Canine Roundworms

Filed Under: Dogs, Parasites

The canine roundworm or Toxocara canis is the most common parasite seen throughout the U.S. Roundworms are a spaghetti-type worm that are passed out in the stool. The occurrence of roundworms is especially common in young puppies and their nursing mothers.

Symptoms of infection include mucoid diarrhea, stunted growth, and an unthrifty coat. The most characteristic symptom is that of a pot-bellied appearance. Less commonly pneumonia, intestinal obstruction and even death may occur. Pneumonia may result from the third stage larva that migrates through the lungs.

Infection occurs by the spread of embryonated eggs in the stool and transplacental transmission (through the placenta) from the mother to the puppy. Transmammary transmission (through the mother’s milk) can occur but is uncommon in the dog. Toxocara larva in the soil may be consumed by chickens, rabbits, cockroaches, earthworms and mice, which become paratenic (intermediate) hosts when the larva becomes encysted within the body tissues. Predation (hunting and consuming) on those infected paratenic hosts may then lead to a pet becoming infected. The entire life cycle (time from infection before sexually mature adult worms are present) takes 28 to 35 days.

Numerous products are effective against roundworms and include Nemex® manufactured by Pfizer, Rid Worm® marketed by Hartz, Drontal Plus® produced by Bayer Animal Health, and Panacur C® marketed by Intervet. Several heartworm preventatives also prevent and treat roundworm infections and include Heartguard® from Merial, Interceptor® manufactured by Novartis, and Revolution® marketed by Pfizer. Nemex® can be used in puppies as young as two weeks of age. Cases with concurrent pneumonia should be treated with antibiotics, antihistamines, and in some cases corticosteroids may be indicated.

Currently both the Companion Animal Parasite Council (CAPC; and the Centers for Disease Control and Prevention (CDC; recommend deworming puppies beginning at two weeks of age and continuing every two weeks through eight weeks of age, which is termed strategic deworming. After eight weeks of age, it is suggested that puppies be placed on a heartworm and/or flea control product that will also have activity against other parasites. Owners should only feed their pets commercially prepared or cooked food and minimize predation, thereby effectively eliminating the paratenic host as a source of infection. Active infections in puppies can be easily detected by a fecal floatation or centrifugation by your local veterinarian.

Not all cases of roundworms can be easily detected. Second-stage larvae may become encysted in the somatic tissues of the mother dog until parturition (the process of giving birth). It is widely assumed that hormonal changes in the mother will cause the roundworm infection to again become active and thereby cause infection in the puppies. For this reason Strategic deworming is advocated although their may be no clinical signs of infection or appearance of eggs in a stool sample.

Roundworm infections can be Zoonotic (transmittable to humans). A report from 1985 found that 5% of children in the United States had a positive antibody titer to Toxocara spp. The study also demonstrated that in some population groups up to 30% of children 1 to 11 years of age were infected with Toxocara. Human exposure to roundworms may come from only two sources: dog or cat feces.

Visceral larva migrans (infection in humans due to canine roundworms) may occur in people due to the ingestion of infective eggs with second-stage larva of Toxocara. This Zoonotic infection occurs most commonly in children 16 to 32 months of age because children at this age are more likely to ingest infective eggs from the soil or place fingers in their mouths after handling puppy feces. The soil of public parks in cities tends to be heavily contaminated with infective eggs that can remain infective in soils for several years. The larvae can migrate to several tissues including the lungs, liver, brain and the eyes. Blindness, enlargement of the liver, or pneumonitis (inflammation of lung tissue) can result from this migration through the various organ systems. Routine hygiene by simply washing hands after handling puppy feces will prevent the occurrence of visceral larva migrans. Daily disposal of dog feces and excluding dogs from the area where small children play outside, especially sandboxes, will greatly reduce the possibility of Zoonotic infection. Canine feces should never be used to fertilize a vegetable garden.


Dryden, Michael. “Total Parasite Control: Piecing the Puzzle Together”. Parasite Developments. Proceedings from the 2007 NAVC Conference and 2007 Western Veterinary Conference. Supplement to Veterinary Forum. Vol. 24, No. 3 (A). March 2007. p. 20.

Bowman, Dwight. “Gastrointestinal Parasites: A Broad Approach”. Parasite Developments. Proceedings from the 2007 NAVC Conference and 2007 Western Veterinary Conference. Supplement to Veterinary Forum. Vol. 24, No. 3 (A). March 2007. pp. 13-15.

Georgi, Jay. “Parasitology for Veterinarians.” Second Edition. W.B. Saunders Inc. 1974. p 267, 268 and 270.

Topics: roundworms

Symptoms: eggs in feces

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