Parasitic Worms in Cats by Dr. Carl Hannigan

Spring has a tendency to spark “parasite season” in the veterinary world.

Roundworms (nematodes)

There are two types of roundworm parasites that can affect cats, ascarids and hookworms and are so called because they are round in section.


In adult cats symptoms are usually subclinical. Symptoms of diarrhea, vomiting and distended abdomen with poor coat and failure to thrive are often seen in kittens and younger cats.

Two types of ascarids can infest cats. They are Toxocara cati and Toxascaris leonine. Both adult forms of these parasites live in the small intestines of cats. Microscopic worm eggs are passed into the environment in the faeces (stool) and may be swallowed by other cats that can pick the eggs up on their feet (the eggs are sticky) from soil and even grass and later lick them off.

In the case of T. cati, after the eggs hatch, the larval forms migrate through the wall of the intestine to the blood stream and are carried to the liver and lungs where they feed on those tissues and grow larger as they do.  After feeding in the lung tissue, the adult worms migrate into an airway, are coughed up and swallowed.  They then attach themselves to the lining of the intestine where they lives absorbing nutrients and excreting millions of microscopic eggs.

In some cases, the larval forms of the worm will form a dormant cyst in the muscles of female cats.  If the cat become pregnant, when she produces the hormone prolactin  (induces milk production), the larvae become activated, are carried in the blood stream to the mammary gland where they then burrow through the tissue into the milk duct and are then delivered in the milk to the nursing kittens.  In this way, the kittens can become infested with this parasite.

On rare occasions, it is possible for people to become infested with this parasite.  During the migratory phase the larvae may migrate through other tissues of the body causing disease.  This is why it is important to deworm cats on a regular basis.  Even indoor cats can become infested from eating worm eggs tracked in on people’s shoes.

In the case of T. leonina the larvae mature in the wall of the small intestine and thus no migration through tissues takes place.

Because of the potentially debilitating effect of roundworms, all cats should be dewormed on an annual basis if indoors and every three months if outdoors.


Rare in Alberta but in heavy infestations can cause bloody diarrhea, poor coat and weight loss in adults living in unclean and crowded conditions.

Three types of Hookworms can infest cats.  They are two Ancylostoma species and Uncinaria stenocephala.  They are uncommon in Alberta because they usually occur in warmer damper climates.  They can be transmitted to cats through the intestine after the egg hatches in the same manner as the ascarid worms or the larvae can infect the cat or people by skin penetration whereupon they migrate to the lungs, are coughed up and swallowed then become adults in the intestines.  In people they can cause a condition called Cutaneous Larva Migrans.


Diagnosis of Roundworms

Unfortunately, there is no surefire diagnostically accurate test for roundworms and so regular deworming of susceptible individuals and kittens or those individuals where worm infestation is suspected may be the best course of action.

Tape Worms (Flat Worms)

Severe infestation can cause weight loss, diarrhea, vomiting, itching around the anus and occasional intestinal obstruction.  Normally affects adult cats.

There are two types of tape worms which can infest cats. They are Dipylidium caninum and Taenia taeninaeformis and the adult forms of both live in the small intestine. All tape worms require an intermediate host (another species) to complete their life cycle. For D. caninum the larval form is ingested by eating fleas, for T. Taeninaeformis the larval form is ingested by eating rodents (mice and rats).

Diagnosis of Tape Worms

Tapeworm segments can sometimes be seen in the faeces (stool).  There is no specific diagnostic test for tape worms other than visualization of the segments which usually look like a grain of wet rice.

Outdoor cats should be dewormed every three months. If the cat is a mouser deworming may have to be done monthly.


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Last updated: November 1, 2020

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