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Roundworms in Dogs

Signs, Diagnosis and Treatment of Roundworms in Dogs

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

Roundworms (Toxocara canis) from a dog

Flukeman / Wikimedia Commons

About Roundworms in Dogs
Roundworms are intestinal parasites that are common in dogs. There are two species of roundworms that infect dogs, one of which can cause health problems in humans, too.

Roundworms are round, up to seven inches long, and white to pale brown in color (they look a little like spaghetti noodles). They hang out in the intestines, soaking up nutrients from the dog's diet.

The medical term for Infection with roundworms is ascariasis.

The Roundworms and Their Life Cycles

Toxocara canis

  • seen commonly, especially in puppies
  • larvae can infect humans
  • complex life cycle


Toxocara canis worms can infect dogs 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.
  • ingesting eggs (which hatch into lavae after being eaten)
  • eating rodents who have been infected with the larvae


In puppies, the larvae (including those that are swallowed and ingested as eggs which then hatch) migrate through the body tissues. Eventually reaching the lungs, they make their way up the windpipe, and are coughed up, then swallowed. Once swallowed this time, the larvae then become adult roundworms in the intestines. These produce numerous eggs, which are passed in the feces. Eggs only become infective after a week or two in the environment.

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, and 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 mom's intestines, so she then sheds numerous eggs that can also infect the pups. T. canis roundworms have a life cycle that makes them very effective at infecting puppies.

Toxascara leonina

  • less common
  • more often seen in older dogs
  • usually do not infect humans
  • also seen in cats

Toxascaris leonina has a much more straightforward life cycle. The eggs are shed in the feces, and once ingested they develop into adult roundworms in the intestines (no migration required) that eventually shed more eggs. The eggs are only infective after several days in the environment. Rodents can also become infected with T. Leonina, and dog can become infected from eating an infected rodent.


Signs and Symptoms of Roundworms
Roundworms (both types) can produce any of the following symptoms, though symptoms may only appear with heavy infections:
-puppies will sometime vomit up roundworms or pass them in their feces
-failure to gain weight, or weight loss
-dull hair coat
-pot-bellied appearance
-diarrhea
-sometimes coughing due to the larval migration through the lungs

Rarely, very heavy infections can lead to blockage of the intestines, which will make a puppy very ill (vomiting, lethargy).

Diagnosis of Roundworms
The eggs of roundworms are detected under the microscope in a routine check of a stool sample (the test process is called fecal flotation). It is possible to tell the difference between the species of roundowrm based on the appearance of their eggs. Of course, when roundworms appear in vomit or stool, the diagnosis is pretty clear!

Treating Roundworms
Treatment is the same, regardless of the species. 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 need to be repeated to deal with any residual larvae as they mature (e.g. at 2 week intervals). The number of treatments necessary will depend on the age of the dog and the situation, and will be recommended by your vet. If you have a pregnant dog, consult your vet for advice on deworming the mom and pups. Once dewormed, many of the monthly medications designed for parasite control contain medication that will prevent roundworm infections on an ongoing basis -- options that can be discussed with your vet. If your dog is not on one of these preventatives, your vet will recommend a regular deworming 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 Dog Roundworms
The larvae of Toxocara canis roundworms can infect people as well as dogs. This happens when eggs are ingested, and is most common in children who may not practice the best hygiene, and may pick up eggs on their hands when playing in the yard, for example. Good hygiene is important, clearly, both in terms of hand washing practices and keeping pet waste picked up.

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 possibly blindness. Migration of roundworm larvae through human tissues is called "visceral larva migrans" while migration to the eyes is called "ocular larva migrans." Proper prevention of roundworm infection is important to prevent these human health problems.


Please note: this article has been provided for informational purposes only. If your pet is showing any signs of illness, please consult a veterinarian as quickly as possible.

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