Roundworms are the most common intestinal parasites found in dogs – and especially in young puppies, that are often infected at birth, or soon thereafter.



  • The larvae of one of the roundworms – Toxocara canis – lie dormant in the tissues of most adult female dogs. Hormonal changes during pregnancy can activate these larvae, which then migrate through the placenta to infect the unborn puppies.

  • Some immature forms of canine roundworms can also be transmitted through the nursing mother’s milk, although this is a less common route of infection.

  • Puppies can also become infected by eating dirt or feces containing worm eggs and larvae, by licking contaminated fur or paws, or by drinking contaminated water.


Most adult dogs become infected through contact with worm eggs and larvae in the environment.

Life Cycle

After being swallowed, roundworm eggs hatch in the dog's intestines. The larvae then invades the intestinal wall, and are carried by the blood to the liver.

From there, they migrate to the lungs, are coughed up, and are re-swallowed.

The parasites eventually make their way to the small intestine, where they spend their adulthood and produce eggs (more than 100,000 eggs per day!).

While most infected adult dogs show no sign of infection, they continue to shed worm eggs in their feces and can quickly contaminate a large area, contributing to the spread of infection.

Roundworm eggs can survive for many months or even years in the environment, even in extreme conditions - which may help explain why intestinal worm infections are so widespread.


Roundworms in dogs can result in a potbelly, poor coat, slow growth, vomiting and diarrhea.

Although not common, heavy infections (especially in puppies), can lead to intestinal blockage, and even intestinal rupture, at times causing death.

Health Risk To People

Canine roundworm infections are zoonotic – they can be transmitted from animals to humans.

Although direct contact with animals may increase the risk of transmission, most infections in humans occur from accidentally eating worm eggs and larvae in the environment.

Because of their play habits – and their tendency to put things in their mouths – young children are more vulnerable to worm infections.

Most canine roundworm infections go undetected in humans. While roundworm infections are not fatal in humans, they can lead to serious health problems if left untreated.

In some cases, roundworms have been known to migrate through body organs, occasionally causing damage to the liver, the lungs and the brain.

If larvae enter the eyes, blindness can result. This phenomenon, called visceral larva migrans (or ocular larva migrans), is serious enough to justify strict control measures for roundworms. The Centers for Disease Control (CDC) report that over 10,000 such cases occur each year in the U.S.


Since most puppies have roundworms, it is recommended that you give your dog a broad-spectrum dewormer that is effective against the major canine intestinal parasites, according to the following schedule:

  • at 6, 8, 10 and 12 weeks of age;
  • once a month until the age of 6 months (recommended by some dog health experts);
  • at least every 6 months after the age of 6 months.


For information on how to choose a good roundworm treatment for dogs and administer it to your dog, click here.