There are two species of roundworms that can affect dogs, Toxocara canis and Toxascaris leonina. Because both are treated with the same medication, determining which species is present is not necessary during diagnosis, which can be done with a fecal flotation exam.

Dogs can become infected with roundworms by consuming worm eggs that have been passed from a host's feces and infected the soil, consuming an animal (such as a rodent) that is carrying developing worms, nursing from an infected mother, or during embryonic development in an infected pregnant dog.

Once consumed, the eggs of the Toxocara canis hatch in the dog's intestinal tract and the young worms make their way into the other body tissues where they can remain for years. The worms typically encyst in a dog's liver, then migrate to the lungs where heavy infections can lead to pneumonia. Coughing results when the worms are in the upper airways and the worms are coughed up then swallowed and renter the intestinal tract once again where they mate and start the whole process over again.

In pregnant dogs, the larvae migrate to the uterus, rather than the mother's lungs, than infect the unborn puppies and migrate to the lungs of the puppies. In nursing mothers, the larvae migrate to the mammary gland and enter the milk consumed by the puppies. Preventing mother to puppy transmission is usually very difficult, and puppies are often automatically dewormed as a precaution.

Roundworm infection can lead to diarrhea, vomiting, and a pot-bellied appearance. Very heavy infections can also result in pneumonia and obstruction of the intestine. In many cases, you may see no symptoms at all, though some dogs will vomit up a worm. The worms are long and white, similar to spaghetti, and can be as long as seven inches in length. Fecal testing can determine if roundworms are present.

Deworming products are generally effective in treating roundworm infections. However, when deworming, only those worms in the intestinal tract are affected. Because of this, a second and sometimes even a third deworming is usually required.

Humans can be infected with the Toxocara canis roundworms if they inadvertently consume worm eggs in soil. In humans, this leads to a serious condition known as Visceral Larva Migrans which can result in extreme inflammatory reactions and even blindness. Proper hand-washing and monthly pet deworming can help to prevent roundworm infections in humans.