Parasites That Cause Diarrhea in Cats

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Parasites That Cause Diarrhea in Cats

Diarrhea is a commonly encountered health problem in cats. The severity of diarrhea can range from soft stool to a full-blown watery or bloody diarrhea, depending on the cause. In cats, one of the most common causes of diarrhea is intestinal parasites. In some areas, the prevalence of parasitic infection is as high as 45%. This means that nearly half the cats in certain areas are infected with at least one species of intestinal parasite. With the high number of stray cats in most places, control of parasite transmission can be challenging. So, what types of parasites cause diarrhea in cats? And what can you do to keep your furry friend healthy? Continue reading to find out!

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Types of Intestinal Worms in Cats

Several gastrointestinal (GI) parasites affect cats, but they can be grouped into two main types: helminthic parasites and protozoal parasites.

Helminthic parasites are worms that stay inside the intestinal tract of cats. They survive at the expense of the cat’s health and get their nourishment from the cat’s tissues and ingested food. They attach themselves to the wall of the intestines, causing damage and inflammation that often results in diarrhea.

On the other hand, protozoal parasites are single-celled organisms that invade and penetrate the outer lining of the intestinal wall. They proliferate and infect the cells along the mucosal lining of the intestine causing damage and diarrhea.

Helminth Parasites in Cats


Roundworms, or ascarids, are the most common type of helminthic parasites found in cats. Roughly 25% to 75% of cats are affected by roundworm infection, with more cases reported in kittens than adult cats. These worms have round bodies and tapered ends, are cream-colored, and can grow to around 3-5 inches long.

Unlike other worms, ascarids are not attached to the intestinal wall. They get their nourishment from the food their host ingests. Roundworm infections don’t usually cause severe symptoms in adult cats but cause GI signs like vomiting and diarrhea in kittens. If left untreated, roundworm infection can be fatal to young kittens.

Roundworms, much like other intestinal parasites, are transmitted via the fecal-oral route. Adult female roundworms lay fertile eggs which are excreted by the host cat through their feces. Infection happens when another cat ingests food or water contaminated by the feces of an infected cat.

Rodents can also become infected and can serve as intermediate hosts. Roundworm eggs hatch inside the rodent’s body and become larvae, but do not mature into adult worms. If the infected rodent is ingested by a cat, the larvae will migrate and become mature roundworms inside the cat’s intestine, resulting in roundworm infection.

Kittens can also become infected by nursing from an infected mother (queen). Roundworm larvae can pass through the queen’s milk and can cause infection to kittens nursing from her.


Hookworms are blood-sucking intestinal worms. They appear as slender worms around half an inch in length, with a “hook” at the anterior end of their body. They attach to the cat’s intestinal wall and feed on its blood, causing hemorrhage and bloody diarrhea in infected cats. They are less common than roundworms and prevalence among cat populations varies greatly depending on the geographical location.

Hookworms are also transmitted via the fecal-oral route, but unlike roundworms, adult hookworms pass on larvae instead of eggs. Transmission happens when a cat ingests food and water contaminated by the feces of an infected cat. Transmission can also happen when larvae penetrate the skin of other cats. It will migrate to the lungs where it will become mature larvae before migrating to the cat’s intestine where they come an adult.


Tapeworms are flat-shaped worms that have a small head connected to a segmented body. Each segment, called proglottids, is shaped like sesame seeds or rice grains. They contain numerous tapeworm eggs and break off from the rest of the body once it matures. Among all the intestinal worms that affect cats, tapeworms are the only helminth parasite that requires an intermediate host to complete its life cycle.

Proglottids are passed out via the cat’s feces and are eaten by fleas or rodents in the environment. Cats become infected when they ingest the fleas or rodents that are infected with tapeworm. Since tapeworms need fleas to become infective, good flea control is necessary to prevent tapeworm infection in cats.

Protozoal Parasites


Coccidiosis in cats is caused by a one-celled organism called Isospora sp. Most adult cats can tolerate coccidia in their system, and infection is often self-limiting. But infected kittens often present with severe diarrhea and continuous vomiting, which can eventually be fatal if not addressed accordingly.

Isospora’s cysts (infective stage) are excreted via the cat’s feces and become mature in the soil or ground. Infection happens via fecal-oral transmission when a cat ingests food or water contaminated by the feces of an infected cat.


Giardia sp. is a highly motile one-celled organism that infects cats and causes a condition called Giardiasis. Infection is uncommon, with around 5% prevalence in some regions, but it can spread easily in a multiple cat household. It mostly affects and causes disease in cats younger than 1 year of age.

Like Isospora sp., Giardia sp. produces infective cysts that are excreted via the feces of an infected cat. Their cysts are much more resistant against common household cleaning and disinfecting materials and can persist in the environment for a relatively long time. Infection occurs when other cats eat or drink materials contaminated with their infective cyst.

Treatment and Management of Intestinal Worms in Cats

Most helminthic parasite infections can be treated with common deworming medications. A series of treatments usually done every 2 weeks for at least 3 to 4 doses is needed to completely eradicate the infection. Tapeworms, having a segmented body, will require a continuous daily dosage of deworming medicine to kill the entire parasite.

Protozoan parasites are treated with anti-protozoal medicine such as metronidazole. Most protozoal infection cases can be completely eradicated in about 5-7 days of medication, but some species like Giardia sp. can become resistant and will require longer treatment.

Cleanliness and good hygiene are the most effective ways to manage and control the spread of intestinal parasites in cats, especially in a multiple-cat household. Making sure that the water and feeding bowls don’t contain contaminants is often enough to stop the transmission of these parasites.

Regular deworming doses help prevent intestinal worm infection in cats. For kittens, deworming is started at around 6-8 weeks of age and is done every 2 weeks for a total of at least 4 doses. Adult cats need to be dewormed every 3 to 6 months, depending on the degree of exposure and risk factors. For control of tapeworm infection, external parasite treatment for fleas is also needed to prevent the further spread of the infection.

Read more:

Common Intestinal Parasites in Cats

Diarrhea in Cats 101

How to Protect Your Cat from Fleas

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Published: 7/13/2021
Last updated: 7/29/2021

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