Roundworms (Intestinal Worms) in Cats and Dogs

Roundworms (Intestinal Worms) in Cats and Dogs

Animals Affected

Cats, dogs, puppies, and kittens.


Roundworms (often simply called worms) are extremely common in pets.   In dogs and cats, worms may cause no symptoms whatsoever, or they may cause diarrhea and other gastrointestinal problems.  Many safe and effective treatments and preventatives are available to keep pets free of worms.

Roundworms can spread to humans, especially children, and cause serious disease. Pets with roundworm infestations pose a public health risk.  However, healthy pets who receive regular roundworm preventatives are extremely unlikely to pose a risk to human beings.


  • Many pets show no symptoms.
  • When pets do exhibit symptoms, diarrhea is the most common one.  Diarrhea caused by roundworms can range from mildly loose stool to severe, watery bowel movements.
  • Kittens and puppies may have a pot-bellied appearance, and may fail to grow and thrive as they should.
  • Vomiting may occur in pets that are infested with roundworms.
  • Pets with roundworm infestations may have poor quality hair coats.
  • Weight loss may be a sign of roundworms.
  • Rarely, pets may pass worms in vomit or stool.

Risk Factors and Prevention

All pets are at risk of contracting roundworms, including indoor cats.

  • Youth is a risk factor. Puppies and kittens have extremely high rates of roundworm infestation because the worms can pass from the mother to her babies through the placenta or through the milk.
  • Exposure to worm eggs in the environment or through vectors (animals that can carry eggs from one location to another) increases the risk of infestation.  Active dogs may be exposed to eggs in outdoor areas.  Cats who hunt may be exposed to eggs or larvae through their prey.  Insects such as houseflies can carry eggs from the environment into the home.
  • Geography plays a role in risk.   Some areas have higher incidences of worms than others.  However, roundworms are present worldwide.
  • Preventative medications, including Interceptor®, HeartGard Plus®, Revolution® (in cats), and Advantage Multi® are effective against roundworms. When administered monthly, these products generally prevent roundworm infestations in pets.


  • Puppies and kittens with severe diarrhea due to roundworm infestations may become dehydrated, fail to grow, and even die.
  • Pets with roundworms may suffer weight loss or fail to thrive.
  • Roundworms can cause chronic diarrhea, and may contribute to house soiling.
  • Pets with roundworm infestations release eggs into the environment via the feces. These eggs can then spread the infestations to other pets or to human beings.


Roundworms are most commonly diagnosed by identification of their eggs in feces.   This is done by preparing the feces in a special manner and evaluating with a microscope. Fecal tests for roundworms not 100% accurate because the worms do not continuously shed eggs.  If only small numbers of eggs are present they may not be detected by the test.

Young puppies and kittens suffer an exceptionally high rate of roundworm infestation. Because of this, many experts recommend that veterinarians and owners assume all pets in this age group have worms regardless of the results of fecal tests.


The preventative medications listed above are effective in removing and preventing roundworms in most cases.  Other medications may be employed in deworming protocols. These include pyrantel pamoate (Drontal®, Strongid®) and fenbendazole (Panacur®).

Be sure to follow your veterinarian’s instructions carefully when deworming your pet.  This will help to ensure the efficacy of the procedure.

The Companion Animal Parasite Council, a group of experts in veterinary parasitology, recommends deworming all puppies and kittens at two week intervals beginning at a few weeks of age.  This is recommended regardless of the outcome of fecal tests.

The Companion Animal Parasite Council recommends that all adult pets receive a monthly roundworm preventative (usually in combination with a heartworm preventative). In addition to keeping individual pets free from worms, monthly preventatives reduce contamination of the environment.  This helps to reduce the risk of worms in both pets and humans.


In pets diagnosed with roundworms, repeated stool samples are recommended to monitor the effectiveness of treatment.

Many deworming protocols involve repeated doses of medication.  These doses should be administered according to the instructions of your veterinarian.

After receiving deworming medicines, some pets will pass dead or dying worms in their feces.


Although roundworms are a significant threat to public health, it should be emphasized that healthy pets who receive regular roundworm preventatives pose very little risk to human beings.

Worm eggs shed in the feces are not capable of infecting pets or humans until one to three weeks have passed.  Therefore, it is extremely unlikely that a human or pet will contract worms through direct contact with an infested individual.  Most cases of worms are contracted through exposure to mature eggs in the environment.  Regularly removing feces from the environment or the litter box helps to reduce the risk of exposure for both people and pets.

Roundworms are spread to people when mature eggs, originally passed through the feces, are consumed orally by a human being.

Other names for roundworms include Toxocara and Toxascaris.

Copyright © Eric Barchas, DVM. All rights reserved.
The contents of this page are provided for general informational purposes only. Under no circumstances should this page be substituted for professional consultation with a veterinarian.