Roundworm, Canine

By Dr. Karen Burgess

Roundworms are an intestinal parasite that lives in the small intestine of the dog and live off of partially digested food.

Where do roundworms come from?
There are several ways that dogs become infected by Toxocara canis, or the common roundworm.

  • Eating microscopic worm eggs from the environment (example-infected dog defecates, another dog steps on soil containing roundworm eggs and then licks paws thereby ingesting the eggs)
  • Puppies that nurse from an infected mother’s milk
  • Eating an infected prey animal (usually rodent), these animals are carriers and themselves do not become sick but merely pass the worm on
  • While in utero from an infected mother

The life cycle of the roundworm is multistep and complicated, but ultimately adult roundworms are found in the intestine.

During development the immature worm migrates through the body and can encyst (essentially allowing it to “hibernate”) in a variety of tissues. When female dogs become pregnant the encysted worms are reactivated, released into the bloodstream and eventually pass on to the puppies through the placenta, milk, and mother’s feces. Puppies less than two weeks old can have dangerous roundworm infections.

What problems do roundworms cause?
Infection with roundworms can cause vomiting, diarrhea, weight loss, and potential death in the very young or weak. The adult roundworm is very long (up to seven inches) and can lead to intestinal obstruction. Migrating worms can cause respiratory signs including pneumonia.

How is roundworm infection diagnosed?
It should be assumed that all puppies are infected with roundworms and intestinal dewormings should start as early as two weeks of age; ideally mother dogs are dewormed even during pregnancy. A microscopic examination of a stool sample can also reveal roundworm eggs.

Are humans at risk from roundworms?
Oral consumption of roundworm eggs by humans can lead to major medical problems related worms migrating through the body. This is of particular concern with young children who are more likely to be exposed to fecal oral contact while playing outside or in sandboxes. In particular, human roundworm infections can cause serious liver disease and even blindness (ocular larval migrans).

How is my pet treated for roundworms?
Several very safe oral medications are available to treat roundworm infection. These treatments temporarily anesthetize the adult worm causing it to “let go” of the gut and then pass with stool. It is not uncommon to see worms in the stool after treatment that may even be moving. Repeated dewormings are necessary to create worms that are not yet adults in the intestine.

How is roundworm disease prevented?
Roundworm eggs are not immediately infective, so picking up pet feces immediately is the best prevention. Once in the environment, eggs can last for weeks to months and are quite resistant. Monthly heartworm preventative typically also contains a dose of intestinal dewormer providing added protection from environmental exposure. Control of rodents in the environment is also recommended.