Roundworms: Cats & Kittens

By Wendy C. Brooks, DVM, DipABVP
Educational Director,

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 is not 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.

How Infection Occurs

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.
adult Toxocara worms

Life as a Roundworm

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









Step One: Toxocara eggs are passed in the host’s feces. If a fecal sample is tested, the eggs can be detected. The embryonic worm develops in the outdoor environment inside its microscopic egg for one month before it becomes able to infect a new host. If environmental conditions are favorable, it takes about a month for the egg to become infective but Toxocara eggs are famous for weathering harsh environmental conditions. Eggs can remain infective for months to years.

Note: Fresh feces are not infectious.

Step Two: 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.

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 after infection. Those larvae that 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 traveling upward towards the host’s throat. A heavy infection can produce a serious pneumonia. When they get to the upper airways, they cause 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 encyst 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 infect 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 to 5 weeks after infection has first occurred. From here the cycle repeats.

Why is Infection Bad?

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 unthriftiness 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.

How do we know if our Cat is Infected?

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 egg37a2115340e4431e8a7b0f43a5b

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 and white, and are 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.

How do we Get Rid of Roundworms?

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 and praziquantel (active ingredients in Drontal® and Drontal Plus®)
  • Pyrantel pamoate (active ingredient in Strongid®, Nemex®, Heartgard Plus® 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®)

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 any of these 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 with one treatment can attack immature worms still in the process of migration as well as the intestinal adults. All other dewormers require repeat deworming.

What about Toxascaris Leonina?

The life cycle of Toxascaris leonina is not nearly as complicated. They do 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 that takes 2 to 3 months. Unlike ToxocaraToxascaris can infect hosts of other species, though with Toxascaris the larvae can develop into third stage larvae in these other hosts while with Toxocara larval development is arrested in species other than cats.


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

For more Information

The Companion Animal Parasite Council has an educational site for cat owners on parasites that includes roundworms.