Toxocara canis (T. canis, also known as dog roundworm) is a worldwide-distributed helminth parasite that primarily infects dogs and other canids, but can also infect other animals including humans. The name is derived from the Greek word toxon 'bow, quiver' and the Latin word caro 'flesh'. T. canis live in the small intestine of the definitive host. This parasite is very common in puppies and somewhat less common in adult dogs. In adult dogs, infection is usually asymptomatic but may be characterized by diarrhea. By contrast, untreated infection with Toxocara canis can be fatal in puppies, causing diarrhea, vomiting, pneumonia, enlarged abdomen, flatulence, poor growth rate, and other complications.

As paratenic hosts, a number of vertebrates, including humans, and some invertebrates can become infected. Humans are infected, like other paratenic hosts, by ingestion of embryonated T. canis eggs. The disease (toxocariasis) caused by migrating T. canis larvae results in two syndromes: visceral larva migrans and ocular larva migrans. Owing to transmission of the infection from the mother to her puppies, preventive anthelmintic treatment of newborn puppies is strongly recommended. Several anthelmintic drugs are effective against adult worms, for example fenbendazole, milbemycin, moxidectin, piperazine, pyrantel, and selamectin.

T. canis is dioecious, having morphology distinctly different between the male and female. Male worms measure 4 to 6 cm (1.5" to 2.3"), typically smaller than female worms who measure at 6.5 to 15 cm (2.6" to 5.9"). The male's posterior end is curved ventrally and the tail is bluntly pointed. The male has a single tubular testis. They also have simple spicules, which allow for direct sperm transfer. In the female, the vulva is about one-third the body length from the anterior end. The ovaries are very large and extensive. The uteri contain up to 27 million eggs at a time.

Both males and females have three prominent lips. Each lip has a dentigerous ridge. The lateral hypodermal chords are visible with the naked eye. No gubernaculum is present. In both sexes there are prominent cervical alae. The adult T. canis has a round body with spiky cranial and caudal parts, covered by yellow cuticula. Toxocara canis is gonochoristic. The cranial part of the body contains two lateral alae (length 2 to 3.5 mm, width 0.1 mm). The eggs are brownish and almost spherical.T. canis eggs have oval or spherical shapes with granulated surfaces, are thick-walled, and measure from 72 to 85 μm. The eggs are very resistant to various weather and chemical conditions typically found in soil.^{[citation needed]}

Eggs are deposited in feces of dogs, becoming infectious after 2–4 weeks. Dogs ingest infectious eggs, allowing the eggs to hatch and the larval form of the parasite to penetrate through the gut wall. In dogs under 3 months of age, the larvae hatch in the small intestine, get into the bloodstream, migrate through the liver, and enter the lungs. Once in the lungs, the larvae crawl up the trachea. The larvae are then coughed up and swallowed, leading back down to the small intestine, where they mature to adulthood. This process is called tracheal migration. In dogs older than 3 months of age, the larvae hatch in the small intestine and enter the bloodstream, where they are carried to somatic sites throughout the body (muscles, kidney, mammary glands, etc.) where they become encysted second stage larvae. This process is called somatic migration. At the height of pregnancy, the encysted eggs in an infected female dog will migrate from the mother to the developing fetus, where they will reside in the liver. After parturition, the larvae migrate from the pup's liver to the lungs to undergo tracheal migration. Alternatively, the migrating larvae in the mother may encyst within the mammary glands, becoming active during lactation and passing directly to the nursing puppy via the milk. Larvae transmitted in this manner do not migrate once they are within the small intestine of the puppy; they will develop directly into the adult stage in the small intestine. Once infected, a female dog will usually harbor sufficient larvae to subsequently infect all of her litters, even if she never again encounters an infection. A certain amount of the female dog's dormant larvae penetrate into the intestinal lumen, where molting into adulthood takes place again, thus leading to a new release of eggs containing L₁ larvae.

Another possible route of infection is the ingestion of paratenic hosts that contain encysted larvae from egg consumption, allowing the parasite to escape from the paratenic host and grow to adulthood within the small intestine of its definitive host, the dog.

Four modes of infection are associated with this species. These modes of infection include direct transmission, prenatal transmission, paratenic transmission, and transmammary transmission.

Transmammary transmission occurs when the suckling pup becomes infected by the presence of L₃ larvae in the milk during the first three weeks of lactation. There is no migration in the pup via this route.