ROUNDWORMS IN CATS & KITTENS
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.
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. A classical source of infection is a child's outdoor sandbox, in which outdoor cats may defecate.
HOW INFECTION OCCURS:
LIFE AS A ROUNDWORM:
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.
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.
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."
Note: When cats are dewormed, this affects only worms in the intestinal tract.
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.
WHY IS IT GOOD TO KNOW THE LIFE CYCLE?
There are some important "take home points" from this information:
HOW DO WE KNOW IF OUR CAT IS INFECTED?
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:
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
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.
WHAT ABOUT TOXASCARIS LEONINA?
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.
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:
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.
FOR MORE INFORMATION:
The Companion Animal Parasite Council has put up an educational site for cat owners on parasites including Roundworms:
Page last updated: 1/21/2023