continuous or come and go, particularly in kittens. The stool itself often looks distinctive soft, poorly formed, pale, foul-smelling, containing mucus, and appearing fatty and occasional vomiting can also occur.

Coccidiosis: Very Common, Usually Silent

Coccidiosis is caused by an invasion of intestinal tissue by any of several protozoa, and while infection is very common in cats, signs of actual illness are much less common. Almost every cat becomes infected with the coccidia Isospora felis at some point in its life, usually with no noticeable effect on health.

Cats become infected by eating infective egg cysts, called oocysts, which enter the environment in the feces of an infected animal. These oocysts can't cause a new infection until they develop further under the right environmental conditions outside the body. Coccidia more commonly infect kittens and tend to affect animals that have been weakened in some way which is why signs of illness are most often seen during the stress of weaning, alongside other infections, or when the immune system is suppressed.

When coccidiosis does cause visible illness, the most common signs in severe cases are diarrhea (sometimes bloody), weight loss, and dehydration. Because most cats clear the infection on their own without treatment, vets often reserve medication for cats that are genuinely showing signs of illness rather than treating every positive test result.

Amebiasis: Rare But More Serious

Amebiasis is caused by the amoeba Entamoeba histolytica, and while it is common in people, it is only occasionally seen in cats and dogs, and rare in other mammals. Humans are the natural host for this parasite and the usual source of infection for pet cats, who become infected by eating food or water contaminated with feces containing the infective cysts.

Once inside the large intestine, E. histolytica can sometimes cause no obvious signs at all, but it also has the capacity to invade the intestinal lining directly, producing inflammation, ulceration, and bleeding. Short-term disease can range from severe diarrhea that may prove fatal, to an illness that becomes long-lasting, to a case that resolves spontaneously. Cats with a long-term infection may show weight loss, poor appetite, straining to defecate, and diarrhea, and these signs may be continuous or may come and go over time.

Signs Your Cat Might Have a Protozoal Infection

Because protozoal infections in cats vary so widely in severity from completely silent to genuinely serious — relying on symptoms alone to guess at the cause is unreliable. Watch instead for a combination of signs: chronic or recurring diarrhea, poor appetite, unexplained weight loss, straining to defecate, and stool that looks unusually pale, greasy, mucus-covered, or foul-smelling.

My Cat's Stool Tests Came Back Negative for Worms — Could It Still Be Parasites?

Yes. A standard fecal exam looking for worm eggs doesn't automatically test for protozoa like Giardia, and because Giardia cysts are shed intermittently, even a protozoa-specific test can come back negative on a day when your cat happens not to be shedding. If diarrhea continues despite a "clean" fecal result, ask your vet specifically about repeat testing or a Giardia antigen test, since the timing of shedding is a genuine limitation of a single sample.

How Vets Diagnose Giardia, Coccidia, and Other Protozoa

Diagnosis for all three protozoal diseases depends on directly identifying the organism, its cysts, or its eggs in a fecal sample or, for amebiasis, sometimes a biopsy of affected tissue. For Giardia specifically, a diagnosis is usually made by identifying Giardia cysts or Giardia antigen in stool samples, though because cysts aren't shed continuously, several fecal tests may be needed before a definitive answer emerges.

For coccidiosis, vets combine the results of fecal examinations with clinical signs and any visible intestinal abnormalities to confirm the diagnosis, since infection alone (without illness) is extremely common and doesn't always warrant treatment. Amebiasis diagnosis relies on finding active or dormant E. histolytica in feces or in biopsies of affected areas in the colon and if the infection has spread outside the intestine, the parasite can be genuinely difficult to detect through routine testing.

Treatment: What Actually Works

Treatment approaches differ meaningfully across the three conditions, which is exactly why an accurate diagnosis matters more than guessing based on symptoms.

Medications built around metronidazole, such as METROGYL SYRUP or FLAGYL SYRUP, are commonly prescribed by vets for protozoal gastrointestinal infections including giardiasis, working against the anaerobic organisms responsible for the diarrhea. During and after a course of antiprotozoal medication, a gut-support supplement like GUTWELL POWDER, which combines probiotics, prebiotics, and a digestive enzyme complex, can help support normal stool quality as your cat's gut recovers always alongside, never instead of, your vet's prescribed treatment plan.

Bathing and thoroughly rinsing a cat during Giardia treatment can also help, since cysts contaminating a cat's hair may be a separate source of reinfection even after the intestinal infection has cleared.

Why Kittens and Multi-Cat Homes Are at Higher Risk

Kittens face higher risk from all three protozoal diseases because their immune systems are still developing and because the stress of weaning specifically tends to trigger visible illness from infections that might otherwise stay silent. Cats in crowded conditions, such as catteries, shelters, or busy multi-cat households, face particularly elevated risk of infection and reinfection with Giardia, since cysts shed in feces contaminate shared litter boxes, floors, and grooming areas far more easily than in a single-cat home.

This is a genuinely relevant pattern in Indian homes with multiple cats or frequent fostering, where litter box sharing and close contact between cats of different ages and health statuses can quietly sustain a cycle of reinfection even after one cat is successfully treated. Prompt removal of feces and thorough cleaning of shared spaces meaningfully reduces this risk.

Can You Catch Giardia From Your Cat?

The picture here is genuinely mixed and depends on which type of Giardia is involved. Cats may be infected with a type of Giardia that only infects cats, but they may occasionally carry a type that also infects people and it is currently unclear whether the same species can move between domestic animals and humans in every case. Some Giardia species appear to infect a variety of mammals, while others are limited to a single species.

Given this uncertainty, sensible hygiene is the practical answer regardless of which type is involved: disposing of cat feces promptly and washing your hands properly after handling an infected cat or its feces meaningfully reduces any potential risk to your household, particularly for young children or anyone with a weakened immune system.

Preventing Reinfection After Treatment

Breaking the cycle after treatment means addressing the environment, not just the cat that was diagnosed. For Giardia specifically, prompt removal of feces limits environmental contamination, since cysts in feces are a direct source of reinfection, particularly in crowded conditions. Cysts can also contaminate a cat's coat, so shampooing and thoroughly rinsing an infected cat during treatment helps remove any cysts clinging to the fur that could otherwise cause reinfection after the intestinal infection clears.

For coccidiosis, good sanitation matters most in homes with multiple cats: removing feces frequently, preventing fecal contamination of food and water, disinfecting shared bowls and litter trays regularly, avoiding raw meat, and keeping insects under control, since sanitation is especially important in settings like catteries or multi-cat households. If your cat has also been dealing with vomiting, appetite loss, or general lethargy alongside a protozoal diagnosis, our guides on cat vomiting and early illness signs walk through which symptoms need urgent attention versus ongoing monitoring at home.

FAQ

What's the difference between worms and protozoa in cats?
Worms like roundworms and tapeworms are multicellular parasites, sometimes visible in stool. Protozoa such as Giardia and coccidia are single-celled organisms that can only be detected under a microscope during a fecal exam, and a standard worm-focused stool test doesn't automatically screen for them.

Is Giardia in cats serious?
It depends on the cat. Many cats carry Giardia with no visible signs at all, while others develop long-term diarrhea, poor appetite, and weight loss, particularly kittens. It's rarely life-threatening on its own, but the chronic diarrhea it causes needs proper diagnosis and treatment rather than being dismissed as ordinary stomach upset.

Do I need to treat my cat for coccidia if they're not showing symptoms?
Not necessarily. Coccidiosis is extremely common in cats, and most infected cats eliminate the infection spontaneously without ever becoming ill. Vets often reserve treatment for cats showing actual signs like diarrhea, weight loss, or dehydration rather than treating every cat that tests positive.

Can protozoal infections come back after treatment?
Yes, particularly Giardia, since cysts in the environment or on a cat's coat can cause reinfection even after successful treatment. Prompt feces removal, cleaning shared litter areas, and bathing the cat during treatment all reduce this risk, especially in multi-cat households.

How is amebiasis in cats different from Giardia or coccidia?
Amebiasis is caused by a different organism, Entamoeba histolytica, and is much less common in cats than Giardia or coccidia. It can cause more serious, persistent, or even fatal diarrhea in some cases, and diagnosis can be more difficult if the infection has spread beyond the intestine.

References

1. Disorders Caused by Protozoa of the Digestive System in Cats — Merck Veterinary Manual
2. Parasites in Cats — Merck Veterinary Manual