Roundworms in Cats & Kittens

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There are two species of roundworms affecting cats and kittens: Toxocara cati and Toxascaris leonina. Both are treated with the same medication protocol so when eggs are seen on a fecal flotation exam it may not be necessary to determine which species is present. T. leonina can infect both dogs and cats so identifying this roundworm might be helpful in indicating which pets in the household are at risk for further contagion.



toxocara adult

adult Toxacara worms

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

  • Consuming infective worm eggs from soil in the environment (generally through normal grooming).
  • Nursing from a 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.


Toxocara cati has one of the most amazing life cycle in the animal kingdom. It is crucial to understand this life cycle if effective treatment is to be pursued.

Life Cycle Toxocara cat captioned

STEP ONE: Toxocara eggs are passed in the hostís 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 one month 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. Eggs can remain infective for months to years.

Note: Fresh feces is not infectious.

STEP TWO: The egg containing what is called a ďsecond stage larvaĒ is picked up orally by a cat or by some other animal. The egg hatches in the new hostís intestinal tract and the young worm burrows its way out of the intestinal tract to encyst in the hostís 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 hosts." The cat is called the "definitive host."

STEP THREE: These second stage larvae can remain encysted happily for years. If the host is a cat, though, most larvae waste no time encysting and continue their migration straight to the lungs. The majority of the incoming larvae have reached the cat's lungs by the third day post-infection. Those larvae that do stay behind encysted do so in the cat's liver. Once they get to the lung, they develop into "third stage larvae" and burrow into the small airways ultimately travelling upward towards the hostís 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 hostís throat where they are swallowed thus entering the intestinal tract for the second time in their development.

mother nursing kittensIf 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 motherís 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.



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: Once back in the intestine, the larvae complete their maturation 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.


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 hostís food and can lead to unthriftiness and a classical ďpot-belliedĒ 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.


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 frequently simply assumed to be infected and automatically dewormed.

Toxocara egg

Toxocara egg

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 vetís 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.


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:

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, 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 is the only one
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 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.

Life Cycle Toxascaris cat captioned

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


This site has been launched by Bayer largely to promote AdvantageMulti®, its new topical deworming/flea control product. The site contains general information on intestinal parasites of pets and potential contagion to humans:

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


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Roundworms in Dogs & Puppies


Roundworms in Humans

Page last updated: 1/9/2012