Veterinarian-written / veterinarian-approved articles for your cat.

Single Celled Parasites of Cats



Giardia is a “protozoan” or primitive single-celled parasite that lives in the small intestine of cats. Giardia cysts are found in water, and transmission occurs via intake of cysts by mouth. Some people refer to this condition as Beaver Fever because outbreaks were historically associated with ponds, but now city water supplies are sometimes also contaminated—a long way from the wild.

Giardia causes smelly, loose bowel movements, bloated abdomen, gassy frothy stool, increased gut sounds, and sometimes lethargy and reduced appetite. Most cats appear totally normal. People may also develop similar symptoms if infected with this parasite. This parasite is nationally reportable in people. Fecal floatation may or may not reveal the cysts of the parasite; special fecal smears are much more commonly used to view the parasites in their trophozoites form; a swimming cell. The smear will require fresh samples of material be taken right from the back end, or having been passed very recently. There is also a laboratory test to identify this parasite.


This parasite, Cryptosporidium felis is a cause of diarrhea in cats. Some cats may not develop obvious signs though. Cysts are identified using high power microscopic examination during the fecal flotation test. The cysts are resistant to cold and heat in the environment and can survive for months outside of their host. If the cat has chronic virus infections like Feline Leukemia Virus, the infection can be severe. There is no really effective treatment for this parasite.

In people, this is a nationally reportable condition. In immune compromised people, this can be a life threatening illness. Healthy people may develop short term diarrhea, sometimes quite severe.


Isospora felis is the name of the coccidian parasite that most commonly infests cats. These parasites have a narrow host range. Coccidiosis can result in watery, sometimes bloody chronic diarrhea, often in large amounts, or the cat may appear totally normal. Some cats may carry the parasite for many years and periodically shed out the cysts but experience no obvious illness. Infection with coccidia causes more problems in kittens than in adults. Stress can increase the risk of a coccidian infection or increase the severity of the signs. Cats become infested if they eat infected mice or pick up cysts in the environment. Diagnosis is made by observing oocysts in the fecal test performed by your veterinarian, though some cats with diarrhea are not yet shedding the cysts so a false negative test can occur.


This disease is caused by Toxoplasma gondii, another digestive tract coccidian parasite. Cats are the definitive host for this parasite, but the cysts can infest any warm blooded animal, though their life cycle is incomplete in these other hosts. Intake of cysts by the oral route leads to infection if the cysts have had time to become infectious (takes between 24 hrs - 5 days). The parasite goes into other body tissues such as muscle, liver and brain as well as living in the digestive tract. Trans-placental transmission is possible in cats. The cysts are shed by the cat out in their stool for a few weeks. The lag between taking in the infectious cysts and the start of this shedding phase is variable, but generally about 20-48 days later, the cat will be shedding the organism.

In healthy adult people, signs of toxoplasma infection often appear similar to the flu, with sore muscles, fever and swollen glands. Women who are pregnant should not clean litter boxes or handle or eat undercooked or raw meat while pregnant since a new infection in an expecting woman may lead to serious hazards for her developing fetus. With protective gloves and careful washing up afterwards, those women living alone can reduce their risk, but not eliminate it.

In cats, there are usually not any clinical signs associated with the parasite, but in some cats, signs of toxoplasmosis include lethargy, depression, weight loss, loss of appetite, fever, and eye and nervous system symptoms. An immature or diseased immune system predisposes an animal to the system-wide disease signs associated with toxoplasmosis.

Diagnosis will involve fecal and blood tests, and sometimes additional testing will be recommended by your veterinarian. Treatment involves appropriate antibiotics and supportive care.

One can help prevent toxoplasmosis infection in cats by not feeding them raw meat or raw animal by-products, and keeping them inside to prevent hunting.


These are intestinal parasites that are often seen in cats and kittens with diarrhea. Unlike many of the single celled parasites, they do not form cysts. There is some question as to whether they cause the diarrhea disease, or just worsen it when combined with other agents such as Giardia.

Pentatrichomonads and Tritrichomonas foetus have been studied recently, and the latter has been confirmed to produce long term (6 months-2 years) diarrhea when co-infection with Giardia has occurred.


The Hepatozoon americanum protozoa parasite is only rarely seen in cats. This parasite is limited to warm climates. The parasite enters the cat via a tick bite, or if she eats the tick during grooming. Infection may lead to no signs at all, or range to a severe disease with poor appetite, weight loss with muscle wasting, fever, anemia and diarrhea. The liver may become enlarged, and a muscle biopsy is used to diagnose the condition. Therapy is supportive.

You May Also Like These Articles:

Uncommon Feline Parasites

Worms in Cats: Feline Intestinal Parasites

Cat Fleas: Does My Cat Have Fleas?

How to Treat Fleas in Kittens

Rabies in Cats

Bloodstream Parasites of Cats

FIP: Feline Infectious Peritonitis in Cats

Disclaimer: This website is not intended to replace professional consultation, diagnosis, or treatment by a licensed veterinarian. If you require any veterinary related advice, contact your veterinarian promptly. Information at is exclusively of a general reference nature. Do not disregard veterinary advice or delay treatment as a result of accessing information at this site. Just Answer is an external service not affiliated with

Notice: Ask-a-Vet is an affiliated service for those who wish to speak with a veterinary professional about their pet's specific condition. Initially, a bot will ask questions to determine the general nature of your concern. Then, you will be transferred to a human. There is a charge for the service if you choose to connect to a veterinarian. Ask-a-Vet is not manned by the staff or owners of, and the advice given should not delay or replace a visit to your veterinarian.