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Internal parasites in cats

There are two main types of internal parasites that your cat may be susceptible to: protozoan parasites and worms. 
Reading timeInternal parasites in cats
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Kittens are especially prone to suffering from internal parasites because their immune system is still developing. Read on to find out more about how parasites can affect your cat.

What are protozoan parasites?

Protozoan parasites are single-celled organisms with the ability to invade and live within the tissue of cats and other animal species.

Potential symptoms of protozoan parasites can include severe diarrhea, incomplete digestion, dehydration and weight loss. Some of the most common types of protozoan parasites that affect cats are:

  • Giardia—microscopic protozoa which fix onto the mucus membrane of the small intestine. Cats catch these parasites when they ingest cysts (a parasite's version of eggs) either through sniffing, eating or drinking.
  • Coccidia—ingested by swallowing cysts in the feces of other animals. 
  • Tritrichomonas blagburni—a very common cause of chronic diarrhea in kittens.

Toxoplasma is another protozoan parasite which spreads when cats eat infected animals such as birds and mice or come into contact with feces from animals with the parasite. While most animals will not develop symptoms as a result, it can cause a condition called toxoplasmosis. A clinical disease, acute infections can affect the nervous system and even cause pneumonia. 

Toxoplasmosis is considered a public health hazard and can be very dangerous to humans and other animals - especially pregnant women. Find out more about Toxoplasmosis.

How can a cat become infected by protozoan parasites?

A cat or kitten can become infected in one of the following ways:

  • By eating the infective form of the parasite—in the form of a cyst
  • By grooming or licking fur contaminated with feces
  • Sharing dirty litter trays
  • Eating something contaminated with feces

Symptoms of protozoa in cats

Only your vet can identify these parasites and prescribe the appropriate treatment. That being said, infected cats may present a number of symptoms to be aware of, including:

  • Diarrhea
  • Bloody stool
  • Swelling and redness around the anus
  • Mucus present in the feces

If you are concerned about your cat as they're showing any of the above symptoms, always make sure you take them to the veterinarian.

How to treat protozoan parasites in cats

Many protozoan parasite infections will clear up on their own. However, your veterinarian may prescribe oral medication to counteract the side effects of an infection. Always consult your veterinarian on the best treatment.



Worms are parasites that will infest the intestines of a cat or kitten.

There are five types of worms commonly found in cats that cause gastrointestinal issues in cats:

  • Roundworms: These worms are in the kitten's small intestine, where they form balls and can cause intestinal obstructions. They can be detected by the presence of eggs in the cat's stool or in anal areas. Cats are infected when they ingest worm eggs or eat infected animals.
  • Tapeworms: Cats catch tapeworms most commonly by ingesting infected fleas but can also be infected by eating rodents and rabbits. They fix onto the walls of the intestine and cause bloating and diarrhea, and sometimes damage to the coat. You may see tapeworms in your cat's stool, stuck to the fur around their tail or where they sleep. They look like grains of rice but are not to be confused with pinworms which look similar but cannot infect cats and dogs.
  • Hookworms: Like other parasites, hookworms attach themselves to the intestines of cats and live on the blood of the animal, but can also be found in the lungs. Cats are infected when they ingest hookworm larvae or the parasite can also burrow into the skin of the animal. Severe infections can be fatal if left untreated, especially for young kittens.
  • Heartworms: Although not as susceptible as dogs, cats can also get heartworms from infected mosquitos. They grow in the lung and heart arteries 3-4 months after infection and cause issues with a cat's breathing. Infection can be fatal, but can also be prevented with medication.
  • Stomach worms: There are two types of stomach worms - Ollanulus tricuspis and Physaloptera. They attach themselves to your cat's stomach lining and can cause vomiting and other stomach issues. Cats catch Physaloptera stomach worms when they eat an infected insect or animal such as a mouse, or if they get in close contact with another infected animal.

How can a cat become infected by worms?

Infestation can vary between each type of worm; however, the most common way a cat or kitten will get worms is by coming into contact with eggs or feces from another infected cat.

Some outdoor cats may hunt rodents and become infected by worm larvae living in the animal's tissue.

Litters of kittens can develop worms if their mother is infected. This occurs through nursing when worm larvae pass to the kittens through their mother's milk.

Symptoms of worms in cats

It's impossible to tell if a cat or kitten has worms just by looking at them; however, there are a number of symptoms that may accompany a worm infestation to be aware of:

  • Diarrhea
  • Weight loss
  • Dry or coarse fur
  • Vomiting
  • Blood in the stool
  • A bloated abdomen
  • Lethargy
  • Worms visible in stool or around the anus

Treatment and prevention of worms in cats

Your cat or kitten should receive regular deworming treatments during their initial growth period.

Your veterinarian will make specific recommendations for routine deworming based on your cat's lifestyle, focusing specifically on whether they go outdoors and if they'll come into contact with other cats.

To ensure your kitten stays healthy while their immune system develops, it's important to speak to your veterinarian about both their vaccination schedule and any treatment plans for internal parasites.

Can cats pass parasites to humans and dogs?

Many parasites can be passed between cats, other animals and humans. Certain parasites are known as "zoonotic", which means they can spread from animals to humans.

Tapeworms, hookworms, toxoplasma and roundworms can all be passed from your cat to humans and other pets such as dogs. Spreading most commonly happens if you stroke a pet and eat without washing your hands. Or, if a dog is playing in soil contaminated by another animal. However, when it comes to hookworms, the larvae can burrow into your skin after contact.

It is recommended to wash your hands after petting your cat and after scooping their litter box - and if you suspect that they have parasites, please consult your veterinarian.

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