Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




 kitten with cat


There are two species of roundworms affecting cats and kittens: Toxocara cati and Toxascaris leonina. Toxocara cati is generally felt to be more harmful to cats but Toxascaris leonina is able to infect both dogs and cats alike so it is helpful to distinguish their eggs under the microscope so as to know which medications to use and which additional pets are at risk. There is overlap in treatment protocols with some dewormers but not with others.

Even though roundworms are occasionally passed or vomited up, most of the time these worms are unseen in the cat's intestinal tract; do not expect to see any sign of them in the litter box or in fecal material. You will most likely never know they are present unless tests are conducted. We will present the Toxocara cati information first followed by information for Toxascaris leonina.



Roundworm infection can have numerous negative effects. It is a common cause of diarrhea in young animals and can cause vomiting as well. Sometimes the worms themselves are vomited up which can be alarming as they can be quite large with females reaching lengths of up to seven inches. The worms consume the host’s food and can lead to un-thriftiness and a classical “pot-bellied” appearance. Very heavy infections can lead to pneumonia as the worms migrate and, if there are enough worms, the intestine can actually become obstructed.

It should also be noted that human infection by this parasite is especially serious (see below). It is important to minimize the contamination of environmental soil with the feces of infected animals so as to reduce the exposure hazard to both humans and other animals. A classical source of infection is a child's outdoor sandbox, in which outdoor cats may defecate.


In cats, there are three ways by which infection with Toxocara canis occurs:

  • Consuming infective worm eggs from soil in the environment (generally through normal grooming).
  • Nursing from an infected mother cat that was herself infected in late pregnancy (most kittens are infected this way).
  • Consuming a prey animal (usually rodent) that is carrying developing worms.

Note: dogs cannot be infected with Toxocara cati.
They have their own roundworm: Toxocara canis.

Adult Toxocara cati

Adult Toxocara cati
(Photocredit: CDC & Alaska State Public Health Library)


Toxocara cati has one of the most amazing life cycle in the animal kingdom. Having an understanding of how roundworms develop from eggs to adults will help ensure that a proper deworming schedule is maintained and that roundworm control plans makes sense. Let us begin at the beginning with the egg.

diagram of how roundworms develop from eggs to adults
(original graphic by

Unembryonated worm eggunembryonated worm egg
(Photocredit: CDC Division of Parasitic Diseases)


Toxocara eggs are passed in the host’s feces (if a fecal sample is tested, it is possible to detect the eggs and confirm roundworm infection). The embryonic worm develops in the outdoor environment inside its microscopic egg for 2-4 weeks before it becomes able to infect a new host. Toxocara eggs are famous for weathering harsh environmental conditions so they may remain infectious for a long time after their one month egg development is complete and long after any visible fecal matter has melted into the surrounding dirt. Eggs can remain infective for months to years.

Note: Fresh feces is not infectious. It takes a month out in the world before the worm embryo is infective.
In this time, visible fecal matter has melted into the soil and it is the soil/dirt that has the infectious eggs.

embryonated worm eggembryonated worm egg
(Photocredit: CDC Division of Parasitic Diseases)

cat with rodent in its mouth

The egg containing what is called a “second stage larva” is picked up orally by a cat or by some other animal. The egg hatches in the new host’s intestinal tract and the young worm burrows its way out of the intestinal tract to encyst in the host’s other body tissues. If the new host is a cat, the life cycle proceeds. If the new host is a member of another species, such as a rodent, the larvae wait encysted until the new host is eaten by a cat. These prey animals that carry worm larvae are called, "paratenic" or "transport" hosts. The cat is called the "definitive host."

cat licking paw
In the process of grooming, a cat

can consume roundworm eggs from soil

Carolyn Wil via wikimedia commons)

STEP THREE: THE YOUNG WORM MIGRATES THROUGH THE CAT'S BODY: At this point, the second stage larva has entered the cat's body and broken out of the intestinal tract. Some larvae simply migrate to the nearby liver and encyst there. Others move to the mammary gland (see later) but the majority will head for the cat's lungs and will have made it there (to the lungs) within three days of entering the cat. Once they get to the lung, they develop into "third stage larvae" and burrow into the small airways ultimately traveling upward towards the host’s throat. A heavy infection can produce a serious pneumonia.  When they get to the upper airways, their presence generates coughing. The worms are coughed up into the host’s throat where they are swallowed thus entering the intestinal tract for the second time in their development.

If the host is a nursing mother, second stage larvae can migrate to the mammary gland instead of the lung. Kittens can thus be infected by drinking their mother’s milk. Larvae that had encysted in the liver and gone dormant will re-awaken during the host's pregnancy, continuing their migration just in time to infect the nursing kittens. In this way, a well dewormed mother cat can still find herself infecting her kittens.

drawing of a cat nursing kittens
(original graphic by


Note: When cats are dewormed, this affects only worms in the intestinal tract.
It does not affect encysted larvae. It is very difficult to prevent
mother to kitten transmission and routine deworming is not adequate.


STEP FOUR: WORMS MATE AND LAY EGGS OF THEIR OWN: Once back in the intestine, the larvae mature into adults and begin to mate. The first eggs are laid about one week after the fourth stage larvae have arrived in the intestine and about 4-5 weeks after infection has first occurred. From here the cycle repeats.


There are some important "take home points" from this information:

  • Fresh feces is not infectious to cats but dirt could be.
  • Cats can get roundworms from cleaning dirt off their fur.
  • Cats can get roundworms from hunting.
  • Mother cats readily give roundworms to nursing kittens.
  • There is a whole body worm migration which means that all the worms in the process of migration must be killed off if the cat is to be "roundworm free." In other words, one deworming treatment is not enough.



You may not know and this is one of the arguments in favor of regular deworming. Regular deworming is especially recommended for cats that hunt and might consume the flesh of hosts carrying worm larvae. Kittens are simply assumed to be infected and automatically dewormed on a regular schedule.

Of course, there are ways to find out if your pet is infected. If a cat or kitten vomits up a worm, there is a good chance this is a roundworm (especially in a kitten). Roundworms are long, white and described as looking like spaghetti. Tapeworms can also be vomited up but these are flat and obviously segmented. If you are not sure what type of worm you are seeing, bring it to your vet’s office for identification.

Fecal testing for worm eggs is a must for kittens and a good idea for adult cats having their annual check up. Obviously, if there are worms present, they must be laying eggs in order to be detected but, by and large, fecal testing is a reliable method of detection.

Toxocara egg

Toxocara egg
(Photocredit: CDC Division of Parasitic Diseases)



Numerous deworming products are effective. Some are over the counter and some are prescription. Many flea control and/or heartworm prevention products provide a monthly deworming which is especially helpful in minimizing environmental contamination. Common active ingredients include:

  • Pyrantel pamoate (active ingredient in Strongid®, Nemex®, Drontal®, Heartgard® and others)
  • Piperazine (active ingredient in many over the counter products)
  • Fenbendazole (active ingredient in Panacur®)
  • Selamectin (active ingredient in Revolution®)
  • Emodepside (active ingredient in Profender®)
  • Moxidectin (Active ingredient in Advantage Multi®)
  • Eprinomectin (active ingredient in Centragard®)
  • Milbemycin oxime (active ingredient in Interceptor®)

There are two important concepts to keep in mind about deworming. Medications essentially anesthetize the worm so that it lets go of its grip on the host intestine and passes with the stool. Once it has been passed, it cannot survive in the environment and dies.

This means that you will likely see the worms when they pass, so be prepared
as they can be quite long and may still be alive and moving when you see them.

The other concept stems from the fact that larvae in migration cannot be killed by most deworming products. After the worms are cleared from the intestine by a deworm treatment, they will be replaced by new worms completing their migration. This means that a second, and sometimes even a third deworming is needed to keep the intestine clear. The follow-up deworming is generally given several weeks following the first deworming to allow for migrating worms to arrive in the intestine where they are vulnerable.

 Do not forget your follow-up deworming.

At this time the emodepside product and the eprinomectin product
are the only ones that can attack immature worms still in the process of migration,
as well as the intestinal adults with one treatment.

All other dewormers require repeat deworming.


The Companion Animal Parasite Council recommends that
routine deworming of kittens begins at age 2 weeks to prevent hookworm infection
and then the kittens should be placed on a monthly heartworm preventive
that includes protection/deworming against Toxocara cati.

For details on the CAPC roundworm guidelines, visit:



The life cycle of Toxascaris leonina is not nearly as complicated. This parasite does not migrate through the body in the way that Toxocara does. Instead, the Toxascaris second stage larva is consumed and simply matures in the intestine, a process which takes 2-3 months. Unlike Toxocara, Toxascaris can complete its life cycle in many host species besides the domestic cat. There is no encysting or arrested development as with Toxocara. The life cycle is simple: the egg is passed, it develops for a week, it is eaten by a new host, it develops for 2-3 more months in the host intestine, it finds a mate and produces eggs of its own.

 diagram of how Toxascaris leonina roundworms develop from eggs to adults
(Photocredit cat: original graphic by
(Photocredit squirrel:
(Photocredit Toxascaris egg: Joel Mills via Wikimedia Commons)

Note: Toxascaris leonina can infect both dogs and cats alike.

As with Toxocara cati diarrhea can result from the presence of Toxascaris leonina and deworming is needed to control symptoms and prevent contagion. Many of the newer deworming products are not labeled for use against Toxascaris leonina, which does not necessarily mean that they do not work but here are the reliable classics that have been used for decades with efficacy:

  • Pyrantel pamoate (active ingredient in Strongid®, Nemex®, Drontal®, Heartgard® and others)
  • Piperazine (active ingredient in many over the counter products)
  • Fenbendazole (active ingredient in Panacur®)

Because Toxocara has a body migration, a sequence of deworming is typically performed so as to get all the larvae that were migrating at the time of the earlier deworming.  Because Toxascaris does not migrate, only one deworming should cover it. Of course, a known infection of either worm suggests a contaminated environment which is why the Companion Animal Parasite Council recommends regular deworming for pet dogs and cats.


The Companion Animal Parasite Council has put up an educational site for cat owners on parasites including Roundworms:


two dogs on grass

Roundworms in Dogs & Puppies

mother playing with toddler
(Photocredit: U.S. Fish and Wildlife Service)

Roundworms in Humans


 Page last updated: 1/21/2023