Canine Roundworms

roundworm_infection_1_2009Roundworms (nematodes or ascarids) are parasites that live freely in the intestine, feeding off of partially digested intestinal contents. Their name is derived from their tubular or “round” shape.

Toxocara canis and Toxascaris leonina are two important species of roundworms in dogs. Toxocara canis causes more significant disease and also may be transmitted to humans.

How did my dog get roundworms?

Infected dogs shed the microscopic roundworm eggs in their feces. Other dogs may become infected by sniffing or licking infected feces. Roundworm eggs can also be spread by other animals such as rodents, earthworms, roaches, house flies and birds. In these animals, the roundworms are merely transported, and do not mature into adults; if a dog eats this “paratenic” host, the roundworm is liberated and completes its maturation.

In the dog, the roundworm undergoes a complicated life cycle before mature roundworms are found in the intestinal tract. This involves several stages and includes migration through various bodily tissues. Roundworm larvae (immature worms) can migrate and encyst throughout the host’s body. This is important in the female dog because these encysted larvae will start to develop during pregnancy and ultimately cross the placenta into an unborn puppy. Thus puppies can be born with roundworms and can pass fertile eggs from adult worms in their stools by the time they are approximately eleven days old. Roundworm larvae may also enter the mother’s mammary glands and be transmitted through the milk.

Are roundworms a threat to my dog?

Roundworms are most threatening to puppies. The most common consequence of roundworms is growth reduction. Since roundworms eat partially digested food in the intestinal tract, if there is a large burden of roundworms they will rob the growing puppy of vital nutrients.


The life cycle of Toxocara canis is more complicated than that of other nematode worms found in dogs. Roundworms can complete their life cycle in immature dogs, but as the pup’s immune system matures (usually by 6 months of age), the larval stages will become “arrested” or encyst in the pup’s muscles. Roundworm larvae swallowed by adult dogs usually encyst in the dog’s tissues. Few roundworms will mature in the bowel of infected adult dogs. Consequently few eggs are passed in the stool leading to difficulty in definitive diagnosis and treatment.

In intact female dogs, encysted Toxocara canis larvae can resume development after estrus, and she can shed eggs in the stool at that time. With dogs that have been spayed or castrated, development of a roundworm infection may indicate that there is an underlying immune system disease that has allowed the encysted larvae to complete their life cycle.

Toxascaris leonina can complete their life cycle in any age of dog, or in cats.

How are roundworms diagnosed?

In puppies, clinical signs such as stunted growth, potbelly, and recurrent diarrhea are a good indication of roundworm infection. To diagnose roundworm infection, a fecal ova and parasite screen will be run.  A small amount (~2grams) of fresh feces (not more than 24 hours old) is needed to run the test.   This sample will be sent to a reference lab which uses a testing process called centrifugal floatation with microscopic examination to look for parasite eggs . This is the most accurate way to get a diagnosis as simple fecal flotations may miss parasite eggs.  In some cases, it may take more than one test to find the parasite as they are not always shed in the fecal mater continuously. Occasionally, intact adult roundworms can be found in the dog’s stool or vomit.

Are roundworms a danger to my family or me?

Roundworms can be a health risk for humans. The most common source of human infection is by ingesting eggs that have come from soil contaminated with cat or dog feces. As many as 10,000 cases of roundworm infection in humans have been reported in one year in the United States. Children are at an increased risk for health problems should they become infected. A variety of organs, including the eyes, may be affected as the larvae migrate through the body. In suitable environments, the eggs may remain infective to humans and cats for years.

How are roundworms treated?

Treatment is simple and effective. There are many safe and effective preparations available to kill adult roundworms in the intestine.

Many heartworm preventives contain medications that are effective against intestinal roundworms help prevent future infections. Some of these preparations only the adult worms and do not affect migrating or encysting larvae. New products have recently been developed that will also affect the larval stages. Your veterinarian will advise you on the best treatment and prevention plan for your pet.

What about roundworm eggs shed in the environment?

Initially the eggs are not infective. After a period, which may vary from weeks to months, the eggs develop into infective larvae. Under ideal conditions, this takes approximately four weeks. These infective larvae can remain viable in the environment for long periods of time and are particularly resistant to changes in temperature and humidity.

The best environmental treatment is prevention. Remove your dog’s feces as soon as possible to prevent the spread and transmission of roundworms.

What is the most effective strategy I can use to control infection in my dogs, protect my family and reduce contamination of the environment?

Deworm pregnant dogs in late pregnancy, after the 42nd day or after six weeks of pregnancy. This will help reduce potential contamination of the environment for newborn puppies.

All puppies should be appropriately dewormed as recommended by your veterinarian. The first deworming should be given at two to three weeks of age. Note that this is prior to the time most puppies are seen for first vaccines. Deworming is then repeated every 2 weeks until 12 weeks of age, then monthly until 6 months of age in puppies. It is entirely appropriate to present new puppy for initial examination and deworming only. Routine deworming of adult dogs is also recommended.  Dosing ranges from monthly during warmer months and periodically during cooler months to year round monthly treatments depending on risk.  Your vet can help you determine the most appropriate deworming program for your dog.

It is also advisable for all dogs to have a fecal examination once to several times a yearly depending on their risk.  This way we can make sure our dewoming programs are working and also pick up other parasites not currently being treated for.

The use of a heartworm preventative product monthly that is effective against roundworms will stop the shedding of eggs into the environment.

Rodent control is important since rodents can serve as a source of infection.

Dogs should be prevented from defecating in children’s play areas and there should be prompt disposal of all dog feces, especially in gardens, playgrounds and public parks.

Practice strict hygiene particularly with children. Do not allow them to play in potentially contaminated environments and ensure proper and frequent hand washing.

This client information sheet is based on material written by: Ernest Ward, DVM (modified by MAH Jan/2106)

© Copyright 2009 Lifelearn Inc. Used and/or modified with permission under license.

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