If you need a more accessible version of this website, click this button on the right. Switch to Accessible Site


You are using an outdated browser. Please upgrade your browser to improve your experience.

Close [x]


Roundworms are relatively large robust worms, about 3 inches on average and up to 5 inches in length. Roundworms are very common and it has been speculated that 85% of puppies are infected with this intestinal parasite. They are diagnosed by standard microscopic fecal exams and false negatives occur when no eggs have been shed into the sample presented. If a dog or puppy vomits up a worm, there is a good chance this is a roundworm (especially in a puppy). Roundworms are long, white and described as looking like spaghetti.  Fecal testing for worm eggs is a must for puppies and a good idea for adult dogs having their annual check up. If there are worms present they must be laying eggs in order to be detected and fecal testing is a reliable method of detection.


Infection occurs by ingestion of eggs when contaminated feces are shed into the environment (generally through normal grooming). However, the vast majority of infection occurs through transmission of the worm through the placenta from the bitch to the pup. As a consequence almost all puppies are positive for roundworms. Puppies and kittens can also be infected through nursing.  Infection can also occur through consuming a prey animal (usually rodent) that is carrying developing worms.  (See the roundworm life cycle below)

Clinical signs:

  • Abdominal pain
  • Bloating or classical “pot-belly” appearance
  • Dull coat, diarrhea
  • Weight loss 
  • Migration of worms can cause respiratory signs that mimic upper respiratory infections. Occasionally, serious pneumonia results.
  • Adult dogs rarely show gatrointestinal signs but can compromise their overall condition, especially if other diseases are active.


How do we know if our dog is infected? You may not know and this is one of the arguments in favor of regular deworming. Regular deworming is especially recommended for dogs that hunt and might consume the flesh of hosts carrying worm larvae. Puppies are frequently simply assumed to be infected and are automatically dewormed.

Treatment of roundworms in puppies consists of oral medication at 4, 6 and 8 weeks, followed by a microscopic fecal exam at 11-12 weeks. In adults, annual fecal exams are recommended to remove asymptomatic infections. Numerous deworming products are effective. Many flea control and/or heartworm prevention products provide a monthly deworming which is especially helpful in minimizing environmental contamination. 

There are two important concepts to keep in mind about deworming. Medications essentially anesthetize the worm so that it lets go of its grip on the host intestine and passes with the stool. Once it has been passed, it cannot survive in the environment and dies.This means that you will likely see the worms when they pass so be prepared as they can be quite long and may still be alive and moving when you see them.

The other concept stems from the fact that all the larvae in migration cannot be killed by any of these products. After the worms are cleared from the intestine, they will be replaced by new worms completing their migration. This means that a second, and sometimes even a third deworming is needed to keep the intestine clear. The follow-up deworming is generally given several weeks following the first deworming to allow for migrating worms to arrive in the intestine where they are vulnerable.

*Roundworms do have a serious zoonotic (a disease that normally exists in animals but that can infect humans) threat when worm migration enters vital tissue of man, in particular the eye. Children are at the greatest risk because they tend to put their fingers into their mouth without proper hygiene. The high incidence of roudworms and the serious health concern in children make a convincing argument for vigorous round worming schedules in puppies.


Life Cycle:

Step One:

Toxocara eggs are passed in the host's feces. If a fecal sample is tested, the eggs can be detected. The embryonic worm develops in the outdoor enviroment inside its microscopic egg for one month before it becomes able to infect a new host. If environmental conditions are favorable, it takes about a month for the egg to become infective, but Toxocara eggs are famous for weathering harsh environmental conditions. Eggs can remain infective for months to years. 

Step two:

The egg containing what is called a “second stage larva” is picked up from the dirt by a dog or by some other animal, usually in the course of normal grooming. The egg hatches in the new host’s intestinal tract and the young worm burrows its way out of the intestinal tract to encyst in the host’s other body tissues.


Step three:

These second stage larvae can remain encysted happily for years. If the host is a dog, the larvae mostly encyst in the host’s liver. When the time comes to move on, the larvae excyst and migrate to the host’s lungs where they develop into “third stage larvae.” They burrow into the small airways and travel upward towards the host’s throat. A heavy infection can produce a serious pneumonia.  When they get to the upper airways, their presence generates coughing. The worms are coughed up into the host’s throat where they are swallowed thus entering the intestinal tract for the second time in their development.

If the host is pregnant, the larvae do not migrate to the lung after they excyst; instead they home to the uterus and infect the unborn puppies. The second stage larvae make their way to the puppies’ lungs to develop into third stage larvae.

If the host is a nursing mother, second stage larvae can migrate to the mammary gland instead of the lung after excyst. Puppies can be infected by drinking their mother’s milk, though, due to the intrauterine cycle described above, the litter would probably already be infected.

Note: When dogs are dewormed with traditional dewormers, this affects only worms in the intestinal tract. It does not affect encysted larvae.

Step four:

Once back in the intestine, the larvae complete their maturation and begin to mate. The first eggs are laid about one week after the fourth stage larvae have arrived in the intestine and about 4-5 weeks after infection has first occurred. From here the cycle repeats.