Roundworms in Puppies and Dogs

Causes, Treatment, and Prevention

Signs of Roundworms in Dogs

The Spruce / Kelly Miller

There are several species of roundworms that can infect dogs, but the most common species we see is Toxocara Canis. These canine parasites usually live in the intestines, soaking up nutrients from the dog's diet. They are most commonly found in puppies but can also affect adult dogs. When left untreated, a roundworm infestation can lead to stunted growth in puppies as the worms take up essential nutrients. Dogs contract roundworms by ingesting them through a variety of sources or through their mothers at birth. Owners of dogs and puppies with roundworms may observe symptoms like vomiting, changes in weight, lethargy, coughing, diarrhea, and more.

What Are Roundworms?

Roundworms are intestinal parasites in dogs that feed off their host's nutrients, and they are the most common gastrointestinal worm to affect canines. These parasites are round, up to 7 inches long, and white to pale brown in color. They somewhat resemble spaghetti noodles.

The medical term for infection of roundworms is ascariasis. Toxocara canis is the most common type of roundworm in dogs and puppies; it can also infect humans. Other Toxocara species exist, like Toxocara Cati (which infects cats) and Toxocara Leonina (which can infect both cats and dogs, but it's uncommon).

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

Symptoms of Roundworms in Puppies and Dogs

Adult dogs can have roundworms inside their bodies that are unable to mature into adult worms thanks to the dog's immune system. Symptoms are more noticeable in puppies younger than six months old. Once the puppy is old enough, roundworm larvae become encysted in the bodily tissues, unable to mature. Roundworm infections 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

Roundworms are usually passed through the body via feces, but it's possible for your dog to vomit these worms as well. 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.

Dull fur is another common symptom as the worms begin using up the dog's important nutrients. In adult dogs, roundworm larvae are usually inactive. Once a dog's immune system is suppressed (due to pregnancy or other diseases), the larvae can infect the dog once again.

Causes of Roundworms

Roundworms can infect puppies 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 from areas with previous roundworm fecal contamination, which will hatch into larvae after being eaten.
  • Puppies may eat rodents ​that are infected with roundworm larvae.

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.

Diagnosing Roundworms in Puppies and Dogs

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. Roundworm eggs are not visible to the naked eye, so even if you can't see worms in your pet's feces, they can still have an infection that needs to be diagnosed by your veterinarian.

It is possible to identify the species of roundworm based on the appearance of their eggs. When roundworms appear in vomit or stool, the diagnosis is easier to make. It is common for puppies to have other types of infections as well (such as hookworms), which makes it important for the veterinarian to run fecal floats to make sure no other parasites are present.


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 best option for your dog. Medications will only affect adult roundworms, however, so they 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 its situation. The treatment course will be determined by your vet. If you have a pregnant dog, consult your vet about treatment options to deworm both the mom and her pups.

Prognosis for Puppies and Dogs With Roundworms

Thankfully, most dogs and puppies that experience roundworm infestations can recover fully with treatment. It's possible for roundworms to be fatal in young puppies with weaker immune systems, so if there are any signs that your puppy's mother or littermates may have contracted these worms, begin treatment with your veterinarian right away. In adult dogs, treatments are usually very effective (even if they require repetition).

Are Roundworms Contagious to Humans?

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. Roundworm larvae migrating through the tissues can cause a fever, elevated white blood cell levels, or enlarged liver.

Most cases are not serious, but when they are, organ damage is possible as a 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.

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

How to Prevent Roundworms

Roundworms can be prevented in dogs through medication, behavioral changes, and hygiene practices.

Preventative Medication

Many of the monthly medications designed for parasite control, including some heartworm medications, 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. 

Sanitary Procedures

Pick up your pet's waste promptly. Always wash your hands after handling waste from your dogs, and prevent your dog from eating any waste left in the yard or on walks.

Monitoring Behavior

While it's not always possible at all times, monitoring your dog to ensure it doesn't eat any small animals is an important step to preventing roundworm infections. Some dogs are more prone to this behavior—especially those bred to hunt—so it will take extra care on the owner's part to prevent occurrences.

  • How long do roundworms live outside the body?

    Roundworm can live for up to two weeks on bedding, carpet, or floors.

  • How easy is it to get roundworms from a puppy?

    It's possible for humans to get roundworms from a puppy, but it's very rare. It's not something you should worry about unless you spend time working with puppy feces.

  • What kills roundworms?

    To kill roundworms, your veterinarian will prescribe a dewormer medication given to your dog multiple times to kill the worms and their larvae. Medications only kill adult roundworms, so it's important to follow your veterinarian's instructions closely for repeated treatments.

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.
Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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