Roundworms in Puppies and Dogs

Beagle puppy lying down
Dan Steel / Getty Images

There are two species of roundworms that infect dogs, one of which can cause health problems in humans, too. They are most often found in the intestines, soaking up nutrients from the dog's diet. There are two common species of roundworms that may infect dogs, Toxocara canis and Toxocara catis.

What Are Roundworms?

Roundworms are intestinal parasites that are common in dogs. These parasites are round, up to 7 inches long, and white to pale brown in color. They look a little like spaghetti noodles. The medical term for infection with roundworms is ascariasis.

Toxocara canis is a very common type of roundworm, and is often seen in puppies; it can also infect humans. 

Toxocara catis is a far less common species of roundworm. It is more often seen in older dogs and cats than in puppies. It is also less likely to infect humans. While its life cycle is different from that of Toxocara canis, symptoms are similar.

Roundworms (Toxocara canis) from a dog by Flukeman / Wikimedia Commons
Flukeman / Wikimedia Commons

Signs and Symptoms of Roundworms

Both types of roundworms can produce any of the following symptoms, though symptoms may only appear with heavy infections:

  • Vomiting
  • Lethargy
  • Roundworms in feces
  • Failure to gain weight or weight loss
  • Dull fur
  • Pot-bellied appearance
  • Diarrhea
  • Coughing due to larval migration through the lungs
illustration of signs of roundworms in dogs
Illustration: Kelly Leigh Miller. © The Spruce, 2018 

Causes of Roundworms

Roundworms can infect dogs in a number of ways:

  • Puppies can be infected with larvae while still in their mother's uterus (a very common route of infection for puppies).
  • Puppies can ingest larvae through their mother's milk.
  • Puppies may ingest eggs which hatch into larvae after being eaten.
  • Puppies may eat rodents ​who have been infected with the larvae.

In puppies, the larvae (including those that are swallowed and ingested as eggs) migrate through the body tissues. Eventually reaching the lungs, they make their way up the windpipe and are coughed up, then swallowed. Once swallowed, the larvae become adult roundworms in the intestines. These produce numerous eggs, which are passed in the feces.

In adult dogs, the situation is usually different: After ingestion, the larvae migrate through the intestinal wall and into other tissues in the body, where they enter a dormant state. In a pregnant female, the larvae become active again. The larvae can cross the placenta into the pups and be secreted in the milk after birth. They can also produce an active adult roundworm infection in the mother's intestines; when this happens, the mother sheds numerous eggs that can also infect the pups. 

Diagnosis of Roundworms

Roundworm eggs are detected under the microscope in a routine check of a stool sample (the test process is called fecal flotation) at the veterinarian's office. It is possible to identify the species of roundworm based on the appearance of their eggs. Of course, when roundworms appear in vomit or stool, the diagnosis is easy to make.


Treatment is the same, regardless of the species of roundworm. There are a number of medications that can be used to treat roundworms, and your vet can help you pick the one right for your dog. Medications will only affect adult roundworms, however, so should be repeated at two-week intervals to deal with any residual larvae as they mature.

The number of treatments necessary will depend on the age of the dog and their situation. The treatment course will be determined by your vet. If you have a pregnant dog, consult your vet for advice on deworming the mom and pups. Many of the monthly medications designed for parasite control will prevent roundworm infections on an ongoing basis. If your dog is not on one of these preventative medications, your vet will recommend a regular protocol to keep roundworms at bay. Keeping pet wastes picked up, and preventing pets from eating rodents can also help prevent infection with worms.

People and Roundworms

The larvae of Toxocara canis roundworms can infect people, as well as dogs. This happens when eggs are ingested and it is most common in children. The larvae don't develop into adult roundworms in people, but the larvae migrating through the tissues can cause inflammation, especially in young children. Most cases are not serious, but in serious cases, organ damage is possible as result of the migrating larvae (e.g., liver, lung, brain), and sometimes the larvae can reach the eyes, leading to visual disturbances and even blindness. 

How to Prevent Roundworms

Proper prevention of roundworm infection is important to prevent these human health problems. Good hygiene, including carefully cleaning up after dogs and washing hands, can prevent infection.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.